Note: Descriptions are shown in the official language in which they were submitted.
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Methods and Systems for Sensing Water in Liquids
This application claims the benefit of U.S. provisional patent application
60/043,566, filed April 14, 1997; U.S. provisional patent application
60/060,058, filed
September 25, 1997; and U.S. provisional patent application 60/074,315, filed
February
11, 1998; the disclosures of all of which are hereby incorporated by
reference.
TECHNICAL FIELD
The present invention relates to methods and systems for sensing water in
liquids.
More particularly, the present invention relates to methods and systems for
sensing water
in oils, hydraulic fluids, and other liquids i:n which it is desirable to
monitor and/or
control water content.
BACKGROUND OF THE INVENTION
Liquids, including oils, such as transformer oils, motor oils, transmission
fluids,
etc., may become contaminated with water and other materials during use.
Liquids
contaminated with water and other materials cause corrosion, wear, and
mechanical
damage to devices using contaminated liquids. Accordingly, it is desirable to
sense and
remove contaminants such as water from the: liquids.
Conventional methods for determining the water content of liquids, such as
Karl
Fischer titration, involve sampling the liquid, sending the sample to a lab,
and adding
a reagent to the sample to measure the water content. This method is
undesirable for a
variety of reasons. First, it is time consuming. Second, the water content of
a liquid
may change from the time a sample is obtained to the time when the results of
the test
are returned from a lab. Thus, a water content test which cannot be performed
on-site
and within a reasonable time after sampling may be unreliable.
One way to characterize the water content of a liquid is relative water
content,
such as percent relative humidity (%RH). Percent relative humidity is a
measure of the
water content of a material relative to the saturation point of the material
at a given
temperature. For example, the percent relative humidity of an oil may be 50%
at 100
degrees Fahrenheit. This indicates that the oil is 50% saturated at that
temperature, or,
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alternatively, a 50% RH reading indicates that the oil can absorb the same
amount of
water already in solution before the oil becomes saturated and water comes out
of
solution at that temperature.
In liquids in which the operating temperature varies over a wide range, it may
be
desirable to measure the absolute water content rather than the relative water
content
because relative water content varies with temperature. In other instances in
which the
operating temperature of a liquid remains relatively stable, %RH provides a
useful
measure of the water content of a liquid.
For a fixed absolute water content in a liquid, the %RH decreases with
increasing
temperature and increases with decreasing temperature. This temperature
dependency
of % RH measurements may lead to a false indication that the water content of
a liquid
is within acceptable limits. For example, a technician may measure the % RH of
a
transformer oil during operation when the oil is at an elevated temperature
and find that
the %RH is 50% . Since the oil is only 50% saturated, the technician may
conclude that
the water level is within acceptable limits. However, after the transformer
shuts down,
the temperature of the oil decreases, and the %RH rises. If the % RH rises to
100% , the
water dissolved in the oil will come out of solution and condense to form
droplets in the
oil. The undissolved water may combine with other contaminants in the oil and
cause
corrosion of the transformer. Undissolved water in transformer oil can also
reduce the
dielectric strength of the oil and may cause arcing. Thus, a low %RH
measurement for
a liquid in which the temperature may decrease may not indicate a safe water
content for
the liquid. Accordingly, there exists a need in the industry for a water
sensing system
which compensates for the temperature dependency of %RH measurements.
Another problem that exists is the presence of undissolved water in liquids
used
in hydraulic, lubrication, and transmission systems. Undissolved water may
cause wear
and damage to mechanical components. For example, if undissolved water in a
hydraulic liquid freezes, the water expands and may cause damage to
components, such
as valves. Undissolved water may also break down a lubricating liquid and
cause wear
of mechanical components. Undissolved water can also react with additives or
the
lubricating liquid itself to form acids which may corrode mechanical
components.
Furthermore, undissolved water itself can corrode mechanical components
through
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oxidation. Accordingly, there exists a need in the industry for accurately and
conveniently determining the water content of a liquid.
As stated above, in some instances, it may be appropriate to measure absolute
water content, and in other instances, it may be appropriate to measure
relative water
content, such as % RH. Features which are needed in a sensor which provides
either
measurement is the ability to function in an on-line, i.e., real time,
environment where
water content is measured in-line, the ability to provide quick and accurate
measurements, and the ability to exist inside the liquid without significantly
decreased
performance. A partial solution to th.e problems associated with conventional
water detection techniques is to remove the water from a liquid without
sensing the water
content. A purifier, e.g., a spinning disk purifier, a nozzle purifier, or a
tower purifier,
may be used to remove water from liquids. Typically, a liquid is circulated
through the
purifier to remove water and other contaminants from the liquid. Purification
results
may be verified through conventional water detection techniques, such as Karl
Fischer
titration. Because of the time and reliability problems associated with
conventional water
detection techniques, these techniques may be undesirable for on-line
purification
operations. Thus, there exists a need in the: industry for a liquid
purification system
which is capable of rapidly and accurately sensing the water content of a
liquid during
purification.
Summary of the Invention
According to a first aspect of the present invention, a water sensing system
is
capable of being coupled to a liquid to measure the water content of the
liquid. The
system may produce one or more output signals indicative of the water content
of the
liquid. The output signals may be used to monitor and control the water
content of a
liquid. Monitoring and controlling the water content of a liquid may be
performed from
a location proximal to or removed from the liquid.
According to another aspect of the present invention, a water sensing system
for
sensing water in a liquid includes a water sensor capable of being coupled to
the liquid
to produce a first signal having a first value indicative of a water content
of the liquid.
A processing circuit is coupled to the water sensor and arranged to produce an
output
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signal in accordance with the relationship between the first value and at
least two
predetermined threshold values.
According to another aspect of the present invention, a water sensing system
for
sensing water in a liquid includes a water sensor capable of being coupled to
the liquid
to produce a first signal having a first value indicative of a water content
of the liquid.
A processing circuit is coupled to the water sensor and arranged to produce an
output
signal based on the first value.
According to another aspect of the present invention, a water sensing system
for
sensing water in a liquid includes a water sensor capable of being coupled to
the liquid
to produce a first signal having a first value indicative of a relative water
content of the
liquid. A temperature sensor is capable of being coupled to the liquid to
produce a
second signal having a second value indicative of a temperature of the liquid.
A
processing circuit is coupled to the water sensor and the temperature sensor
to produce
a third signal in response to the first and second signals having a third
value indicative
of an absolute water content of the liquid.
According to another aspect of the present invention, a liquid purification
system
for sensing and removing water from a liquid includes a water sensor capable
of being
coupled to the liquid to produce a first signal having a first value
indicative of a water
content of the liquid. A processing circuit is coupled to the water sensor to
produce an
output signal. A purifier is coupled to the processing circuit to remove water
from the
liquid in response to the output signal.
According to another aspect of the present invention, a method for sensing
water
in a liquid includes sensing a relative water content value of the liquid,
sensing
a temperature of the liquid, and electronically converting the relative water
content value
to an absolute water content value. Electronically converting the relative
water content
value tv an absolute water content value may comprise using a look-up table,
or other
electronic means, such as an electronically implemented algorithm.
According to another aspect of the present invention, a method for removing
water from a liquid includes sensing a water content of the liquid, producing
a signal
indicative of the water content, and actuating a purifier to remove water from
the liquid
in response to the signal indicative of the water content.
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According to another aspect of the present invention, a method for calibrating
a
water sensor includes measuring the water content of a first medium having a
first known
water content value using a water sensor probe to produce a first output
signal,
measuring the water content of a second meduum having a second known water
content
value using the water sensor probe to produce a second output signal, and
electronically calibrating the water sensor using the first and second known
water content
values and the first and second output signals.
According to another aspect of the present invention, a method for calibrating
a
temperature sensor used in a water sensing system includes m a a s a r i n g t
h a
temperature of a temperature sensor probe using an external device, and
electronically
calibrating the temperature sensor using the temperature measured by the
external device.
Brief Description of the Drawings
Figure 1 is a block diagram of a water -sensing system according to an
embodiment of the present invention.
Figure 2 is detailed block diagram of a water sensing system according to
another
embodiment of the present invention.
Figure 3 is a detailed partial block/1>artial circuit diagram of a water
sensing
system according to the embodiment of Figure 2.
Figure 3a is a partial block/partial circuit diagram of a light intensity
control
circuit according to an embodiment of the present invention.
Figure 3b is a perspective view of a housing according to the embodiment of
Figure 3.
Figure 4 is a block diagram of a water sensing system according to another
embodiment of the present invention.
Figure 5 is a frontal view of a display of a water sensing system according to
an
embodiment of the present invention.
Figure 6 is a partial block/partial circuit diagram of a noise isolation
subcircuit
according to an embodiment of the present invention.
Figure 7a is a timing diagram illustrating a digital input signal of a noise
isolation
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circuit according to an embodiment of the present invention.
Figure 7b is a timing diagram illustrating another digital input signal of a
noise
isolation subcircuit according to an embodiment of the present invention.
Figure 7c is a timing diagram illustrating an intermediate signal of a noise
isolation subcircuit according to an embodiment of the present invention.
Figure 7d is a graph of an output signal versus time according to an
embodiment
of the present invention.
Figure 8 is a block diagram of a water sensing system according to another
embodiment of the present invention.
Figure 9 is a detailed partial block/partial circuit diagram of the embodiment
of
Figure 8.
Figure 10 is a frontal view of a display and a block diagram of a water
sensing
system according to the embodiment of Figure 9.
Figure 11 is a block diagram of a water sensing system including calibration
circuitry according to an embodiment of the present invention.
Figure 12 is a flow chart of a water content calibration program according to
an
embodiment of the present invention.
Figure 13 is a flow chart of a temperature calibration program according to an
embodiment of the present invention.
Figure 14 is a block diagram of a liquid purification system according to
another
embodiment of the present invention.
Detailed Description of the Preferred Embodiments
One example of a water sensing system embodying the present invention is
illustrated in Figure 1 and generally comprises a water sensor 2, a
temperature sensor
4, and a processing circuit 5. The water sensor 2, the temperature sensor 4,
and the
processing circuit 5 cooperate to measure the water content and the
temperature of a
liquid. The system illustrated in Figure 1 may measure relative water content
of a
liquid, the absolute water content of a liquid, or both.
The sensors 2 and 4 are also preferably capable of operating in extreme
environments. For example, both the water sensor 2 and the temperature sensor
4 are
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preferably capable of being immersed in a liquid without malfunctioning. The
sensors
2 and 4 are preferably corrosion resistant: and capable of withstanding
extreme
differences in temperature. For example, for power transformer applications,
the sensors
2 and 4 are preferably capable of measuring the water content and temperature
of
transformer oil when a transformer is operating and the oil is at an elevated
temperature,
and when the transformer is not operating and the oil is at a low temperature.
The range
of operating temperatures may be extreme if the transformer is located in a
low
temperature environment. Additionally, the internal electronics of the sensors
2 and 4
are preferably sealed from the liquid being sensed in accordance with the
relevant hazard
classification/NEMA rating.
The water sensor 2 comprises a probe: and internal circuitry associated with
the
probe to produce a signal indicative of the water content of a liquid. In one
embodiment, the water sensor 2 comprises a, capacitive probe that produces a
voltage
indicative of the % RH of the liquid based on the capacitance between
interdigitized
electrodes of the probe. Alternatively, the water sensor 2 may comprise a
resistive
probe that measures the water content of a liquid based on the AC resistance
of the
probe. Any type of water sensor with one or more of the above-described
characteristics
is within the scope of the invention.
The temperature sensor 4 also connprises a probe and excitation circuitry
associated with the probe to measure the 'temperature of a liquid. In a
preferred
embodiment, the temperature sensor 4 comprises a resistance temperature
difference
(RTD) sensor and associated excitation circuitry. An RTD sensor comprises a
material,
such as platinum. The resistance of platinum changes in response to a
temperature
change. The excitation circuitry produces a current through the material. The
sensor
output may be the voltage across the materials, which changes as the
resistance changes.
Alternatively, the temperature sensor 4 may comprise a thermocouple. Any type
of
temperature sensor with one or more of the above-described characteristics is
within the
scope of the invention.
In a preferred embodiment, the water sensor 2 and the temperature sensor 4 are
located in the same probe. Locating the water sensor 2 and the temperature
sensor 4 in
the same probe is preferred because the water content and the temperature can
be
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measured from the same location in a liquid. However, the present invention is
not
limited to a water sensor and a temperature sensor located in the same probe.
For
example, the water sensor 2 and the temperature sensor 4 may be located in
separate
probes. In addition, excitation circuitry associated with the water sensor and
the
temperature sensor may be located remotely from the probe, e.g., in a computer
that
monitors the outputs from the sensors 2 and 4.
Although the illustrated embodiment includes a single water sensor 2 and a
single
temperature sensor 4, multiple sensors may be included and are within the
scope of the
invention. For example, a plurality of water and temperature sensors may
produce a
plurality of output signals indicative of the water content and temperature of
the liquid.
Using a plurality of sensors provides redundancy, which increases reliability.
The output
signals from the plurality of sensors may be processed to eliminate errors due
to one or
more faulty measurements. For example, analog to digital conversion circuitry
may be
coupled to the output of each sensor, which may in turn be coupled to voter
logic to
select a water content value on which two or more sensors agree.
Another embodiment of a water sensing system may include a plurality of water
sensors and temperature sensors coupled to a single fluid system or a
plurality of fluid
systems. For example, a plurality of water sensors 2 and temperature sensors 4
may be
coupled to a single fluid system to measure the water content and temperature
of a
liquid, e.g., a hydraulic fluid, at a plurality of locations in the fluid
system. The output
signals from the sensors may be processed in any suitable manner for
communicating
water content information from the various locations. In a preferred
embodiment, the
output signals from the plurality of sensors are multiplexed to enable a
single processing
circuit to process outputs from the plurality of water sensors. In an
alternative
embodiment, each of the plurality of sensors may include its own processing
circuitry
to increase processing speed and provide redundancy.
The processing circuit 5 may perform various functions, such as comparison,
temperature compensation, calibration, noise isolation, voltage conversion,
and output
control. In order to perform these functions, the processing circuit 5 may
comprise a
plurality of subcircuits. For example, the processing circuit 5 may comprise
an output
subcircuit to drive one or more output devices, a comparison subcircuit to set
threshold
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values and compare sensed data to the thresholds, a noise isolation subcircuit
to prevent
coupling of electrical noise to external devices, a temperature compensation
subcircuit
to perform temperature compensation for %RH measurements, a calibration
subcircuit
to electronically calibrate the water sensor 2 and the temperature sensor 4,
and a voltage
conversion circuit for converting the output voltage from the water sensor 2
into %RH
values. The processing circuit 5 may include various combinations of
subcircuits
according to a desired application.
Exemplary types of liquids in which tlae water sensing system of Figure 1 may
be used to sense the water content include oils, such as transformer oil,
lubricating oil,
hydraulic fluid, gasoline, kerosene, transmission fluid, diesel fuel, and fuel
oil. One
particular class of hydraulic fluids in which embodiments of the present
invention may
be used to determine water content is water g'.lycols, such as water ethylene
glycols. In
addition, embodiments of the present invention may be used to determine water
content
in other types of liquids, including, for example, the water content in liquid
hydrocarbons such as liquid propane.
Another example of a water sensing system is shown in Figure 2. In the
illustrated embodiment, the water sensing system includes a water sensor 2, a
temperature sensor 4, and a processing circuit 5. The water sensor 2 and the
temperature sensor 4 may comprise any of the sensors previously described. The
processing circuit 5 includes a temperature compensation subcircuit 6, a
comparison
subcircuit 7, and an output subcircuit 8.
The temperature compensation subcircuit 6 may perform temperature
compensation for % RH measurements to account for the temperature dependency
of
%RH and/or to allow conversion between '% RH and absolute water content or
vice
versa. For example, the temperature compensation subcircuit 6 may convert the
output
of the sensor 2 from relative water content, e.g., in %RH to absolute water
content,
e.g., in parts per million (ppm). The conversion may be effected directly by
sensing the
temperature, sensing the % RH, converting the %RH value to a ppm value using
any
suitable means, for example, a look-up table or a temperature compensation
algorithm,
and displaying the absolute water content value. The term "look-up table" is
not limited
to data arranged in a tabular format. For example, the term "look-up table"
may include
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one or more computer databases which include temperature compensation data. In
another alternative, temperature compensation may be performed indirectly by
measuring
RH and temperature, varying one or more threshold values with which the %RH
value
is compared based on the temperature.
In order to qualify various levels of water content, the comparison subcircuit
7
stores one or more threshold values and compares the sensed water content
values to the
threshold values. For example, if the water content of the liquid is above a
predetermined threshold, the comparison subcircuit may produce a high output
signal,
and if the water content is below a predetermined threshold, the comparison
subcircuit
may produce a low output signal, or vice versa. Alternatively, the comparison
subcircuit
may be omitted and the output from the sensors may be displayed directly in
analog
format, digital format, or both.
The output subcircuit 8 communicates water content information to external
devices based on the output from the comparison subcircuit 7. In some
embodiments,
the output subcircuits may receive external inputs, for example, from a
computer to
allow the sensing system to interface with the computer. The output subcircuit
8 may
produce audible, visible, or a combination of audible and visible output
information to
the sensor operator. For example, the output subcircuit 8 may produce a signal
which
drives a visual display, which displays water content, temperature, or both to
the
operator. The output subcircuit 8 may produce a signal which drives a strip
chart
recorder, or an alarm, which indicates that the water content of the liquid is
above an
acceptable level. The output subcircuit 8 may include a computer interface
which allows
the operator to view and analyze the water content of the liquid. The output
subcircuit
8 may also produce a signal which controls one or more external devices 10,
e.g.,
relays, pumps, filters, purifiers, heaters, and/or dryers to remove water or
other
contaminants from the liquid.
Figure 3 is a detailed partial block/partial circuit diagram of a water
sensing
system according to the embodiment of Figure 2. In the illustrated embodiment,
the
water sensor 2 outputs a voltage proportional to the %RH of the liquid. The
comparison
subcircuit may comprise a pair of potentiometers 3, 3' to set threshold
voltages, VTl and
V.~, to compare with the water sensor output voltage.
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Using potentiometers allows the threshold voltage values to be varied and
allows
the system to be adapted to sense water in dlifferent types of liquids.
However, the
present invention is not limited to using potentiometers to set the threshold
voltages. For
example, in a preferred embodiment, the potentiometers may be replaced by a
microprocessor, and the threshold voltages may be programmable, e.g., using
software
or hardware. In still another alternative embodiment, if the water sensing
system is
designed for use in a known single type of liquid, the threshold voltages may
be fixed
using resistors having a predetermined fixed value.
In the present embodiment, the threshold voltages VT, and V.~ are preferably
initially set according to the liquid type and according to a baseline
operating temperature
of the liquid. For example, VT, and V.,.z ma;,~ define lower and upper
threshold water
content values for a specified type of liquid at its average operating
temperature.
Although the illustrated embodiment depicts two threshold voltages, more or
fewer than two threshold voltages are within the scope of the present
invention. For
example, a single threshold voltage may be used to define a single water
content
threshold. In an embodiment including a single water content threshold, when
the water
content of the liquid is above the threshold, the sensing system may activate
an alarm or
other external device, e.g., a purifier. Alternatively, when the water content
is above
the threshold, the sensing system may deactuate an external device using the
water-
containing liquid to prevent damage to the device. If only a single threshold
voltage is
used, it may be preferable to include a timer. The timer may be used to delay
the
deactivation of the alarm or other external device for a period of time after
the transition
from upper to lower water content values. However, in an even more preferred
embodiment, the sensor circuit includes at lleast two threshold voltages to
provide a
hysteresis between upper and lower water content values.
The two threshold voltages used in the present embodiment define three ranges
of water content values for a liquid. For example, one range, e.g., a low
range, may
be defined for water sensor output voltages from about OV to about VTI.
Another range,
e.g., an intermediate range, rnay be defined for water sensor output voltages
greater than
or equal to about VTl and less than about V~~Z. Yet another range, e.g., a
high range,
may be defined for sensor output voltages greater than or equal to about V.~.
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In the present embodiment, the low range represents a lower range of water
content values for which purification, i.e., removal of water from the liquid,
may not
be required. The intermediate range represents an intermediate range of water
content
values for which purification may or may not be required depending on the
previous
range, as will be discussed below. The high range represents a high range of
water
content values for which purification may be required.
In addition to representing an intermediate range of water content values, the
intermediate range provides a hysteresis between the high and low ranges. For
example,
VTl preferably defines the low/intermediate threshold below which the sensing
system
deactuates an external water removal device, such as a purifier. V~ preferably
defines
an intermediate/high threshold above which the sensing system actuates the
external
water-removal device. If VT, is equal to or close to V.,~, the external water-
removal
device may oscillate frequently between ON and OFF states, which is
undesirable
because it wastes energy and stresses mechanical components of the external
water-
removal device. Accordingly, V.~ is preferably set somewhat higher than VTl to
define
a hysteresis between the high and low water content ranges and reduce the
likelihood of
frequent oscillation between ON and OFF states.
The comparison subcircuit according to the illustrated embodiment includes
voltage comparators 9 and 9' to compare the output from the water sensor 2 to
the
threshold voltages VTl and VT2. The comparator 9 produces a LOW output signal
when
the output Vse,~ from the water sensor 2 is greater than or equal to about VT,
and a HIGH
output signal when Vse,~ is less than about VT,. The comparator 9' produces a
LOW
output signal when Vse,~ is greater than or equal to about V.L2 and a HIGH
output signal
when Vse,~ is less than about V.,.Z. The output signals from the comparators
may be used
directly to communicate water content information to the operator. However, in
the
present embodiment, the output signals from the comparators are further
processed by
an output subcircuit, as will be discussed below.
In a preferred embodiment, the comparators are replaced by a microprocessor
and
the comparison function is implemented using software. For example, a
microprocessor
may include an analog to digital converter that converts the output voltage
from the
sensor to a digital value. A comparison routine compares the digital value to
threshold
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values set in software. The microprocessor produces one or more output signals
based
on the comparison.
The temperature compensation subcircuit 6 comprises any type of circuit or
algorithm to compensate for the temperature .dependency of %RH measurements.
In a
preferred embodiment, the temperature compensation subcircuit is implemented
in
software. The temperature compensation subcircuit according to the illustrated
embodiment performs temperature compensation indirectly by adjusting the
threshold
values VT, arid V.,~ when the temperature of the liquid varies from a baseline
temperature
at which the threshold values VTl and V.L2 are; initially set.
IO Because the % RH of a liquid increases with decreasing temperature, the
temperature compensation subcircuit preferably decreases the threshold values
VTl and
V.,.z when the temperature of the liquid increases above the baseline
temperature.
Similarly, when the temperature of the liquid falls below the baseline
temperature, the
temperature compensation subcircuit preferably increases the threshold values
VTl and
V.,.z. The amount of increase or decrease of the threshold values in response
to
temperature changes depends on factors such as the type of liquid and the
additive
content of the liquid. In order to determine the amount of increase or
decrease of the
threshold values, conventional laboratory methods for determining absolute
water
content, such as Karl Fischer titration, ma;y be used. Absolute water content
data
obtained using laboratory methods can be interpolated to generate a look-up
table or an
algorithm to be used for adjusting the threshold values.
In a preferred embodiment, the look-up table or algorithm which adjusts the
threshold values is stored in a memory and accessed by a microprocessor
implementing
the temperature compensation program. Thf: temperature compensation program
reads
the output from the temperature sensor 2 and adjusts the threshold values
based an the
liquid temperature, as described above. The: present invention is not limited
to using a
temperature compensation program. In an ailternative embodiment, the look-up
table or
temperature compensation algorithm may be implemented by an analog circuit or
a logic
array that calculates the amount of adjustment of the threshold values.
The temperature compensation subcircuit may be configured to vary the
threshold
voltages for a single liquid type. Alternatively, the temperature compensation
subcircuit
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may be configured to vary the threshold voltages for a plurality of liquid
types. In
embodiments configured for multiple liquid types, the system may include an
external
switch to allow the operator to select a liquid type. In a preferred
embodiment, the
temperature compensation subcircuit is programmable so that the sensing system
can be
adapted to perform temperature compensation for any liquid type. By varying
VT, and
V.,~, the aforementioned problem of temperature dependency of % RH
measurements may
be attenuated.
The output subcircuit of the present embodiment communicates water content
information to an operator. An output subcircuit according the present
embodiment may
be variously configured. For example, in the illustrated embodiment, the
output
subcircuit comprises a logic subcircuit 12. The logic subcircuit 12 receives
the outputs
from the comparators 9, 9' and produces control signals C1-C3. If both of the
inputs
to the logic subcircuit 12 are LOW, the control signal C1 is HIGH. If the
output from
comparator 9 is LOW and the output from comparator 9' is HIGH, the control
signal C2
is HIGH. If the output from comparator 9 is HIGH and the output from
comparator 9'
is HIGH, the control signal C3 is HIGH. In a preferred embodiment, the logic
subcircuit comprises a decoder. Because the control signals C 1-C3 are
produced by
separate outputs from the decoder 12, none of the control signals will be HIGH
at the
same time.
Although the control signals C1-C3 may be used directly to communicate water
content information, the output subcircuit according to the present embodiment
preferably
further comprises one or more drivers to drive one or more display devices.
For
example, in the illustrated embodiment the output subcircuit includes a driver
14 coupled
to a plurality of lights to visually display water content data to an
operator. The driver
14 may comprise a lamp driver or an LED driver, depending on the type of
lights used
to indicate water content levels.
In operation, when the water sensor output voltage Vge,~ is less than both VTl
and
VT2, the outputs from both comparators are HIGH and the control signal C3 is
HIGH.
Since the sensor output voltage Vse~ is less than both threshold voltages, the
water
content of the liquid is in the low range. Accordingly, when the control
signal C3 is
HIGH, the driver 14 actuates a light, preferably a green light 20. The green
light 20
14
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may comprise an incandescent lamp or an LED. In a preferred embodiment, the
green
light 20 comprises an LED.
When the water sensor output voltage is between VTl and V.,.2, the output from
the comparator 9 is LOW, the output from the comparator 9' is HIGH, and the
control
signal C2 is HIGH. Since the water sensor output voltage is between the
threshold
voltages, the water content of the liquid is in the intermediate range.
Accordingly, when
the control signal C2 is HIGH, the driver 14 actuates another light,
preferably a yellow
light 18. The yellow light may comprise an incandescent lamp or an LED. In a
preferred embodiment, the yellow light comprises an LED.
When the water sensor output voltage its greater than both VTl and V.,.z, the
output
from both comparators is LOW and the control signal C1 is HIGH. Since the
water
sensor output voltage is greater than both threshold voltages, the water
content of the
liquid is in the high range. Accordingly, when the control signal C 1 is HIGH,
the driver
14 actuates yet another light, preferably a red light 16. In a preferred
embodiment, the
red light 16 may comprises an LED.
The following truth table illustrates tree relationship between the signals:
A B C1 C2 C3
Low Range ~ 1 1 0 0 1
Intermediate Range ~ 0 1 0 1 0
High Range ~ 0 0 1 0 0
where A and B are the outputs from the corrtparators 9, 9', respectively.
The green, yellow, and red lights visually communicate water content
information
to an operator in a manner that facilitates intf:rpretation of the water
content of a liquid.
Using familiar colors to display water content also reduces the likelihood of
interpretation errors and increases the efficiency with which sensing may be
performed.
However, the present invention is not limited to colors or even to visual
displays. For
example, water content information may be conveyed numerically using a display
screen
or audibly using an alarm.
In addition to providing visual output, the output subcircuit may control one
or
more external devices. For example, in the illlustrated embodiment, the output
subcircuit
controls a relay, which may be used to actuate and deactuate an external
device such as
a purifier. In one embodiment, the relay may be controlled exclusively by the
control
l:i
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signals C1-C3, which are indicative of the water content of a liquid.
Alternatively, the
output subcircuit may include a timer subcircuit 31 which controls the relay
to operate
a purifier for a set period then deactuates the purifier. In a preferred
embodiment, the
relay is controlled by a combination of the control signals C1-C3 and the
timer 31.
In order to allow the control signals C1-C3 to influence the operation of the
relay
and to provide the hysteresis between the high and low water content ranges,
the output
subcircuit preferably further comprises an SIR flip-flop 24. The control
signal C 1 is
connected to the SET input of the S/R flip-flop 24 to set the output Q of the
flip-flop 24
when the water content of a liquid is in the high range. The control signal C3
is
connected to the RESET input of the flip-flop 24 to reset the output Q when
the water
content of the liquid is in the low range.
The output Q of the flip-flop 24 may be used to actuate and deactuate an
external
device directly. For example, in an embodiment without a timer subcircuit,
when the
output Q becomes HIGH, the water content is in the high range, and the relay
actuates
an external device. When the output Q becomes LOW, the water content is in the
low
range, and the relay deactuates the external device.
Since the control signal C2 is not connected to the flip-flop 24, the output Q
of
the flip-flop 24 does not change when the water content enters the
intermediate range.
More particularly, when the water content transitions from the high range to
the
intermediate range, the output Q of the flip-flop 24 remains HIGH. When the
water
content changes from the low range to the intermediate range, the output Q of
the flip-
flop 24 remains LOW. In this manner, the flip-flop 24 provides a hysteresis
between
the high and low water content ranges.
Operating an external device based solely on water content may be desirable in
some instances in which water is the only impurity to be removed from a
liquid.
However, it may be desirable to remove other impurities, such as particulate
contaminants from the liquid. Particulate contaminants may be removed by
filtering the
liquid before or after the liquid enters a purifier, for example, by passing
the liquid
through a filter upstream or downstream of the purifier. Since the time
required to
reduce particulate contaminants to an acceptable level may not be equal to the
time
required to reduce water content to an acceptable level, the output subcircuit
preferably
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includes an OR gate 26 to OR the output from the SIR flip-flop 24 with the
output from
the timer 31.
The timer subcircuit is preferably configured to produce a HIGH output signal
as the timer counts down a predetermined time period and a LOW output signal
before
the count begins and after the predetermined time period expires. Producing a
HIGH
output signal forces the output of the OR gate: 26 to be HIGH regardless of
the output
of the flip-flop 24. Thus, during the predetermined time period while the
counter is
counting down, an external device remains actuated regardless of the water
content of
the liquid. If the timer expires, the output of the OR gate becomes LOW and an
external
device is controlled by the output Q of the flip-flop 24.
The predetermined time period may be varied according to a desired level of
purification; i.e., the time period may be increased if the application
requires cleaner
liquid. In addition, the predetermined time period may be varied
automatically. For
example, the present invention may include a sensor for sensing particulate
contamination
in a liquid. The sensor may be coupled to th,e timer-to vary the time period
according
to the particulate contaminant level. A start/reset signal may be provided to
the timer
by the sensor circuit or by the external device.
Although the illustrated embodiment depicts the output subcircuit as
implemented
by hardware components, the present invention is not so limited. In a
preferred
embodiment, some or all of the components crf the output circuit are
implemented using
software. For example, the timer circuit, the S/R flip-flop, and the decoder
may be
omitted and the output signals may be controlled by an output control program.
The relay according to the present embodiment may be variously configured. For
example, in the illustrated embodiment, the relay comprises a coil 30 and a
switch 32.
A relay driver 28 is included to control the position of the switch. An
external device
such as a purifier may be connected in series with one of the terminals of the
switch.
The relay may, for example, control an external relay or switch which supplies
operational power to the external device.
In the illustrated embodiment, the switch is shown in the OFF position. When
the output D1 of the OR gate is HIGH, the relay driver energizes the coil
which changes
the switch to the ON position. Accordingly, a device connected in series with
the
1 i'
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normally open (N.O.) terminal of the relay may be supplied with operational
power.
When the output of the OR gate is LOW, the switch changes to the OFF position
and
deactuates an external device.
In order to provide an indication of the position of the switch during
operation
of the sensing system, the output subcircuit preferably further comprises a
switch
position indicator light 22. In a preferred embodiment, the switch position
indicator light
22 produces a color, preferably not red, yellow, or green, e.g., white. The
light 22 may
comprise an incandescent lamp or an LED. In a preferred embodiment, the lamp
22
comprises an LED. The light 22 is controlled by the output D1 of the OR gate
26.
When D1 is HIGH, the switch is in the N.O. position, and the light 22 is ON.
When
D1 is LOW, the switch is in the N.C. position, and the light 22 is OFF.
Providing a visual indication of the relay position may be useful when the
position
is not evident from the operation or non-operation of an external device. For
example,
if the sensing system is coupled to a liquid and not to an external device and
the water
content is in the intermediate range, the relay may be closed or open,
depending on
whether the previous range was low or high. Absent some external indicator,
the
position of the switch may not be known. Accordingly, the light 22 provides a
visual
indication of the position of the relay which may otherwise be unknown.
Although the illustrated embodiment includes a single relay, the present
invention
is not limited to such an embodiment. For example, another embodiment of the
present
invention may include two or more relays to control a plurality of external
devices. Any
number of relays is within the scope of the invention.
In addition to controlling one or more relays and providing visual outputs,
the
output subcircuit may directly interface with an external device, such as a
programmable
logic controller or a computer. Accordingly, the output circuit may include
one or more
drivers or regulators for converting the outputs from the sensors into a
format suitable
for interfacing with the external device. For example, the output subcircuit
may include
one or more regulators to interface with a programmable logic controller that
includes
a standard 4-20 mA interface. The output circuit may also include one or more
analog
to digital converters for interfacing with an external microprocessor.
Moreover, in
embodiments in which the output subcircuit drives an external device, e.g., a
purifier,
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the output subcircuit preferably provides an indication of the operating state
of the
external device. For example, if the output. subcircuit controls a purifier
and also
includes a computer interface, the output subcircuit preferably transmits the
operating
state, i.e., ON or OFF, of the purifier to the computer. The output subcircuit
may also
transmit the water content information, for example, %RH, absolute water
content, or
both, to the computer.
The water sensing system preferably includes a power supply 34 to convert AC
line voltages to voltages usable by the device. The power supply 34 may be
part of the
sensor circuit or a separate component. T'he power supply 34 may be variously
configured. In a preferred embodiment, the power supply is configured to
automatically
convert 115 V, 60 Hz, 220 V 60 Hz, and 240 V, 50 Hz signals to the appropriate
DC
levels, e.g., +5 V DC for logic devices. This feature allows the device to
work under
both U.S. and European power systems without requiring an external switch.
Alternatively, the water sensing system may iinclude an internal power source,
such as
a battery.
According to a further aspect, the ouyut subcircuit may include a light
intensity
control circuit 36 to control the power supplied to the lights to control the
luminous
intensity of the lights. Features of the light intensity control circuit are
discussed below.
The light intensity control circuit preferably includes a potentiometer or
other power
adjustment device to allow an operator to adjust the luminous intensity of the
lights
according to the lighting conditions of an operating environment.
In a preferred embodiment, the water sensing system preferably maintains a
substantially constant average or perceived lwminous intensity of the
indicator lights and
other lights used to display water content information, even when the power
supply
voltage varies. For example, when the water aensing system is connected to
external AC
power systems, the line voltage input to the power supply may vary from about
90 VAC
to about 250 VAC. In addition, power surge,> or interruptions in a power
supply system
may cause the voltage supplied to the water sensing system to vary. This line
voltage
may be rectified and used to power indicator LED's and LED's used in segmented
displays that display water content and temperature information. Because the
luminous
intensity of LED display devices varies with applied current, preferred
embodiments of
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the present invention include an LED intensity control circuit for maintaining
a
substantially constant average current through the LED's by switching the
LED's on and
off.
Figure 3a illustrates an example of a light intensity control circuit
according to
a preferred embodiment of the present invention. In the illustrated
embodiment, a
transformer T1 and a bridge circuit B1 convert AC line voltage from an
external power
source into an unregulated 7-30 VDC voltage. For example, when the external
line
voltage is about 90 VAC, the signal is about 7 VDC. When the external line
voltage is
250 VAC, the signal is about 30 VDC. If the 7-30 Volt unregulated DC voltage
were
used directly to power the LED's, the intensity of the LED's Dl-Dn would vary
when
the water sensing system is connected to different power systems. Accordingly,
in order
for the water sensing system to provide uniform luminous intensity, a
microcontroller
U1 calculates and controls the duty cycle of an LED driver control signal C1
to maintain
a substantially constant average current through the LED's by switching the
LED's on
and off.
In a preferred embodiment, the microcontroller U 1 comprises a Motorola
68HC 11. The 68HC 11 microcontroller includes an internal AID converter and
internal
memory devices. A voltage divider circuit comprising resistors Rl and RZ
divides the
unregulated 7-30 VDC signal into a level suitable for input to the A/D
converter of the
microcontroller U 1. The present invention is not limited to the 68HC 11
processor. For
example, another microcontroller with an external AID converter and external
memory
circuits is within the scope of the invention.
A duty cycle calculation algorithm stored in the memory of the microcontroller
U1 calculates required duty cycle to maintain a constant average luminous
intensity of
the LED's in response to the input to the AID converter. When the line voltage
decreases, the algorithm increases the duty cycle. When the line voltage
decreases, the
algorithm decreases the duty cycle. As a result, a substantially constant
average
luminance intensity is maintained. In a preferred embodiment, the relationship
between
the duty cycle and the unregulated DC voltage is linear. For example, the duty
cycle
may be inversely proportional to the unregulated DC voltage. One approximation
for
the duty cycle may be:
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Duty Cycle = 7 VDC'./V~~reg (1),
where V"~eg 1S the unregulated DC voltage. For example, when the unregulated
DC
voltage is low, e.g., about 7 VDC, the duty cycle is high, e.g., about 100%,
according
to the approximation. When the unregulated DC voltage is high, e.g., about 30
VDC,
the duty cycle is low, e.g., about 23%, according to the approximation. In
other words,
according to the approximation, the product of the duty cycle of the signal C
1 and the
unregulated line voltage is maintained substantially constant.
An LED driver IC 500 receives the sil;nal C1 and controls the current through
the LED's by switching the diodes on and off in response to the duty cycle of
the signal
C 1. The switching frequency of the diodes is preferably selected such that
the switching
is undetectable by the human eye. In a preferred embodiment, the switching
frequency
is at least about 100 cycles per second, and more preferably at least about
200 cycles per
second. _
The LED driver IC 500 may be variously configured. In a preferred
embodiment, the LED driver IC comprises a plurality of serial-in/ parallel-out
shift
registers. The number of outputs of the shift registers corresponds to the
number of
LED's. The duty cycle of the signal C1 controls the switching of all of the
outputs.
More particularly, in the illustrated embodiment, each LED is connected in
series
with a 300 Ohm resistor R3-Rn. Each 300 Ohm resistor R3-R~ is connected to the
unregulated 7-30 VDC node. The forward voltage drop across each of the LED's
when
the LED's are on is about 2 V. The current through one of the diodes is given
by the
following expression:
1. _ ~ Yanzs~y-yf ? *D
avg
3
where D is the duty cycle of the signal C1.
In a preferred embodiment, the average current through the diodes is
maintained
to be about 17 mA to maintain a desired average luminous intensity level. For
example,
in order to maintain a 17 mA average curreno, when the unregulated voltage is
7 V, the
duty cycle of the signal C 1 is preferably about 100 % . If the voltage V~,~eg
is about 18.5
21
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V, the duty cycle of the signal C1 is about 31 % in order to maintain an
average current
of 17 mA.
The present embodiment is not limited to maintaining a 17 mA average current
through the LED's. For example, the circuit may include one or more external
controls
to allow the operator to vary the duty cycle of the signal C 1 to make the
diodes appear
brighter or dimmer. Alternatively, a potentiometer may be coupled to the LED's
to
regulate the average current through the LED's.
The present invention is not limited to the light intensity control circuit of
Figure
3a. For example, a 7 V regulator may be used to convert the unregulated 7-30
VDC
signal into a constant voltage of 7 V to be supplied to the LED-resistor
series circuits.
This arrangement would result in a substantially constant current through the
LED's.
However, when the Line voltage is greater than 7 V, energy is wasted in the
regulator.
In contrast, according to the present embodiment, the LED's are switched on
and off in
response to the power line voltage and the energy loss is negligible. Thus,
the present
embodiment provides a constant average luminance intensity for the LED's and
conserves energy. In yet another alternative embodiment, in which the LED's
are
replaced by incandescent lamps, the present invention may include an analog or
digital
circuit for regulating the RMS current for powering the lamps to maintain a
substantially
constant perceived luminous intensity.
According to a further aspect, the water sensing system preferably includes a
housing. The housing comprises any housing suitable for containing and
protecting
electronic components. In a preferred embodiment, the housing is splash proof
to protect
the sensor circuit. For example, the housing may substantially conform to the
appropriate NEMA specification for the packaging of electronic devices. The
housing
may comprise a removable lid to allow replacement and repair of the sensor
electronics.
The lid preferably includes a gasket to reduce the likelihood of liquid entry
around the
lid. The housing may include an inlet for an AC power cord in embodiments in
which
the housing receives power from an external AC supply. Alternatively, the
housing may
include an internal power supply such as a battery. In embodiments which
include a
battery, the AC power cord may be omitted or included as an additional source
of
power. The housing further comprises a sensor cable inlet to allow a sensor
cable to
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communicate with the interior electronics. The housing is preferably portable
and may
comprise a hand held unit.
In embodiments of the water sensing system that include indicator lights,
e.g.,
as shown in Figure 3, the housing preferably includes a plurality of apertures
to
accommodate the indicator lights. Figure 3b illustrates an exemplary housing
for the
water sensing system including indicator lights according to the embodiment of
Figure
3. Alternatively, in embodiments of the water sensing system that include an
alphanumeric display, the housing may include a window for the display. In
still other
embodiments, the housing may include both apertures for indicator lights and a
window
for a display. In another alternative embodiment, the housing may be omitted.
For
example, the sensing system may comprise a~ circuit card which plugs directly
into an
adapter card socket in a PC. In addition to 'the functional features of the
housing, the
present invention also includes the ornamental features of the housing, as
shown in
Figure 3b and described above.
In some applications, measuring % RH of a liquid is useful without performing
temperature compensation. For example, where the temperature of a liquid
remains
stable, measuring % RH may be useful to indicate whether the water content is
within an
acceptable range. Accordingly, one embodiment of the present invention, as
illustrated
in Figure 4, includes a sensing system without a temperature compensation
subcircuit.
The illustrated embodiment includes a water sensor 2, a temperature sensor 4,
and a
processing circuit 5. The processing circuit 5 includes an output subcircuit
8. The
processing circuit 5 may also include a comparison subcircuit as described
above in the
discussion relating to Figure 3 and a calibration subcircuit. In a preferred
embodiment,
the processing circuit 5 comprises a microcontroller U 1.
The microcontroller Ul processes thc: output signals from the sensors 2 and 4.
For example, the microcontroller may convert the output signals to digital
values to be
displayed by the output subcircuit. The microcontroller may be programmed to
perform
temperature compensation. However, in the illustrated embodiment temperature
compensation is not included. Although the illustrated embodiment depicts a
microcontroller U1, a microprocessor with external memory and support
circuitry is
within the scope of the invention.
2?.
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Any of the previously described embodiments of the water sensor 2 and the
temperature sensor 4 may be used in the present embodiment. One reason for
including
a temperature sensor in an embodiment without a temperature compensation
subcircuit
is to monitor the operating temperature of a liquid. Providing temperature
information
in addition to water content information allows an operator or external device
to
determine whether the water content of a liquid is within a safe range for a
given
temperature. Another advantage of including a temperature sensor is that the
operator
may measure the operating temperature of the liquid for other reasons, such as
preventing overheating or freezing of the liquid.
The output subcircuit 8 may be variously configured. In the illustrated
embodiment, the output subcircuit includes a display, first and second analog
interface
outputs, and a PC interface. The output subcircuit preferably also includes a
noise
isolation subcircuit to isolate the first and second analog interface outputs
from the
ground the sensing system. The output subcircuit preferably also includes a
light
intensity control circuit as illustrated in Figure 3a for controlling the
luminous intensity
of the numbers of the display.
An exemplary display which may be included in the embodiment of Figure 4 is
shown in Figure 5. In the illustrated embodiment, the display is configured to
provide
%RH and temperature data in decimal format to the operator. For example, the
display
may comprise a liquid crystal or an LED display device which displays the
temperature
and the water content to the operator in a digital format. A plurality of
toggle switches
may be included to toggle the display between %RH, degrees Celsius, and
degrees
Fahrenheit. Alternatively, the display may have a fixed format which displays
temperature and water content simultaneously or only water content. In a fixed
screen
format, the switches may be omitted. In yet another alternative, the display
may
comprise a lamp or LED display which indicates when threshold water content
values
are exceeded, as described with respect to Figure 3.
The present invention is not limited by the type of display. For example, the
display may comprise an analog meter including a displacement arm and a
preprinted
background including temperature and/or water content scales. One or more
scales may
be included on the background to allow the sensor to display water content
with varying
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degrees of sensitivity. A switch may be included to switch the display between
temperature and water content and between the various scales of water content
ranges.
Alternatively, the display may include separate analog meters to display water
content
and temperature simultaneously. In another alternative, the subcircuit may
include both
analog and digital display outputs.
The personal computer (PC) interface 11 may be any interface suitable for
connecting the output subcircuit to a personal computer. For example, the PC
interface
may comprise an Industry Standard Architecti~re (ISA) interface, a Personal
Computer
Memory Card International Association (PCMCIA) interface, or any other PC
interface
suitable for input/output access by a microcontroller. In a preferred
embodiment, the
PC interface comprises an RS232 interface. T'he PC interface allows water
content data
to be monitored and analyzed by an external computer, such as a lap-top
computer.
The analog interface outputs illustrated in Figure 4 may be variously
configured.
In a preferred embodiment, the output subcircuit includes first and second
analog
interface outputs A1 and A2 for water content and temperature, respectively.
For
example, the first analog interface output A1 rnay regulate a signal in an
external device
responsive to the measured water content of the liquid. The second analog
interface
output may regulate a signal in an external device responsive to the measured
temperature of the liquid. In a preferred embodiment, the first and second
analog
interface outputs regulate current in a standard 4-20 mA interface.
A standard 4-20 mA interface includes a power supply and a resistor connected
in series. A meter, e.g., a voltmeter, is connected across the resistor to
measure the
voltage across the resistor and determine the current through the resistor.
Two external
leads connect the standard 4-20 mA interface to a sensor circuit, such as a
water sensing
system according to embodiments of the present invention. The sensor circuit
regulates
the current through the resistor such that the meter reads from 4 mA to 20 mA.
In the present preferred embodiment, the first analog interface output Al
regulates the current in a standard 4-20 mA interface to vary between 4 and 20
mA
based on measured water content. For example, when the measured water content
is 0
% RH, the first analog interface output A1 regulates the current in a standard
4-20 mA
interface to be about 4 mA. When the measured water content is 100 % RH, the
first
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analog interface output A1 regulates the current in the standard 4-20 mA
interface to be
about 20 mA. The first analog interface output A1 preferably varies the
current in the
standard 4-20 mA interface linearly with measured water content when the
measured
water content is between 0 % RH and 100 %RH.
The second analog interface output A2 preferably regulates the current in the
standard 4-20 mA interface to vary between 4 and 20 mA based on measured
temperature. For example, when the measured temperature is a minimum value for
a
liquid in its operating environment, the second analog interface output A2 may
regulate
the current in the standard 4-20 mA device to be about 4 mA. When the measured
temperature is a maximum value for the liquid in its operating environment,
the second
analog interface output A2 may regulate the current in the standard 4-20 mA
device to
be about 20 mA. The second analog interface output A2 preferably varies the
current
in the standard 4-20 mA interface linearly with measured temperature when the
temperature is between the minimum and maximum values for a liquid.
Providing first and second analog interface outputs A1 and A2 to regulate
current
in a standard 4-20 mA interface allows the water sensing system according to
the present
embodiment to be used with one or more peripheral devices including such an
interface,
e.g., a programmable logic controller (PLC), a strip chart recorder, or a
computer. A
4-20 mA interface is standard in many industrial devices. Thus, the sensing
system may
interface with devices other than those listed.
When the water sensing system is coupled to external devices that include a .
standard 4-20 mA interface, electrical noise may be coupled through the ground
of the
sensing system to the 4-20 mA outputs and to the external devices. The noise
may lead
to inaccurate water content or temperature readings by the external devices.
The problem of noise increases when the external devices are located remotely
from the sensing system due to differences in ground potential. For example, a
strip
chart recorder may be connected to an electrical outlet in a control room of a
plant. A
water sensing system may be connected to an outlet on the manufacturing floor
to sense
water in a lubricating liquid for a machine on the manufacturing floor. The
ground
potential of the control room outlet may be different from the ground
potential of the
manufacturing floor outlet, e.g., higher or lower. Because of the difference
in potential,
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ground currents may flow between the sensing system and the remote device.
Accordingly, the present embodiment preferably includes a noise isolation
subcircuit
which prevents ground currents and other noise; from external devices from
affecting the
analog interface outputs by electrically isolating the analog interface
outputs from the
ground of the sensing system.
Figure 6 illustrates a noise isolation subcircuit 100 for the water sensor and
a
microcontroller U 1 according to the embodiment of Figure 4. The
microcontroller U 1
receives an output signal from the water sensor and produces output signals O1
and 02
responsive to the output signal from the water sensor. The noise isolation
subcircuit 100
produces an analog signal V;" indicative of the water content of the liquid in
response to
the signals O1 and 02. Although not illustrated, a similar noise isolation
subcircuit is
preferably included to produce an analog signal indicative of the temperature
of the
liquid.
In order to provide electrical isolation, the noise isolation subcircuit 100
preferably includes one or more isolation devices that produce a signal at an
output
terminal of the isolation device which is elecl:rically isolated from an input
terminal of
the isolation device. For example, the isolation device may comprise an
isolation
transformer. In a preferred embodiment, tlhe isolation device comprises an
optical
isolator. In a most preferred embodiment, the noise isolation subcircuit
comprises two
optical isolators U2, U3.
In the illustrated embodiment, each optical isolator U2, U3 includes a light
emitting diode (LED) and a phototransistor to electrically isolate the inputs
of each
optical isolator from the outputs of each optical isolator. Coupling between
the input and
the output is established through an optical linlK rather than an electrical
link. A problem
with using an optical isolator in a circuit which produces an analog signal is
the transfer
function of an optical isolator may be nonlinear. Moreover, the transfer
function may
vary with temperature. Thus, it is difficult to produce an accurate continuous-
valued or
analog signal in a circuit including optical isolators.
In order to solve this problem, the optical isolators are preferably driven in
a
digital mode in which pulse width modulation is used to represent the water
content
values. More particularly, the microcontroller calculates and controls the
duty cycle of
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the signals O1 and 02 used to drive the optical isolators responsive to the
output voltage
level of the water sensor 2. Driving the optical isolators in a digital mode
reduces the
effect of the nonlinearity in the transfer function of the optical isolators
because the
optical isolators are either on or off.
As illustrated in Figures 7a and 7b, the signal O1 is the digital complement
of
the signal 02. That is, when the signal O1 is high, the signal 02 is low, and
vice versa.
The signal O1 drives the optical isolator U2. The signal 02 drives the optical
isolator
U3. The outputs of the optical isolators U2, U3 are commonly connected to
produce a
switched signal V1, illustrated in Figure 7c. Driving two optical isolators in
complement
provides a high switching speed of the signal V 1 with negligible power loss.
The present invention is not limited to using two optical isolators to switch
an
output signal. For example, one optical isolator could be used in combination
with a
pull-down resistor to switch the output signal. However, such a design would
result in
a slower switching speed and increased power consumption. Therefore, using two
optical isolators is preferred.
The noise isolation subcircuit preferably includes a switch U4 coupled to the
commonly-connected outputs of the optical isolators. in a preferred
embodiment, the
switch comprises a CMOS switch because a CMOS switch accurately switches
between
0 V and a reference voltage Vre~, with a reduced offset voltage. A reduced
offset voltage
decreases the power consumption of the noise isolation circuit. Other types of
switches
are within the scope of the invention. For example, a B7T switch may be used
if power
consumption is not critical.
In operation, the signal V 1 controls the switching of the switch U4. More
particularly, when the signal V1 becomes high, and the switch U4 switches to
V~ef
When the signal V 1 becomes low, the switch switches to 0 V . When V 1 becomes
high
again, the switch switches back to V~ef
The noise isolation subcircuit preferably includes a low pass filter to
convert the
output from the switch U4 into a DC voltage. In the illustrated embodiment,
the low
pass filter comprises a resistor R2 and a capacitor C 1. The values of the
resistor R2 and
the capacitor C 1 are preferably chosen such that the cutoff frequency of the
low pass
filter is lower than the pulse width modulation frequency. For example, for a
pulse
28
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width modulation frequency of about 10 kHz, the cutoff frequency of the filter
may be
chosen to be about 1 Hz. Choosing the cutoff frequency of the low pass filter
to be less
than that pulse width modulation frequency provides a smooth output voltage
that
approximates a DC signal.
Figure 7d illustrates the signal V;n output from the low pass filter. In the
illustrated embodiment, a slight ripple is shown in the signal V;~ due to the
switching.
However, the ripple is exaggerated to illustratf; the pulse width modulation
of the input
signals. If the values of Rl and C1 are chosen correctly, this ripple will be
negligible.
The following expression illustrates the relationship between the signal V;n
and the
reference voltage Vref~
Vin = D * Vref
where D is the duty cycle of the pulse width rnodulation of the input signals
O1 and 02
calculated by the microcontroller.
The noise isolation subcircuit preferably includes a voltage-to-current
converter
U5 to regulate the current in an external device: responsive to the signal V;n
from the low
pass filter. Two external leads L1 and L2 allow U5 to connect to an analog
interface,
e. g. , a standard 4-20 mA interface I1. A diiode D 1 prevents damage to the
sensing
system if an operator connects the leads Ll and L2 to reverse polarity. An
output stage
transistor Q 1 regulates current in the interface: I1.
In a preferred embodiment, U5 regulates current in the interface I1 to vary
between 4 and 20 mA according to measured water content, as discussed above.
The
water sensing system preferably includes a similar noise isolation subcircuit
to provide
an isolated 4-20 mA interface output for temperature. The noise isolation
subcircuit of
Figure 6 may be used in combination with any of the circuits or subcircuits of
the water
sensing system of the present invention that produce analog outputs.
Another example of a water sensing system embodying the present invention is
shown in Figure 8. Like the above mentioned embodiments, the water sensing
system
of the present embodiment preferably includes a water sensor 2, a temperature
sensor
4 to collect water content and temperature data and a processing circuit 5
having an
output subcircuit 8 to provide various signals to external devices. In the
illustrated
embodiment, the output subcircuit 8 comprises an analog interface output, a
display
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output, a control output, and an interface connection. Each of the outputs may
be
variously configured as discussed above.
The water sensing system of Figure 8 preferably includes a temperature
compensation subcircuit. However, unlike the temperature compensation
subcircuit of
Figures 2 and 3, which performs temperature compensation indirectly by
adjusting
threshold values, the temperature compensation subcircuit of the present
embodiment
performs temperature compensation directly by converting % RH values to ppm
values
and displaying the ppm values. The temperature compensation subcircuit
receives
output signals from the water sensor 2 and the temperature sensor 4. In a
preferred
embodiment, the output signal from the water sensor 2 is indicative of the %RH
of the
liquid. Thus, for a given instant in time, the % RH and the temperature of a
liquid may
be known. Given the %RH, the temperature and the type of liquid, the
temperature
compensation subcircuit 6 may calculate the water content of the liquid in
ppm. For
example, in a preferred embodiment, the temperature compensation subcircuit
may
comprise a memory device which stores a look-up table to convert from %RH to
ppm.
In an alternative embodiment, the temperature compensation subcircuit may
include an
analog or digital calculating subcircuit which calculates the ppm value. In
another
alternative, the temperature compensation subcircuit may comprise a
microcontroller
programmed to convert the values. Using a microcontroller allows the
conversion
process to be varied or programmed according to the type of liquid and the
additive
content of the liquid.
Figure 9 illustrates a detailed partial block/partial circuit diagram of the
water
sensing system of Figure 8. In Figure 9, the temperature compensation
subcircuit
comprises an electrically erasable programmable read only memory 52 (EEPROM).
The
EEPROM stores a look-up table comprising ppm conversion values for one or more
liquid types. For example, the EEPROM may store conversion values for a
plurality of
liquid types in separate memory storage areas. In an embodiment in which the
EEPROM stores data for a plurality of liquid types, the sensing system may
comprise
an external switch to select a liquid type.
The values for the look-up table may be determined by conventional laboratory
techniques for measuring absolute water content, such as Karl Fischer
titration. For
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example, the ppm values for a group of samplers of a given liquid can be
measured in a
lab. The temperature and %RH of the sample, can be measured using a %RH sensor
and a temperature sensor. The relationship beaween %RH, ppm, and temperature
can
be determined graphically. A look-up table or a temperature compensation
algorithm
may be obtained from the graph.
The temperature compensation subcircuit according to the illustrated
embodiment
further comprises an analog-to-digital (A/D) converter 54 to convert analog
signals from
the water sensor 2 and the temperature sensor 4 into digital values which
correspond to
memory addresses in the look-up table. In embodiments where analog
subcircuitry is
used to calculate the ppm value, the A/D converter may be omitted.
The temperature compensation subcircuit 6 may further comprise a switch 56 to
switch between the output from the water sensor 2 and the temperature sensor
4. The
temperature compensation subcircuit may also include a timing and control
subcircuit 58
to control the position of the switch. The tinning and control subcircuit 58
preferably
includes a plurality of inputs which allow thE: operator to select an output
type. For
example, the timing and control subcircuit may allow an operator to switch
between
RH, temperature, and ppm output. In a preferred embodiment, the A/D converter,
the timing and control subcircuit, and the switch may be replaced by a
microcontroller
which performs equivalent functions. That is., the microcontroller accesses
the look-up
table to convert from %aRH to ppm.
The output subcircuit 8 may be variously configured. For example, the output
subcircuit may include one or more relays and one or more interface
connections. In the
illustrated embodiment, the output subcircuit comprises a digital display
comprising a
plurality of LED or, alternatively, LCD display devices 60 driven by a
plurality of
display drivers 62 to communicate % RH, temperature, and ppm values. The
output
subcircuit may include one or more regulators 64 to receive the signals from
the sensors
2, 4 and regulate the current in a standard 4-~20 mA interface, as described
above. In
a preferred embodiment, the water sensing system includes a water content
output
terminal 66 and a temperature output terminal 67. The output signals from the
terminals
66 and 67 may be isolated, for example, using a noise isolation subcircuit
similar to the
circuit illustrated in Figure 6.
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In operation, the present embodiment includes a plurality of operating modes.
In a temperature operating mode, the switch 56 is connected to the output from
the
temperature sensor, and the output subcircuit displays temperature information
to an
operator. The temperature may displayed in degrees Celsius, degrees
Fahrenheit, or
both. In a %RH operating mode, the switch 56 is connected to the output from
the
water sensor, and the output subcircuit displays %RH to the operator. In a ppm
operating mode, the switch 56 is alternatingly connected to the output from
the water
sensor and the output from the temperature sensor by the timing and control
circuit to
select a %RH value and a temperature value for a particular instant in time.
The
temperature value and the %RH value are digitized by the AID converter and
used to
look up a ppm value in the look-up table in the EEPROM. The ppm value is then
displayed to the operator via the output subcircuit.
Figure 10 illustrates an exemplary configuration of a display 60, a sensor
probe
200, and a circuit board 205 of the water sensing system of Figure 9. In the
illustrated
embodiment, the display 60 comprises separate alphanumeric readouts for
temperature
and water content. Alternatively, as illustrated in Figure 5, a single display
may be used
to display both temperature and water content. The sensor probe 200 includes
both the
water sensor 2 and the temperature sensor 4. A cable 201 couples the sensor
probe 200
to a circuit board 205 which includes the processing circuit 5 and various
subcircuits.
The cable is preferably removably coupled to the circuit board 205 for storage
and
portability. The arrows 206 and 207 indicate outputs for providing temperature
and
water content information to the display 60 and/or to an external device. The
water
sensing system preferably also includes a switch 202 to switch the water
content portion
of the display 60 between %RH and ppm and a switch 203 to switch the
temperature
portion of the display 60 between Fahrenheit and Centigrade or Celsius. The
water
sensing system may also include a plurality of liquid type controls 204 to
select a type
of liquid in which water is being measured.
A water sensing system according to the present embodiment provides reliable
water content information even when the temperature of the liquid being sensed
varies.
By converting from %aRH to ppm, the sensing system provides a temperature-
independent output which can be used to determine whether or not a water
content is
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above an acceptable level. For example, if the sensing system produces an
output signal
indicating a water content of above about 500 p~pm, an operator may determine
that the
water level is too high. By determining a permissible water content based on
ppm rather
than % RH, the problem of temperature dependency of RH sensor measurements is
reduced. Alternatively, under operating conditions where the additive content
or the
saturation characteristics of a liquid is not known, or when the temperature
of the liquid
is relatively stable during operation, %RH may provide a more useful
measurement of
water content.
According to another aspect, the preaent invention includes a method for
determining the saturation level of a liquid, even when the additive content
of the liquid
is unknown. The additive content of a liquid may vary due to a variety of
factors, such
as, contamination of the liquid over time or changes in the additive package
of the liquid.
The additive content affects the saturation level of the liquid. Thus, when
the additive
content is unknown, determining the saturation level may be difficult. As used
herein,
the term "saturation level" defines the water content of a liquid in ppm
corresponding
to 100 %RH.
An exemplary method for determining the saturation level of a liquid according
to the present invention generally includes calibrating a water sensing system
with a
sample of the liquid. For example, if it is desired to measure the water
content of
hydraulic liquid stored in a particular drum, the method includes sampling a
predetermined quantity of the liquid. The present invention is not limited to
any
particular sample quantity. Thus, for example;, the sample quantity may be one
liter of
the liquid. Next, a water sensing system according to any of the embodiments
of the
present invention may be used to measure the %RH of the liquid, i.e., by
inserting the
water sensor probe into the sample.
Once the %RH is measured, a known quantity of water is added to the sample.
The quantity of water added is preferably selecaed to be small enough that the
likelihood
of saturating the sample is small. The quantil:y of water added depends on a
variety of
factors, such as, liquid type, sample volume, and anticipated water content
level. For
illustration, the quantity of water added may be about one milliliter. The
added water
is mixed with the sample, preferably until substantially all of the water is
dissolved.
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Because dissolving water in some liquids, such as oils, may be difficult, it
may be
preferable to first dissolve the water in an intermediate solvent and then
dissolve the
water/solvent solution in the sample. The intermediate solvent should be
selected not to
greatly affect the saturation characteristics of the sample. An exemplary
solvent that
may be used is an alcohol.
Since the quantity of water added to the sample is known and the volume of the
sample is known, the increase in water content in parts per million can be
determined.
For example, adding one milliliter of water to a one liter sample of the
liquid increases
the water content of the sample by 1000 ppm.
The next step includes measuring the %RH of the sample after the water is
dissolved. Again, this measurement is performed by inserting the water sensor
probe
into the liquid. Next, the difference between the two measured %RH values is
determined. The saturation level may then be determined according to the
following
equation:
ppm(sat) _ (1000ppm) * (1000 ~ O~~RHdelta<100~ (2)
RHde1 to
, where ppm(sat) is the saturation level of the liquid and RHdelta is the
difference
between the measured %RH values. The value ppm(sat) indicates the maximum
amount
of water that the liquid can hold in solution in parts per million. The
saturation level is
a useful measurement in determining whether or not the water content of a
liquid is
within a safe range. In order to convert %RH values to ppm values, the %RH
values
may simply be multiplied by the saturation level ppm(sat). For example, if
ppm(sat} of
liquid is 10,000, and the measured % RH at a given time is 10 %o , then the
absolute water
content of the liquid is 1000 ppm.
The present invention is not limited to using equation (2) to calculate the
saturation level for a liquid. Any equation for calculating the saturation
level based on
adding a predetermined quantity of water to a liquid and measuring the change
in % RH
is within the scope of the invention.
The following assumptions are made in the above described method for
determining the saturation level. The first assumption is that the water added
to the
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sample is fully dissolved. The second
assumption is that relationship between ppm and %RH is linear over the
measured range.
For example, if the water added to the sample saturates the sample, the
calculated
saturation level would be inaccurate if a linear assumption was made. The
third assump-
tion is that the temperature of the sample is constant when the first and
second %RH
measurements are performed.
The above described method for determiining the saturation level of a liquid
may
be programmed into the memory of any of the embodiments of the water sensing
system
described herein. In other words, an equation for calculating the saturation
level and a
routine for obtaining the first and second %1tH values, as described above,
may be
implemented in software. These functions are preferably transparent to the
user. Thus,
in order to determine the saturation level for a liquid, a user actuates the
saturation level
calculating routine, e.g., by depressing a ppm key on the housing of the water
sensing
system. Next, the user obtains a sample of the liquid in which it is desired
to measure
the water content and inserts the sensor probe into the sample. The volume of
the
sample may be fixed or variable. If the volume is variable, the user may input
the
sample volume, e.g., using the increment and decrement controls. The
saturation level
calculating routine then records the output voltage from the water sensor and
stores the
value in memory. The user then removes the probe from the sample, dissolves a
predetermined quantity of water in the sample, and reinserts the probe into
the sample.
The quantity of water added may be fixed or variable. If the quantity is
variable, the
user may input the quantity. The saturation level calculating routine records
the second
%RH value and calculates the saturation level using an equation, such as
equation (1),
based on factors such as the sample volume, the volume of water added, and the
measured %RH values. The saturation level calculating routine preferably
stores the
calculated saturation level in memory and uses it to convert %RH values to ppm
values.
Alternatively, the saturation level calculating routine may display the
saturation level to
the user and the user may perform the conversions manually.
In this manner, water sensing systems according to embodiments of the present
invention are capable of calculating the saturation level of liquid and
converting to ppm,
even when the additive content of the liquid is unknown. Thus, when the
additive
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content of a liquid changes due to contaminants that accumulate in the liquid
over time,
changes in the additive package of the liquid, or any other factor causing the
additive
content to be unknown, embodiments of the present invention are capable of
reliably
determining the saturation level. When the additive content of a liquid is
expected to
vary between batches of the liquid, the calibration is preferably performed
for each
batch. However, due to the simplicity of the calibration process, calibration
may be
easily performed in the field by the end user. Once the saturation level is
stored,
subsequent ppm calculations for a liquid may be automatically performed using
the stored
value without further calibration, unless the additive content changes.
According to a further aspect, embodiments of the present invention may
include
a voltage conversion algorithm to convert the output voltage from the water
sensor into
a water content value. The voltage conversion algorithm may be linear or
nonlinear,
depending on the relationship between the water sensor output voltage and the
%RH.
In a preferred embodiment, the voltage conversion algorithm is linear.
The voltage conversion algorithm may be determined by measuring the "actual"
water content values of a plurality of liquids using a conventional laboratory
method,
such as Karl Fischer titration and measuring the water sensor output voltages
corresponding to the water content values. Data relating the water sensor
output voltages
to the water content values is then plotted. A curve-fitting algorithm is used
to
determine the function or algorithm that best approximates the data. The
algorithm is
then programmed into the memory of the processing circuit to convert the
output voltage
of the water sensor into a water content value. Alternatively, the algorithm
may also be
used to generate any suitable circuit, such as a look-up table or logic
circuit, for
converting the water sensor output voltages into water content values.
If the voltage conversion algorithm is determined to be nonlinear, the
complexity
or order of the voltage conversion algorithm depends on factors such as, the
desired
accuracy of the water content measurement, the available memory, and the speed
of the
microprocessor of the water sensing system. As desired accuracy, available
memory,
and processing speed increase, the order of the voltage conversion algorithm
may be
increased to more closely approximate the measured water content data.
The literature distributed with conventional humidity sensors includes an
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algorithm for correcting % RH measurements for minor variations in sensor
output
voltage caused by temperature changes. For a;xample, for one capacitive
sensor, the
correcting algorithm is:
HRH
$RHcczrected- ( 1 , 09 3 - ( 0 . 0 012 * TEIPIp) )
where %RH is the percent relative humidity and temp is the temperature in
degrees
Fahrenheit. However, this conventional correcaing algorithm is designed for
use with
the conventional linear voltage conversion algorithm for converting sensor
output
voltages to % RH in air. If the voltage conversion algorithm is determined to
be different
from the conventional voltage conversion algorithm, the correcting algorithm
may also
require alteration. For example, if the voltage conversion algorithm is
nonlinear, the
correcting algorithm may be nonlinear.
According to a further aspect, the present invention includes circuitry and
methods for calibrating a water sensing system. Unlike other electronic
devices which
may require complex external equipment a.nd/or return to the manufacturer for
calibration, embodiments of the present invention include internal calibration
circuitry
which allows an operator to calibrate a water sensing system, preferably on-
site and
without using complex external calibration equipment.
Figure 11 illustrates a water sensing system including water content and
temperature calibration programs. In the illustrated embodiment, the water
sensor 2 and
the temperature sensor 4 may comprise any o~f the sensors previously
described. The
processing circuit 5 comprises a microcontroller U1. The microcontroller
preferably
includes processing circuitry such as a microprocessor to perform mathematical
operations for the input signals from the sensors and to calculate calibration
values. The
microcontroller also includes a memory M1, preferably divided into read-only
and
random access portions. In a most preferred embodiment, the memory Ml includes
a
read only portion, a random access portion, and an electrically erasable
programmable
read only portion. The read-only portion stares programs, e.g., calibration
programs
and temperature compensation programs. 7.'he random access portion stores
values
generated during the execution of programs, such as digitized sensor output
values. The
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electrically erasable programmable portion stores data used by the programs,
for
example, reference calibration values.
Although the illustrated embodiment depicts a microcontroller including an
internal memory, the present invention is not limited to such an embodiment.
For
example, the microcontroller may be replaced by a microprocessor with external
memory
chips for the three memory portions. The three memory portions may comprise a
single
or separate chips.
A control block 61 includes jumpers, switches, and other controls to control
the
operation of the microcontroller. For example, different combinations of the
jumpers
or switch positions may be used to actuate different calibration programs. The
control
block 61 preferably also includes a value selection control to select and
store calibration
values in the memory M1.
The processing circuit preferably also includes a display 60. In some
embodiments, for example, those that display water content and temperature
values, the
display comprises an alphanumeric display. In other embodiments, for example,
those
that indicate whether one or more thresholds have been exceeded, the display
comprises
one or more indicator lamps or LED's without an alphanumeric display. In still
other
embodiments, the display comprises both an alphanumeric display and indicator
LED's.
If the display comprises an alphanumeric display, the control block 61
preferably
includes a display adjustment control to adjust the displayed value during
calibration.
The display may also include one or more calibration LED's which communicate
calibration status information to an operator during calibration.
The present invention is not limited to using a microcontroller and
calibration
programs to calibrate the sensor. Analog or digital circuitry which performs
the same
or equivalent functions are within the scope of the invention. For example,
the
processing circuit may include one or more analog or digital calibration
subcircuits to
calibrate the water sensor and/or the temperature sensor. The calibration
programs or
subcircuits may be combined with any of the previously described embodiments
of the
water sensing system.
The switches or jumpers for accessing the water content and temperature
calibration programs, the display adjustment control (if included), and the
value selection
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control are preferably internal components of the water sensing system, i.e.,
the
calibration components are on a circuit board within the housing of the water
sensing
system. In an alternative embodiment, the calibration components may be
external.
However, because the calibration components are preferably accessed only
during
calibration, it may be more practical for these components to be internal.
In order to accurately calibrate the water sensor, the water sensor probe is
preferably inserted into a medium that accurately provides a known, constant
%RH value
as a reference point. In a preferred embodvnent, two calibration media are
used to
provide two different reference points for % RH calibration. Exemplary
calibration
media suitable for % P,H calibration include humidity cells in which a
saturated salt
solution produces a known constant %P:H in l:he air above the salt solution in
a closed
container. The container includes a connector to allow the insertion of the
water sensor
probe. According to a preferred embodiment, a calibration cell comprising a
saturated
salt solution of lithium chloride provides a %R.H of 11.3 % at 75 °F
may be used for one
of the reference points. A calibration cell including a saturated sodium
chloride solution,
which provides a % RH of 75.3 % at 75 °F may be used as another
reference point in
performing %RH calibration.
The present invention is not limited to i:he use of calibration cells which
comprise
lithium chloride solutions, sodium chloride solutions, or saturated salt
solutions. Any
medium which provides an accurate %F,H value may be used with the present
invention.
In another alternative, the water sensor may b~e heated to remove
substantially all of the
water from the sensor probe to provide a %P;H of 0%. A %P,H of 0% could be
used
to zero the probe and calibrate the water sen:;or as described below.
Figure 12 illustrates the operation of the water content calibration program,
including steps in which an operator provides input to the program, according
to a
preferred embodiment of the present invention. To initiate the water content
calibration
program, a sensor operator sets the appropriate jumpers or switches on the
circuit board
to a predetermined position to actuate the program stored in the memory for
obtaining
a first measured value for calibration. Next, the operator places the water
sensor probe
in a calibration medium to provide a first reference % P;H value. For example,
the first
calibration medium may comprise a lithium chloride calibration cell that
provides a %P;H
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of 11.3 % at 7S degrees Fahrenheit. The program causes the % RH and
calibration
LED's on the display (illustrated in Figures S and 11) to flash, indicating
that % RH of
the calibration medium is not stable. The program may determine stability by
analyzing
the water sensor output voltage, for example, by measuring variations in the
water sensor
S output voltage.
In embodiments of the water sensing system that include an alphanumeric
display,
the operator uses the display adjustment controls (illustrated in Figure 11)
to adjust a
numeric readout on the display to be equal to the known %RH. Providing an
alphanumeric display thus allows easy adjustment of the water sensing system
for use
with any calibration medium. Alternatively, in embodiments without an
alphanumeric
display, the known %RH for a specific calibration medium is preprogrammed in
the
memory. In embodiments without an alphanumeric display, the operator
preferably
selects a calibration medium with a %RH corresponding to the preprogrammed
value.
Thus, in some embodiments, display adjustment is not required.
1S When the water sensor output voltage becomes stable, e.g., when the
variation
in the output voltage is within a predetermined tolerance, the program stops
the flashing
of the LED's and waits for the operator to store the sensor output voltage and
the display
value. The operator actuates value select control to enter the % RH value of
the medium
and the sensor output voltage into the memory device. Alternatively, the
program may
automatically record the water sensor output voltage when the output becomes
stable.
The program preferably prevents the operator from storing the first water
sensor output
voltage until the output voltage is stable.
After the program records the first %RH value and the corresponding water
sensor output voltage in the memory, the operator preferably executes another
routine
2S for recording a second measured value for calibration. To initiate the
second routine,
the operator actuates the appropriate jumpers or switches on the circuit
board. The
operator places the water sensor probe in a second calibration medium with a
second
known %RH, which is preferably different from the first known %RH. For
example,
the second calibration medium may comprise a calibration cell including a
saturated
sodium chloride solution that provides a known %RH of 75.3 % at 7S degrees
Fahrenheit. The program again causes the calibration LED's to flash until the
%RH in
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the calibration cell stabilizes.
In embodiments having an alphanumeric display, the operator then uses the
display adjustment control to set the number on the display equal to the
second known
%RH. Alternatively, in embodiments without an alphanumeric display, the second
known %RH value is preprogrammed in memory. In embodiments without an
alphanumeric display, the operator preferably aelects the second calibration
medium to
correspond to the preprogrammed value.
When the %RH becomes stable, the program stops the flashing of the LED's.
The program then waits for the operator to store the second water sensor
output voltage
and the second display value, which is equal to the second known %aRH. When
the
LED's cease to flash the operator activates the value selection control to
store the second
known %RH and the second water sensor output voltage in the memory.
Alternatively,
the program may automatically record the water sensor output voltage when the
output
becomes stable. The program preferably prevents the operator from storing the
second
water sensor output voltage until the output voltage is stable.
Once the first and second known %R13 values and the corresponding first and
second water sensor output voltages are stored. in the memory. The program
calculates
calibration factors and stores the calibration factors in the memory device.
Any method
for calculating the calibration factors is within the scope of the invention.
For example,
the program may use statistical analysis, e.g., linear regression analysis, to
calculate one
or more constants or coefficients to adjust the voltage conversion algorithm
that converts
water sensor output voltage to %RH. Once the program calculates the
calibration
factors, the program stores the calibration factors in the memory. After the
program
stores the calibration factors, the operator prei:erably verifies the
calibration by inserting
the probe into both the high and low % RH media and verifies that the display
reads the
correct value for each medium.
The present invention is not limited to using two % RH measurements to
calibrate
the water sensor. More or fewer measurements are within the scope of the
invention.
The present invention preferably uses at least two measurements, e.g., a high
measurement and a low measurement, to increase the accuracy of the calibration
by
calculating high and low calibration factors, from which intermediate
calibration factors
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may be extrapolated.
According to another aspect, the present invention preferably includes
circuitry
and methods for temperature calibration, in addition to circuitry and methods
for water
content calibration. Alternatively, some embodiments of the present invention
may
include circuitry and methods for water content calibration and not for
temperature
calibration. Still other embodiments of the present invention may include
circuitry and
methods for temperature calibration only . In a preferred embodiment including
temperature calibration circuitry and methods, temperature calibration is
performed by
the microcontroller according to a temperature calibration program stored in
the
memory. Figure 13 is a flow chart illustrating the operation of an embodiment
of the
temperature calibration program, including steps in which the operator
provides input to
the program.
In order to perform temperature calibration the operator sets the appropriate
jumpers or switches on the circuit board to a predetermined position. The
program then
waits for the operator to select a unit of measure for temperature
calibration, e.g.,
degrees Celsius or degrees Fahrenheit. Another jumper or switch may be used to
select
the appropriate unit of measure for temperature calibration. The program then
causes
the Fahrenheit/Celsius and calibration LED's to flash. The operator places an
external
temperature measuring device, e.g., a calibrated thermometer, near the probe
to measure
the temperature of the probe. The operator then adjusts the display adjustment
controls
on the display to set the display temperature equal to the temperature
measured by the
external temperature measuring device. When the measured temperature becomes
stable,
e.g., when variation in the output voltage is within a predetermined tolerance
range, the
program stops the flashing of the LED's and waits for input from the operator.
When
the LED's cease flashing, the operator activates the value selection control
to store the
temperature value and the corresponding temperature sensor output voltage. The
program preferably prevents the operator from storing the temperature sensor
output
voltage until the output becomes stable. The program calculates the
temperature
calibration factor and stores it in memory. The operator then preferably
verifies the
temperature calibration by varying the temperature of the probe, e.g., by
heating or
cooling the probe, reading the temperature from the display, and verifying the
display
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temperature using the external temperature measuring device.
Although the illustrated temperature calibration program calibrates the
temperature sensor using a single temperature measurement, the present
invention is not
so limited. For example, the temperature calibration program may utilize two
or more
temperature measurements to perform temperature calibration.
By providing water content and temperature calibration programs, the water
sensing system according to embodiments of the present invention reduces the
need for
complex external calibration equipment. Calibration can be performed on-site
at the
sensing location and measurement accuracy is increased.
In another embodiment of the water sensing system, the processing circuit may
comprise a separate unit from the sensor. For example, the processing circuit
may be
a portable unit designed to process the outputs from a plurality of sensors.
The sensors
may be attached to devices using the liquids in which the water content is
being sensed.
For example, a water sensor may be coupled to the housing of a helicopter
transmission
to measure the water content of the transmission fluid. The processing circuit
may be
capable of coupling to the sensor through an inductive or a direct electrical
connection
to process the output signal from the sensor when the helicopter is on the
ground. In
this arrangement, the processing circuit is not subjected to the harsh
operating
environment caused by vibrations and extreme temperatures experienced during
flight.
A portable processing circuit may also be capable of measuring the water
content of
liquids in vehicles other than helicopters, for example, in watercraft, or in
land vehicles
which may produce a harsh operating environment for a processing circuit or
which may
have power, space, or weight constraints. A portable processing circuit is
preferably
battery powered to enable use in environments where external power is not
available.
In an embodiment in which the sensor probe is separate from the processing
circuitry, the housing for the processing circuitry may include a sensor
interface to allow
the processing circuitry to communicate with .an external probe. The sensor
interface
may comprise a cable, an antenna, or any other means suitable for
communications.
In yet another alternative embodiment, the water sensing system, including the
processing circuit, the water sensor, and the temperature sensor, may comprise
a
portable unit. In such a unit, the processing circuit may include any of the
subcircuits
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previously described, e.g., a comparison subcircuit, a voltage conversion
subcircuit, a
temperature compensation subcircuit, and/or a calibration subcircuit.
The water sensing system according to the present embodiment is preferably
battery-powered to increase portability. In a most preferred embodiment, the
water
sensing system is powered by a nine Volt non-rechargeable battery. Using a
nine Volt
non-rechargeable battery is preferred because such batteries are inexpensive
and readily
available. However, any type of power source suitable for powering the water
sensing
system is within the scope of the invention. For example, the water sensing
system may
be powered using a solar cell.
The water sensing system according to the present embodiment preferably also
includes a display for displaying water content information. Because the
present
embodiment is preferably battery-powered, the display is preferably a low
power display,
such as a liquid crystal display. The display is preferably capable of
displaying numbers
indicative of the water content and temperature and labels, such as, " % RH" ,
"ppm" ,
"°C" and "°F". However, the present invention is not limited to
using a liquid crystal
display. Any display suitable for low power operation is within the scope of
the
invention. For example, the display may comprise an analog meter.
A water sensing system according to any of the embodiments previously
described
may be used to maintain a desired water content level in a liquid. For
example, in
hydraulic fluids, such as water glycols, it may be desirable to maintain a
ratio of water
to glycol. In ethylene glycol/water solutions, the water may evaporate faster
than the
ethylene glycol. As a result, the ratio of water to ethylene glycol may vary
over time.
Since the ratio affects properties of the hydraulic fluid, such as boiling
point, freezing
point, and flame retarding ability, it is desirable to maintain a specified
ratio of water
to ethylene glycol in some hydraulic systems. Accordingly, any of the
embodiments of
the water sensing system previously described may be used to sense the water
content
of a hydraulic fluid and produce an output signal for adding water to the
hydraulic fluid.
Water may be added automatically by a actuating a valve coupled to a hydraulic
fluid
system or manually by an operator until the ratio is within a desired range of
values.
According to a further aspect of the present invention, a water sensing system
according to any of the previously described embodiments may be combined with
one
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or more external devices such as a purifier to remove water and other
contaminants from
the liquid. For example, the water sensing system according to Figures 2 and 3
may
be coupled to a purifier to provide a liquid purification system controlled by
temperature
compensated water content threshold values. Alternatively, a water sensing
system
without a temperature compensation subcircuit, for example, as shown in Figure
4, may
be coupled to a purifier to provide a liquid purification system controlled by
%RH water
content data. In another alternative, a water sensing system capable of
switching
between temperature compensated data and %RHI, for example, as shown in
Figures 8
10, may be coupled to a purifier to provide a liquid purification system with
both %RH
and ppm measurement capability.
A purifier according to the present embodiment can be any type of purifier
suitable for removing water from liquids. For example, the purifier may
comprise a
spinning disk purifier, a nozzle purifier, or a tower purifier. Exemplary
spinning disk
fluid purifiers suitable for removing water andl other contaminants from
liquids are
disclosed in U.S. Patent No. 4,604,109 to Koslow, entitled "Fluid Purifier",
and U.S.
Patent No. 5,133,880 to Lundquist et al., entitled "Fluid Purifier and Method
for
Purifying Fluid Contaminated with a Volatile Contaminant", the disclosures of
which are
hereby incorporated by reference. A spinning disk fluid purifier suitable for
use with
the present embodiment includes a fluid housing, at least one spinning disk
inside the
housing, a fluid inlet, a vapor outlet, and a fluid outlet. In operation,
fluid contaminated
with water enters the housing through the fluid inlet and contacts at least
one of the
spinning disks. After contacting a disk, the contaminated fluid is thrown
radially
outward from the disk in the form of small droplets which travel toward and
contact an
interior wall of the housing. Water present in the fluid is vaporized during
the travel of
the droplets and exits the housing through the vapor outlet. The purified
droplets
coalesce and flow down the interior wall of the housing and exit through the
fluid outlet.
A particle filter is preferably located upstream ~of the purifier to remove
large particles
from the liquid before the liquid enters the purifier. A second filter may be
located
upstream or downstream of the purifier to remove smaller particles from the
fluid.
A tower fluid purifier suitable for use with the present embodiment includes a
vacuum tower chamber maintained under a vacuum to remove contaminants, such as
CA 02286167 1999-10-06
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water, from fluids, such as oils. A vacuum pump creates a vacuum that draws
contaminated fluid into the purifier. The fluid enters the purifier through an
inlet valve
and proceeds through a heater. The fluid enters the vacuum tower chamber
through an
inlet at the top of the vacuum tower chamber. A regulating valve regulates the
flow of
fluid into the chamber. The fluid flows in a downward direction over a
dispersion
material inside the chamber. As a result, the surface area per unit volume of
the fluid
is increased.
Free and dissolved air, liquids, and gases are removed from the fluid by
exposing
the fluid to a low relative humidity atmosphere which is obtained by
maintaining a
vacuum in the chamber. The low pressure in the chamber draws ambient air into
the
chamber. The air enters the purifier through a filter and a restrictor
orifice. The air
enters the chamber through an air inlet at the bottom of the chamber and
proceeds
upwards against the falling flow of contaminated fluid. Water and gases are
removed
from the fluid in the upward air flow and exit at the top of the chamber. An
oil mist
filter at the top of the chamber separates oil from air. Excess oil is drained
from the oil
mist filter to the bottom of the chamber. The purified oil collects at the
bottom of the
chamber. A discharge pump removes the purified oil and returns it to the
reservoir or
system being purified.
A nozzle fluid purifier suitable for use with the present embodiment includes
a
vacuum chamber. A spray nozzle located at the top of the vacuum chamber sprays
a
cone of fluid into the chamber with a thin film and a large surface area. A
vacuum
pump maintains the vacuum chamber at a predetermined vacuum to optimize
purification
for a particular application. Air enters the vacuum chamber through a filter
and a
restrictor. The filter removes contaminants from the air. The restrictor
allows
expansion of air inside the chamber to about three times the ambient volume of
the air.
As a result, the relative humidity inside the chamber is about one third of
the ambient
level.
Gases and water vapor are transferred from the fluid surface to the upward
flowing air, thereby drying and degassing the fluid. The air and contaminants
exit the
chamber through an oil mist separator and to the atmosphere. Purified fluid
collects in
the bottom of the chamber. A discharge pump returns the purified fluid to the
reservoir
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or system being purified.
One example of a liquid purification system embodying the present invention is
illustrated in Figure 14. According to the illustrated embodiment, the system
includes
an external device such as a purifier 40 to remove water from the liquid. The
purifier
40 may include a control circuit 44 to actuate arid deactuate the purifier. A
processing
circuit 5 is coupled to the control circuit to control the actuation and
deactuation of the
purifier. The processing circuit 5 may comprise any of the sensor circuits
including any
of the subcircuits in the previously described embodiments. The processing
circuit may
be physically incorporated with or separate from the purifier control
circuitry. In a
preferred embodiment, the processing circuit S comprises a portable unit
separate from
the purifier 40 but capable of communicating vvith the purifier 40. For
example, the
embodiment of Figure 3 includes a relay that may control the purifier 40.
In order to sense the water content and temperature of the liquid being
purified,
the liquid purification system preferably includes a water sensor 2 and a
temperature
sensor 4. Alternatively, the temperature sensor 4 may be omitted. The sensors
2 and
4 may be coupled to the liquid, e.g., inserted in a liquid reservoir.
Alternatively, the
sensors 2 and 4 may located in the purifier or coupled to a conduit upstream
or
downstream of the purifier 40.
In order to filter particulate contaminants from the liquid, the system
preferably
includes a fitter 42. In a preferred embodiment, the filter 42 comprises a 3
~.m (~i3 >
200) Ultipor~ filter available from Pall Corporation. The filter 42 may be
incorporated
with or separate from the purifier. For example, the filter may be located
upstream,
downstream, or within the purifier 40. In tine illustrated embodiment, the
filter 42
located downstream from the purifier.
In operation, the purifier may be coupled to a liquid reservoir, e.g., an oil
drum,
to purify the liquid. The water sensor 2 senses the water content of the
liquid. If the
water content is above a first predetermined level, e.g., the turn-on level,
the processing
circuit 5 sends a signal to the control circuit 44~ to actuate the purifier
40. The purifier
operates until the water content level is below a second predetermined level
which is
preferably less than the turn-on level to prevent oscillation between ON and
OFF states,
as discussed in the embodiment of Figure 3. Although using two thresholds is
preferred,
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actuation and deactuation of the purifier using a single threshold value is
within the scope
of the invention.
In embodiments in which the processing circuit includes a timer circuit, the
purifier may operate to remove particulate contaminants from the liquid even
when the
water content is below an acceptable level. A timer circuit causes the
purifier to operate
for a predetermined time period. For example, if the water content is reducea
ueiow an
acceptable level before the time period expires, the purifier nonetheless
continues to
operate beyond the time required for water removal in order to remove the
particulate
contaminants. If the time required for water removal is greater than the
predetermined
time period, then the purifier may operate beyond the time period for
particulate
filtration. In this manner, the liquid purification system according the
present
embodiment reliably removes both water and particulate contaminants from a
liquid.
Although the purifier may be actuated and deactuated automatically by signals
from the water sensor and/or a timer, as described above, other methods of
controlling
a purifier using a water sensor are within the scope of the invention. For
example, the
purifier may be actuated by an operator to begin purification of a liquid. The
water
sensor 4 may deactuate the purifier when the water content of the liquid is
below a
predetermined level. In another alternative, the water sensor may actuate the
purifier
when the water content is above a predetermined level and an operator may
deactuate
the purifier after a predetermined time period. Alternatively, the water
sensor may
display the water content to the purifier operator and the operator may
actuate or
deactuate the purifier according to the indicated level. In embodiments in
which the
purifier includes a heater to accelerate the purification process, the water
sensor may be
used to actuate and deactuate the heater when the water content is above or
below a
predetermined level or levels to save energy. Thus, couplW g the water sensor
to a
purifier in any manner to control the purification process is within the scope
of the
invention.
A liquid purification system according to the present invention is preferably
portable and suitable for on-line, i.e., real time and preferably in-line,
sensing of water
and purification of liquids. The combination of the sensors, the processing
circuit, and
the purifier increases the efficiency of the purification system by sensing
the water
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content of a liquid as the liquid is being purified. The need for performing
lab tests such
as Karl Fischer titration is reduced. The combination of sensing and purifying
makes
the present embodiment particularly suitable for operations in which large
numbers of
separate containers require purification because i:he time required to purify
liquid in each
individual container is decreased.
While the invention has been described in some detail by way of illustration
and
example, it should be understood that the invention is susceptible to various
modifications and alternative forms, and is not restricted to the specific
embodiments set
forth. It should be understood that these specific embodiments are not
intended to limit
the invention but, on the contrary, the intention is to cover all
modifications, equivalents,
and alternatives falling within the spirit and scope of the invention.
49
