Note: Descriptions are shown in the official language in which they were submitted.
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PRESSURE TRANSMITTER WITH REMOTE SEAL DIAPHRAGM
AND CORRECTION CIRCUIT THEREFOR
BACKGROUND OF THE INVENTION
This invention relates to a field-mounted
= pressure transmitter having a remote diaphragm for
measuring pressure of a process medium. More
= particularly, the present invention relates to providing
a corrected transmitter output to compensate for errors
caused by the remote diaphragm.
Transmitters sensing process variables, such
as differential pressure, gage pressure and process
temperature,'are known. The transmitters typically are
mounted in the vicinity of a process medium to be
measured, at a process control industry installation.
Transmitters provide an output representative of sensed
process variables. This output is then communicated
over a two-wire current loop to a remote control room.
In many cases, the transmitter has a
transmitter housing that contains a pressure sensor and
one or two diaphragms fluidically coupled to the
pressure sensor. The process medium to be measured is
plumbed to the transmitter housing to contact the
diaphragm(s), and the diaphragm(s) transmit the process
medium pressure to the pressure sensor. In other cases,
the transmitter comprises a remote diaphragm separated
from the transmitter housing by a capillary tube, which
tube is typically flexible and can have a length of a
fraction of a meter or as long as tens of meters. The
process medium contacts the remote diaphragm, which
conveys the exerted pressure to the pressure sensor
disposed in the transmitter housing via a substantially
incompressible fluid filling the capillary tube.
These latter transmitters, utilizing one or
more remote diaphragms, are the subject of the present
invention. Existing remote diaphragms are subject to
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errors caused by temperature changes of the
surroundings. With a constant process medium pressure,
the transmitter output can change with the outside
ambient temperature due to thermal effects on the remote
diaphragm, and the connected capillary tube. The
resulting errors are dependent on the vertical position
of the remote diaphragm relative to the transmitter.
Because the vertical distance between the remote
diaphragm and the transmitter is specific to a selected
installation, predetermination of the thermal effects is
difficult.
Known remote diaphragm transmitters have a
temperature sensor disposed in the transmitter housing,
and the output of such temperature sensor is used by the
transmitter circuitry to provide a relatively accurate
transmitter output, corrected for the thermal response
of the various transmitter components. Such known
transmitters, however, do not correct for temperature
changes of installation-specific remote diaphragm
systems where there is a net vertical separation between
the remote diaphragms.
Therefore, there is a need for a means which
can compensate for the measurement inaccuracies
introduced by thermal effects on a remote diaphragm
transmitter. For highest accuracy, such a means should
be as installation-specific as possible -- i.e., it
should account for the specific installation geometry
desired for the transmitter, as well as for the
particular type of incompressible fill fluid(s) used,
length of the capillary tube(s), and so forth.
Furthermore, such a means should be easy to install and
be compatible with many existing transmitters to reduce
implementation costs.
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SUMMARY OF THE INVENTION
The present invention is directed to a
pressure or flow transmitter having a pressure sensor
disposed in a transmitter housing. The transmitter
includes a diaphragm remote from the transmitter
housing. The diaphragm is connected to the pressure
sensor with capillary tubing containing a fluid. The
diaphragm is disposed at a relative vertical position
from the transmitter. The transmitter contains a signal
processor which provides a transmitter output as a
function of the pressure output, the signal processor
adjusting the transmitter output as a function of the
relative vertical position of the diaphragm.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic elevational view of a
transmitter having a remote diaphragm system in an
installation where a gage pressure or an absolute
pressure of a process medium is measured.
FIG. 2 is a graph of diaphragm stiffness
effect as a function of temperature.
FIG. 3 is a graph of fill fluid density effect
as a function of temperature.
FIG. 4 is a graph showing the combination of
the effects of FIGS. 2 and 3 as function of temperature.
FIG. 5 is a schematic more detailed view of
the transmitter of FIG. 1 showing a first embodiment of
the present invention.
FIG. 6 is a flow chart of one aspect of the
first embodiment of the present invention.
FIG. 7 is another schematic more detailed view
of the transmitter of FIG. 1 showing a first embodiment
of the present invention.
FIG. 8 is a flow chart of one aspect of the
first embodiment of the present invention.
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FIG. 9 is a flow chart of another aspect of a
second embodiment of the present invention.
FIG. 10 is another schematic elevational view
- of a transmitter and remote diaphragm system.
FIG. 11 is another schematic elevational view
of the transmitter of FIG. 1 but having a modified
remote diaphragm system in an installation where a
differential pressure of the process medium is measured.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a typical transmitter 11 having
a remote diaphragm system 12 connected to a transmitter
housing 14. The transmitter 11 measures the pressure of
a process medium 16. The remote diaphragm system 12
includes thin, flexible diaphragm 18 which contacts the
process medium. System 12 also includes backplate 19
which, together with diaphragm 18, define a cavity 20.
Capillary tube 22 couples cavity 20 to a pressure sensor
27 disposed in transmitter housing 14, such coupling
being made via a transmitter housing diaphragm 25 and a
sealed fluid system connecting diaphragm 25 with sensor
27. The sealed fluid system, as well as cavity 20 and
capillary tube 22, is filled with a suitable fluid (not
shown) for transmitting the process pressure to sensor
27. Fluid may include silicone oil, glycerin and water,
propylene glycol and water, or any other suitable fluid
which preferably is substantially incompressible.
When process pressure is applied from process
medium 16, diaphragm 18 is typically displaced thus
transmitting the measured pressure from remote diaphragm
system 12 through a passage in plate 19 and through tube
22 to pressure sensor 27. The resulting pressure
applied to pressure sensor 27, which can be a
capacitance-based pressure cell, causes such capacitance
to change. Sensor 27 can also operate on other known
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sensing principles, such as strain gauge technology.
Circuitry within transmitter housing 14 electronically
converts the capacitance into a linear 4=20 mA
transmitter output signal over wire pair 30 indicative
of the process pressure. Transmitter housing 14 holds
a temperature sensor 28 which measures the temperature
locally at the transmitter housing.
Transmitter housing 14 includes circuitry (not
shown in FIG. 1) which measures and compensates
temperature and pressure, and provides an output over
wire pair 30. The output can be either digital or
analog.
The vertical distance H between diaphragm 25
and diaphragm 18 introduces a fill fluid density effect
error which is a function of both H and temperature (T)
of the fill fluid between diaphragms 25,18. The
pressure measured by transmitter 11 can be expressed as:
Measured Pressure - PPROCESS + PERRORI + PERROR2 Eq. 1
where:
PERRORI Pl (T) s Diaphragm s ti ffness effect Eq. 2
PERRoR2 = P2 (T,H) = Fi1I fluid densi ty effect Eq. 3
and
PPROCESS - Process pressure Eq. 4
According to the invention, the transmitter output is
corrected for at least the fill fluid density effect
error.
The diaphragm stiffness effect occurs when a
' change in process or ambient temperature causes the
volume of fluid in the system to change as a result of
thermal expansion. The change in fill fluid volume as
a result of a change in temperature is given by:
~
SUBsT=3 NEcT(RLH ' 2'
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OV - (Vc+ VT)(C)OT Eq. 5
where OV is the change in fill fluid volume, Vc is the
volume of fill fluid in cavity 20, VT is the volume of
fill fluid in capillary tube 22 and transmitter 11, C is
the coefficient of thermal expansion of the fill fluid,
and AT is the change in temperature of the combined fill
fluid volumes Vc and VT relative to its temperature
during the last transmitter calibration.
An increase in fill fluid volume urges
diaphragm 18 away from backplate 19, causing an
increased pressure by diaphragm 18 against the fill
fluid, which increased pressure is transmitted to sensor
27. Similarly, if the temperature decreases, the volume
of fluid in the capillary tube/remote diaphragm system
decreases and causes a reduction of pressure against
sensor 27. FIG. 2 is a graph of Pg.arl(T) versus T at a
fixed process pressure and diaphragm stiffness.
The fill fluid density effect error PEor2 ( T, H)
is shown in FIG. 3. It is known to nullify the initial
pressure error Pp,ror2 at a given temperature after
installing the transmitter and thereby defining height
H by calibrating or re-zeroing transmitter 11.
Temperature changes relative to the temperature at the
last calibration, however, create subsequent variations
in the pressure measurement by changing the density of
the fill fluid. This "fill fluid density effect, " also
termed "head temperature effect," is dependent on the
distance H, the density of the fill fluid, the
coefficient of expansion of the fill fluid, and the
change in temperature.
The fill fluid density effect can be expressed
as:
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PErroz2 '( H) (SG) (C) AT Eq. 6
where H is the distance shown in FIG. 1, SG is the
specific gravity of the fill fluid in capillary tube 22,
C is the coefficient of thermal expansion of the fill
fluid, and AT represents the difference in the
temperature of the fill fluid relative the temperature
of the fill fluid at the last calibration of transmitter
11.
As set forth in Equation 6 and illustrated
graphically in FIG. 3, the fill fluid density effect
P,,or2 is proportional to the change in temperature. The
slope of the graph of FIG. 3 is given by:
Slope - (H) (SG)(C) Eq. 7
Thus, where To is the temperature of the fill fluid at
the last calibration of transmitter 11, the fill fluid
density effect for any given temperature T is:
pE'zzoz2 a Slope =(T-To) Eq. 8
The fill fluid density and diaphragm stiffness
effects are additive and the total effect on the sensor
27 as a function of temperature is shown graphically in
FIG. 4 which is a graph of equation 1. The present
invention preferably includes correcting both effects.
In one embodiment, transmitter housing 14 is
functionally divided between sensor module 50 and
electronics module 52, shown in FIG. 5. Sensor module
50 performs tasks related to measuring and compensating
process variables. Electronics module 52 performs
necessary calculations, data logging and output control
functions. Pressure sensor 27 capacitance is provided
to Capacitance-to-Digital Application Specific
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Integrated Circuit (ASIC) 54 which converts the
capacitance signal to an intermediate value called
"pressure counts," or "pcounts". Temperature sensor 28
provides a signal representative of temperature within
the transmitter housing to a Resistance-to-Digital ASIC
56 where the temperature signal is converted into an
intermediate value called "tcounts". Pcounts and
tcounts are provided to electronics module 52 wherein
calculation circuit 58 compensates for diaphragm
stiffness and fill fluid density effects, and provides
a corrected signal representative of the process
pressure. The corrected signal is typically subjected
to further processing at circuit 60 and converted at
circuits 62 and 64 to a digital and an analog signal,
respectively, suitable for output from transmitter
housing 14.
The thermal effect from fill fluid density is
dependent on the vertical position of remote diaphragm
systems 12 which can be unique to each installation.
According to the invention, a characterization procedure
is performed for each installation which provides
installation specific data to be used with calculation
circuit 58 to provide a transmitter output more closely
representative of the actual process pressure. One type
of characterization procedure is set forth in the flow
chart of FIG. 6. Information regarding the installation
specifications and environmental data is collected at
S1. Such information can include the vertical distance
H, properties of the remote diaphragm (S), the length of
capillary tubes 22, the type of fill fluid within each
capillary tube 22, typical ranges of process pressure
and operating temperature, and the like. Such
information is input into a computer program which, as
shown at S21 simulates the transmitter behavior in the
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particular installation. In one embodiment, simulated
transmitter output data is calculated using equations
such as Equation 1, Equation 5, and Equation 6, as the
simulated installation is subjected to the temperatures
of -40 F, 0 F, 70 F, 120 F, and 185 F at known process
pressures at S3.
The calculated output datapoints, which can be
plotted on a graph similar to FIG. 4, are fed into a
mathematical program utilizing simple least squares,
weighted least squares, spline, or other known
techniques to fit the datapoints to an equation such as:
PSrrorl+pErrorz - a+bT+cT2+dT3+ Eq. 9
where T is again the fill fluid temperature. Since the
calculated datapoints fed to the program were calculated
based on the vertical distance H, the fill fluid volume
and properties, and so on, the computed coefficients a,
b, c, etc. will also reflect those installation-specific
parameters, and hence are referred to as "installation-
specific" coefficients. These installation-specific
coefficients are then stored in memory at S6 such as
EEPROM 70, or other memory accessible by calculation
circuit 58.
An embodiment of transmitter 11 is shown in
FIG. 7. Transmitter 11 contains manufacturer
predetermined coefficients in EEPROM 80, which
manufacturer predetermined coefficients account for the
response to temperature and pressure of components
within the transmitter housing but not of the remote
diaphragm system with capillary tube. Installation-
specific correction circuit 82 is provided which
corrects the output of circuit 58 for the temperature
response of the remote diaphragm system and capillary
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tube, i.e. the fill fluid density effect and the
diaphragm stiffness effect. Capacitance-To-Digital ASIC
54 is operably connected to calculation circuit 58.
Calculation circuit 58 provides an output substantially
representative of process pressure but susceptible to
substantial errors due to the remote diaphragm
system/capillary tube. Resistance-To-Digital ASIC 56 is
operably connected to calculation circuit 58 and to
correction circuit 82. Memory 83, accessible to
correction circuit 82, contains installation specific
coefficients, a, b, c, and so on from Equation 9 above.
In many applications, the coefficient b, linear with
temperature T, is sufficient by itself to provide
adequate correction. Correction circuit 82 compensates
the output of circuit 58 for P,,ori + PBrror2, thereby
providing a corrected signal.
Operation of this embodiment is set forth in
the flow chart of FIG. 8. The installation-specific
coefficient(s) are preprogrammed into memory 83 at S31.
Signals representative of pressure and temperature are
input into calculation circuit 58 at S32. Calculation
circuit 58 provides an intermediate output which is fed
to correction circuit 82 at S34. The signal
representative of temperature is also provided to
correction circuit 82 at S36. Circuit 82 runs the
temperature value through an equation such as Equation
9, using the installation-specific coefficients to
obtain a pressure value correction, at. S38. The
correction, including the fill fluid density effect and
the diaphragm stiffness effect, is added to the
intermediate output to provide a corrected output at
S40. The corrected output is processed and converted to
digital and analog output signals at S42. In one
embodiment, the diaphragm stiffness effect is corrected
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for in circuit 58 and only the fill fluid density effect
is corrected for in circuit 82.
Another embodiment of transmitter 11 in
providing a corrected output signal based on sensor
inputs and predetermined installation-specific
coefficients is shown in the flow chart of FIG. 9.
Signals representative of pressure and temperature are
converted to pcounts and tcounts at S10, and are
normalized at S12. The normalized signals are run
through a single polynomial equation, which is
preferably 5th order in pressure and 2nd order in
temperature, whose coefficients have been calculated and
stored in EEPROM 70 so as to correct for both the
components within the transmitter housing as well as the
remote diaphragm/capillary tube system, including the
height H dependent fill fluid density effect. The
corrected signal can be further subject to processing at
S16 and is preferably converted to digital and analog
output signals at S18, S20.
In the embodiments shown thus far, the
temperature sensor disposed in the transmitter housing
was used as an indicator of both the temperature of
components within the transmitter housing and the
temperature of the remote diaphragm/capillary tube
system. Such double utilization of the transmitter
housing temperature sensor promotes simplicity and
reliability. Further, the temperature at the
transmitter housing is often a good indicator of
temperature at least of the capillary tube and many
times of the remote diaphragm.
Where more accuracy is required, the
transmitter 11 of FIG. 10 employs a separate,
distributed temperature sensor to measure the average
temperature of the fill fluid contained in capillary
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tube 22. The separate temperature sensor includes wire
90 of sufficient length to follow capillary tube 22 from
transmitter housing 14 to diaphragm system 12 and back
again to housing 14. Wire 90 has an end-to-end
resistance, measured by circuit 29, which is indicative
of its lengthwise-averaged temperature. Wire 90 can
comprise any standard thermal couple material. A
protective layer 93 covers capillary tube 22 and wire
90. Preferably, protective layer 93 also electrically
insulates wire 90. In operation, the output from
circuit 29, rather than the conditioned output from
housing sensor 28, is provided to the correction circuit
82 of FIG. 7.
FIG. 11 shows transmitter housing 14 connected
to two remote diaphragm systems 12, 12B and suitable for
measuring differential pressure of process medium 16.
Capillary tubes 22A, 22B connect the respective remote
diaphragms to diaphragms 25 at transmitter housing 14.
The properties of both remote diaphragms 12A and 12B,
and the type and volume of fill fluid in the capillary
tubes 22A, 22B, and the elevations Hi and H2, are taken
into account in the calculation of the installation-
specific coefficients. If capillary tubes 12A, 12B are
substantially identical and filled with the same type of
fill fluid, then the elevational difference Hl-HZ can be
used to calculate the net fill fluid density effect.
The previously described embodiments of the
present invention have many advantages. Among these
include a significant improvement in transmitter
accuracy and performance over a temperature range.
Additionally, existing transmitters can be supplied with
installation-specific coefficients and correction
algorithms by recharacterizing the transmitter and thus
providing backwards compatibility.
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Although the present invention has been
described with reference to preferred embodiments,
workers skilled in the art will recognize that changes
may be made in form and detail without departing from
the spirit and scope of the invention.
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