Note: Descriptions are shown in the official language in which they were submitted.
CA 02412405 2002-11-25
LIQUID LEVEL GAUGE AND
SPECIFIC GRAVITY CALIBRATION THEREFOR
Background
This application relates to liquid level gauges of the type used in measuring
the fullness
of vessels or the depth of liquids contained in vessels, such as tanks or the
like, and relates in
particular to electronic float-type gauges.
Such gauges utilize a float which rises and falls with the liquid level. Float
movement
may be guided by a fixed guide tube, which may extend into the vessel from the
top thereof. The
float carries a permanent magnet for magnetic coupling to a device in the
guide tube. In one type
of float gauge, the magnetic device may be in the nature of a voltage divider
including a tapped
impedance with a magnetically operated switch, such as a reed switch, at each
tap point. As the
liquid level changes, the switches are sequentially closed by the float
magnet, which is moving
with the liquid level, for applying to a gauge meter or the like a voltage
corresponding to the
liquid level. Thus, when the vessel is full, the float will be adjacent to the
top of the voltage
divider for applying a minimum voltage to the meter and, as the liquid level
drops, the float will
fall with the liquid level, causing sequentially larger voltages to be applied
to the meter,
indicating greater depths of the float in the vessel corresponding,
respectively, to lower liquid
levels.
Such liquid level gauges must be calibrated so that the voltage level
generated by the
float when the tank is full will produce a zero output. This is referred to as
a zero set calibration.
The gauge may also be calibrated so that, when the liquid is at a minimum
level, the float
position will cause a maximum output voltage corresponding to the full range
of the gauge, and
this may be referred to as a range calibration. However, these zero and range
calibrations do not
CA 02412405 2002-11-25
take account of variations in the specific gravity of the liquid, which will
cause variation of the
depth to which the float sinks in the liquid.
Summary
This application is directed to a liquid level gauge which avoids the
disadvantages of
prior gauges while affording additional structural and operating advantages.
An important aspect is the provision of a liquid level gauge with an improved
calibration
apparatus.
In connection with the foregoing aspect, another aspect is the provision of a
liquid level
gauge of the type set forth, which provides a calibration adjustment for
variations in specific
gravity of the liquid.
In connection with the foregoing aspect, a still further aspect is the
provision of a liquid
level gauge of the type set forth, which does not affect a zero-level
calibration setting.
A still further aspect is the provision of a liquid level gauging calibration
method which
accounts for variation in specific gravity of the liquid.
Certain ones of these and other features may be attained by providing a liquid
level gauge
for sensing the level of the surface of a body of liquid in a vessel, the
gauge comprising: a float
member adapted to float on the surface of the liquid in a position which
varies with the level of
the surface, a sensing circuit coupled to the float member for sensing the
position of the float
member, an indicating circuit coupled to the sensing circuit for indicating
the liquid level
corresponding to the float position, and a calibration circuit for varying the
calibration of the
sensing circuit in accordance with changes in the specific gravity of the
liquid so that the float
member will accurately indicate the level of the liquid surface irrespective
of the specific gravity
of the liquid.
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CA 02412405 2002-11-25
Brief Description of the Drawings
For the purpose of facilitating an understanding of the subject matter sought
to be
protected, there are illustrated in the accompanying drawings embodiments
thereof, from an
inspection of which, when considered in connection with the following
description, the subject
matter sought to be protected, its construction and operation, and many of its
advantages should
be readily understood and appreciated.
FIG. 1 is a fragmentary side elevational view in partial section of a liquid
level gauge
mounted on a liquid-containing vessel;
FIG. 2 is a reduced, fragmentary, diagrammatic illustration of the vessel and
liquid level
gauge of FIG. 1;
FIG. 3 is a perspective view of a receiver portion of the liquid level gauge
of FIG. l, with
the cover removed and top hatch closed;
FIG. 4 is a top plan view of the receiver of FIG. ~, with the top hatch open;
FIG. 5 is an enlarged fragmentary, diagrammatic illustration of a transmitter
portion of
the liquid level gauge of FIGS. 1 and 2;
FIG. 6 is a functional block diagrammatic illustration of the liquid level
gauge of FIGS. 1
and 2; and
FIG. 7 is a schematic circuit diagram of the calibration portion of the liquid
level gauge
of FIG. 6.
Detailed Description
Referring to FIGS. 1 and 2, there is illustrated a vessel 10, which may be in
the nature of
a tank ar the like, having a top wall 11 in which is formed a port 12. The
vessel 10 contains a
liquid I3 having a surface 14, the level of which is to be sensed.
CA 02412405 2002-11-25
Referring also to FIGS. 3-5, there is mounted on the top wall 11 a liquid
level gauge 20,
which includes a generally cylindrical housing 21 having an upper end closed
by a circular cap
22, which includes a hinged hatch 23 with a handle knob 24. Formed in the side
wall of the
housing 21 is a generally rectangular aperture 25 adapted to be closed by a
part-cylindrical cover
plate 26. The plate 26 has depending legs with apertures 27 adapted to
respectively receive tabs
27a mounted on the housing side wall 21 below the aperture 26. Provided at the
upper edge of
the cover plate 26 are hooks 28, respectively engageable with latches 29
mounted on the cap 22
for securing the cover plate 26 in place.
The housing 21 is mounted on the vessel top wall 11 by means of a pedestal 30,
which
includes an annular attachment plate 31 fixedly secured to and closing the
lower end of the
housing 21. Integral with the attachment plate 31 acid depending therefrom is
a hollow
cylindrical stem 32, integral at its lower end with an annular mounting plate
33 adapted to be
fxedly secured to the top wall 1 l, as by threaded fasteners 34, so that the
stern 32 is in coaxial
communication with the port 12. Fixed to the inner surface of the vessel top
wall 11 and
depending therefrom coaxial with the port 12 is a hollow guide tube 35.
Disposed coaxially
within the guide tube 35 and extending between the base plate 36 and the
vessel top wall 11 is a
voltage divider tube 37 which communicates with the port 12, the tube 3?
having a guide disk 36
attached to its lower end and being provided with encircling helical
compression springs 3 $ and
39, respectively adjacent to its lower and upper ends. The interior of the
guide tube 35
communicates with the interior of the vessel 10 through ports 36a and through
the bottom of the
tube 35 around the guide disk 36. Disposed within the tube 37 is a voltage
divider impedance
which, for simplicity, is diagrammatically illustrated in FIG. 2 as an
elongal:ed continuous
resistor, although it will be appreciated that it could have other forms, such
as a series of discrete
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CA 02412405 2002-11-25
impedances, non-resistive impedance, and the like. One or more batteries 41
may be connected
across the voltage divider 40. A float 42 encircles the voltage divider tube
37 and is freely
slidable thereaiong, the springs 38 and 39 serving as cushioning stops to
limit travel of the float.
The lowermost and uppermost positions of the float are, respectively,
illustrated in solid line and
broken line in FIG. 1. The float 42 carries one or more cylindrical permanent
magnets 43 (see
FIG. 5)
The voltage divider 40 is provided with a plurality of equidistantly
vertically spaced tap
points (not shown), connected in parallel to one terminal of a meter 44,
respectively through reed
switches 45 (FIG. 5), the other terminal of the meter 44 being connected to
the battery 41. Thus,
it will be appreciated that the voltage divider 40 and the float magnet 43
function as a transmitter
within the vessel 10, connected by three wires, which pass up through the port
12, to the battery
41 and meter 44, which are disposed in the housing 21 externally of the vessel
10, the meter 44
serving as a receiver for the transmitted signals, all in a known manner. It
will be appreciated
that the voltage divider tube 37 is appropriately sealed from exposure to the
liquid 13.
In the illustrated embodiment, the reed switches 45 are arranged in a
vertically staggered
series tapped in a "2-3-2 at-a-time'' sequence. When two adjacent switches,
such as the switches
A and B in FIG. 5, are closed, the effective electrical tap point is midway
therebetween. As the
float 42 rises and closes the next switch C, while holding the first two
switches A and B closed,
the effective tap point is at the midpoint of the switch B, a distance D from
the first tap point. As
the float continues to rise, the switch A opens, so that only the switches B
and C are closed, so
that the effective tap point is midway therebetween, again a distance D from
the second tap
point. Thus, it can be seen that the switches are vertically spaced so that
the sequential tap points
CA 02412405 2002-11-25
are equidistantly spaced a distance D from each other. Accordingly, any
inaccuracy would be
limited to the distance D plus any meter and circuit tolerances.
Referring to FIGS. 6 and 7, the circuitry of the liquid level gauge 20 is
illustrated. The
gauge 20 includes an A/D converter and control circuit 50, which may be an
integrated circuit of
the type sold by Maxim under the designation MAX138. Coupled to the A/D and
control circuit
50 is a level sensing circuit 51, which includes the voltage divider 40 and
the associated reed
switches 45 and float magnet 43. Also connected to the A/D and control circuit
50 is a display
circuit 52, which may include an integrated circuit LCD display which is
preferably positioned
below the housing cap 22 for viewing through the hatch 23 when it is opened,
as illustrated in
FIG. 4. A DC power supply circuit 53, which includes batteries 41, provides a
predetermined
DC V+ supply voltage to the circuits 50-52. It will be appreciated that the
A/D and control
circuit 50 and the display circuit 52 cooperate to perform the function of the
meter 44.
Also connected to the A/D and control circuit 50 is calibration circuitry
including a range
adjustment circuit 54, a zero set circuit 55 and a specific gravity adjustment
circuit 56, Referring
to FIG. 7, the range adjustment circuit 54 includes a potentiometer 57 having
one end thereof
connected to the V+ supply voltage and the other end connected to the
Reference LO and
Common terminals of the circuit 50, the potentiometer 57 having a wiper
connected to the
Reference HI terminal of the circuit 50. A capacitor 58 is connected across
the Reference LO
and Reference HI pins. The wiper may be operable by a suitable control knob
(not shown) in the
housing 21 when the cover plate 26 is removed for performing the range
adjustment calibration.
The zero set circuit 5~ includes a potentiometer 60, having one terminal
thereof
connected to ground and the other connected to the wiper of a potentiometer
62, one end of
which is floating and the other end of which is connected to the V+ supply.
The wiper of the
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CA 02412405 2002-11-25
potentiometer 60 is connected to an In LO terminal of the circuit 50. The
input from the level
sensing circuit 51, i.e., from the tap switches 45, is supplied via a lead 63
through series resistors
64 and 65 to the In HI terminal of the circuit 50. Capacitors 66 and 6 l are
connected in parallel
across the In LO and In HI terminals. The wiper of the potentiometer 60 is
provided with a
manually operable knob 60 accessible through the open aperture 25 in the
housing 21, while the
wiper of the potentiometer 62 is coupled to an actuating knob 62, which may be
coaxial with the
knob 60. The batteries 41 may be mounted on suitable clips on the inside of
the cover plate 26
for connection to the remainder of the circuitry when the cover plate 26 is
mounted in place on
the housing 21.
The display circuit 52 and the A/C and control circuit 50 may be arranged so
that the
display circuit 52 will directly display in inches, the depth of the float 5:?
and, therefore, the
distance of the liquid levell4, below the maximum level point. In this regard,
the "zero" point is
set so that the gauge will display 0.00 inches when the float is at its
highest position, this zero set
being accomplished by adjusting the potentiometer 60. The full range point is
set so that the
gauge will display the maximum depth when the float is at the bottom of the
guide tube, e.g.,
60.0 inches.
During this initial calibration of the zero and fully range points, the
calibration is made
assuming the liquid in the vessel has a maximum specific gravity, e.g., 1.3. A
line may be
scribed on the float 42 so that it can be lined up to the zero point on the
vessel 10 when
calibrating the zero point, and at the 60-inch point when calibrating the full
range point. Thus,
when the liquid 13 has a specific gravity of 1.3, the gauge 20 will be
correctly calibrated and will
provide accurate readings of the liquid level.
7
CA 02412405 2002-11-25
However, if the liquid 13 in the vessel has a specific gravity less than 1.3,
the float 42
will sink lower in the liquid. For example, referring to FIG. l, at a specific
gravity of 1.3, the
float will sink to a level corresponding to the level 14, whereas at a lower
specific gravity it will
sink further so that the liquid level will be disposed higher on the float, as
at 14a.
Even though the liquid level is at its maximum level, if the specific gravity
is below 1.3,
the gauge will display a positive depth. To correct the effect of the
different float buoyancy in
different specific-gravity liquids, the display must be adjusted by the amount
of the float
"sinkage." To achieve this, an external zero offset reference input or a
specific gravity
adjustment is provided by the potentiometer 62. By rotating the knob of the
potentiometer 62,
the zero level can be adjusted, without changing the setting of the zero set
potentiometer 60.
Consequently, the range is also "moved" by this same offset amount. By
providing a separate
potentiometer 62 for specific gravity adjustment, the float line corrections
can be effected
without altering the initial zero set calibration, which is used as an
installation calibration.
A table may be affixed to the housing 21 showing the amount of offset (in
inches) for
different specific gravities, e.g., from 0.5 to 1.3. The specific gravity
adjustment potentiometer
62 is turned until the value displayed on the display circuit 52 is corrected
by the amount shown
in the table. For example, if the liquid 13 has a specific gravity of 1.0, the
corresponding offset
given in a table may, e.g., be 0.2 inches. Thus, the specific gravity
adjustment potentiometer 62
is turned until the display is reduced by 0.2 inches. For example, if the
display 52 were
displaying a level of 30.2 inches, the potentiometer 62 is adjusted until the
display shows the
value of 30.0 inches. The gauge is then considered adjusted for a specific
gravity of 1Ø
8
CA 02412405 2002-11-25
From the foregoing, it can be seen that there has been provided an improved
liquid level
gauge with a calibration system which accommodates adjustment fox different
specific gravities
of liquids being gauged.
The matter set forth in the foregoing description and accompanying drawings is
offered
by way of illustration only and not as a limitation. While particular
embodiments have been
shown and described, it will be obvious to those skilled in the art that
changes and modifications
may be made without departing from the broader aspects of applicant's
contribution. The actual
scope of the protection sought is intended to be defined in the following
claims when viewed in
their proper perspective based on the prior art.
9
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