JP3047104U - Pressure transmitter with fill fluid loss detector - Google Patents

Abstract

(57) [Problem] To provide a pressure transmitter having a highly reliable filled fluid loss detector. SOLUTION: A pressure transmitter (50) is provided with an isolation diaphragm (1) by a filling fluid (22H / 22L) such as oil.
8L / 18H) with a pressure sensor (24). Means (52L / 52H, 56) provide for measuring the position of the isolation diaphragm (18L / 18H) and comparing the measured position to a target position to indicate the amount of leakage of the fill fluid (22H / 22L). Have been. Non-contact devices such as capacitive or ultrasonic technology, as well as contact devices such as switches, use isolation diaphragms (18L / 1
8H). The filling fluid loss device can be incorporated into a two-wire transmitter, which can be made to send an alarm signal to the control unit when filling fluid is detected.

Description

[Detailed description of the invention]

[0001]

[Technical field to which the invention belongs]

 This invention relates to measurement transmitters used in the process control industry, and more particularly, to transmitters having a pressure sensing cell filled with a substantially incompressible fluid such as oil.

[0002]

[Prior art]

 Known process control industry pressure transmitters receive process fluids and provide an output representative of the pressure of such fluids. In transmitters of interest to the present invention, the process fluid pressure acts on the transmitter's flexible isolation diaphragm, pushing it against the backplate. The space between the isolation diaphragm and the back plate, and the passage connecting the space to the pressure sensor, is filled with a substantially incompressible fluid such as silicone oil. This fluid that fills the space is hereafter referred to as "fill fluid". The isolation diaphragm transmits process fluid pressure to the pressure sensor via the fill fluid. The pressure sensor then outputs a signal representative of the fill fluid pressure, followed by a signal representative of the process fluid pressure.

[0003]

[Problems to be solved by the invention]

 However, in some instances, corrosion or physical destruction of the isolation diaphragm or the seal in the transmitter causes some of the fill fluid to leak from the enclosed structure. This loss of fill fluid affects static accuracy and, depending on the amount of fluid lost and operating conditions, affects the response time of the transmitter. This results in static and dynamic output errors individually.

[0004] For many transmitters, the loss of fill fluid can at least be clearly classified into two stages: an early stage and a development stage. In the early stages, where the loss of fill fluid is relatively small, the transmitter is still essentially functioning well. Although small displacements or slow drifts in the transmitter output can be observed, the transmitter operates normally as per its operating specifications. The physical mechanism that causes the drift is the change in fill fluid pressure caused by the outflow of the fill fluid. The changed pressure of the filling fluid causes a displacement in the output of the pressure sensor. Some industries now monitor “zero displacement” (drift) installed on a regular basis to detect loss of fill fluid at an early stage before operation is substantially affected. I do. In practice, this means that the transmitter must be decoupled from processing, calibrated, and a "trend chart" created.

[0005] The development phase of fluid loss can be characterized by the loss of so much fluid that abnormal behavior appears at the output of the transmitter. An example of abnormal operation is a transmitter whose pressure sensor is of the following type: each isolation diaphragm injects or receives an amount of fill fluid to or from the pressure sensor when the differential pressure of the process fluid fluctuates. Get up on the transmitter that is the type of take. When such a transmitter is subjected to rapid changes in the pressure differential of the process fluid, the fill fluid under the isolator is forced into a hole in the backplate as it flows to the pressure sensor. It is. Depending on the amount of fill fluid lost, the rate of pressure increase, and the magnitude of the pressure increase, the fill fluid drawn into the hole will act to push the isolation diaphragm to substantially seal the hole. You. This temporarily traps the fill fluid between the isolation diaphragm and the backplate, preventing more fill fluid from flowing to the pressure sensor. As a result, the transmitter pressure follows the fast process fluid pressure change only for a short period of time, and then reaches its expected value when the fill fluid moves slowly past the "seal" to the pressure sensor. Experience a very slow change towards it. Interestingly, a rapid pressure change in the opposite direction (which causes the fill fluid to be expelled rather than sucked into a hole in the backplate) does not indicate this anomaly. The stage of development of the fill fluid also means that even when a great deal of fill fluid is lost and the isolation diaphragm is pressed against the backplate over the entire surface area of the isolator, the transmitter must be able to fill the pressure sensor with the fill fluid. Due to lack, the expected output for a given applied pressure is not reached.

[0006] Although various devices and methods have been proposed for detecting the loss of fill fluid, it is preferable to conveniently provide for the initial stages of fluid loss before the operation of the transmitter is substantially affected. It is necessary to create a method and device that can reliably detect fluid loss.

[0007]

[Means for Solving the Problems]

 According to one feature of the invention, a transmitter having a pressure sensor coupled to an isolation diaphragm by a fill fluid monitors the position of the isolation diaphragm and thereby monitors the fill fluid for leakage. Includes a position sensor located nearby. The position of the isolator is an indicator of the loss of fill fluid. This is because, at the applied pressure and ambient temperature, the loss of the fill fluid causes the relative position of the isolation diaphragm to change. In a preferred embodiment, the position sensor includes a capacitive or ultrasonic transducer characterized as a "non-contact" position sensor, which does not require contacting the isolation diaphragm for accurate position measurement. In another embodiment, the position sensor includes a contact type sensor, such as a switch, which can have an isolation diaphragm as one of the contact points.

[0008] According to another feature of the invention, the transmitter has a pressure sensor coupled to the isolation diaphragm by a fill fluid. The transmitter also includes a device for monitoring the loss of the fill fluid. Advantageously, the pressure sensor and the fluid monitoring device can use substantially the same measurement technique, for example, a capacity-based technique. The use of the same measurement technique allows for the efficient use of duplicate measurement circuits.

In accordance with yet another aspect of the invention, a method of monitoring a fill fluid loss in a two-wire transmitter includes automatically or periodically measuring a parameter indicative of the fill fluid loss, wherein the parameter is measured. Includes a step to provide an alarm signal when a preset level is exceeded.

[0010]

1, a conventional pressure transmitter 10 has a transmitter housing including a base 12, a partition member 14, and covers 16 screwed at both ends thereof. ing . External piping, not shown, provides two processing fluid inlets, which are in contact with the isolation diaphragms 18L and 18H. The processing fluid supply ports apply pressures PL and PH to the diaphragms 18L and 18H, respectively. A first portion of the fill fluid 22L transmits pressure PL to one end of the pressure sensor 24, and a second portion of the fill fluid 22H transmits pressure PH to the other end of the pressure sensor 24. The fill fluids 22L, 22H fill the spaces 28L, 28H between the isolation diaphragms 18L, 18H and the back plate or stop surfaces 32L, 32H, respectively. As their names suggest, the isolation diaphragms 18L, 18H isolate the pressure sensor 24 from direct contact with the process fluid, but the pressure of the process fluid is transmitted to the sensor via the fill fluid. Is used to make it so. The pressure sensor 24 comprises a capacitive differential pressure cell whose capacity is a function of the differential pressure ΔP = PH−PL. For the purposes of the present invention, other known pressure sensors, such as strain gauges or optical pressure sensors, which can be fluidly coupled to the isolation diaphragm, can be used with the present invention.

A lead 36 connects the pressure sensor 24 to a backup circuit board 38, and the backup circuit board 38 is further connected to a transmitter circuit 42 by a wire 40. An external DC power supply (see FIG. 8) powers circuit 42 via twisted pair 44. The twisted wire pair 44 enters the transmitter 10 through a port 46. The circuit 42 regulates the DC current flowing through the twisted pair 44 4 as a function of the measured capacity between 4 and 20 mA and transmits the processing variable ΔP. The circuit 42 can also be transmitted digitally via a twisted pair 44 to the control unit by a known digital protocol, for example a protocol such as the HART protocol. Transmitter 10 also includes a platinum resistance thermometer (PRT) 48 located on transmitter base 12. The PRT measures the temperature of the pressure sensor 24 and the isolation diaphragms 18L, 18H. Of course, other temperature sensors, such as thermocouples, can be used in the present invention. Circuit 38 receives the output from PRT 48 and circuit 42 uses the output to correct for temperature-induced errors in the capacitance measurement of transmitter output ΔP.

The transmitter 50 of FIG. 2 is the same as the transmitter of FIG. 1 except that it includes means for measuring the relative position of the isolation diaphragms 18L, 18H. Ultrasonic transducers 52L, 52H rigidly adhered to the inner surface of the base 12 of the transmitter emit ultrasonic or vibrational (preferably 1-50 MHz) pulses into the base 12 of the transmitter. This oscillation causes a first reflection at the base / fill fluid interface of the transmitter (ie, the backplate) and a second reflection at the fill fluid / isolation diaphragm interface. The ultrasonic transducer detects the reflections and transmits them separately to the drive / detection circuit 56. The circuit 56 measures the time interval between the reflections, which represents the distance between each isolation diaphragm and the respective backplate. In order to increase the accuracy of the measurement, circuit 56 or one of the other transmitter circuits is based on the assumption that the speed of sound in the fill fluid and the speed of sound in the surrounding transmission mechanism are usually temperature dependent. The measured time interval is corrected by the measured temperature.

An ultrasonic transducer that can be used in the present invention includes Model V208-RM provided by Panametrics Inc. (221 Crescent Street, Waltham, Mass., USA). Piezo film sold by AMP (Valley Forge, PA, USA) can be used in the present invention.

Preferably, the circuit 42 'of the transmitter 50 executes a diagnostic routine to check for leaks of fill fluid at regular intervals controlled by a clock in the circuit 42'. First, the circuit 42 'measures the position or height of the isolation diaphragms 18L, 18H. These two values are stored in memory 58. Circuit 42 'also stores in memory 58 the current value of the temperature measured by PRT 48 and the pressure .DELTA.P measured by pressure sensor 24. During manufacture of the transmitter, a look-up table or set of coefficients is programmed into a non-volatile portion of memory 58. The table or coefficients produce the expected position values for the isolation diaphragm 18L and the isolation diaphragm 18H for a given set of pressure ΔP and temperature values. Temperature is used to compensate for the thermal expansion effect of the fill fluid. The table or coefficient is calculated and stored in memory shortly after the fill fluid chamber is sealed. Then, during the calibration process, it is calculated after measuring the height of the isolator for various applied pressures and temperatures. Then, during operation, the circuit 42 'calculates the expected value of the isolation diaphragm position for the current pressure .DELTA.P and temperature values by calling up the table or by the coefficients.

The circuit 42 'subtracts or compares the expected value with the accumulated value of the measured location of the isolation diaphragm. If the discrepancy for one of the isolation diaphragms is greater than a preset threshold THRESH stored in memory 58, circuit 42 'is driven by a digital signal or by driving the DC output to an alarm level on the order of 28mA. An alert signal can be sent to the control unit. THRESH indicates the maximum allowable displacement of the isolation diaphragm caused by losses in the fill fluid. The maximum displacement corresponds to the maximum amount of fill fluid loss that can be tolerated. THRESH can be set to a value low enough to detect the initial fill fluid loss described above.

The circuit 42 'can be programmed to perform the above-described diagnostic process every N pressure measurements. Where N is a programmable integer value. If the circuit 42 'reads the pressure at a rate of 1 Hz, by setting the N to 2.6 × 10 ^6, would to be able to test approximately once fluid loss in 1 month. The transmitter automatically tests for fluid loss and automatically alerts the control unit if a problem occurs, reducing the need for maintenance operators to periodically check the transmitter. FIG. 3 illustrates the improved operation of the fill fluid detection process described above.

As an adjunct diagnostic, the circuit 42 ′ can accumulate in memory a discrepancy between the measured diaphragm position and the expected diaphragm position. Then, in the steady state, the tendency for such a mismatch over a long period of time can predict the time at which the transmitter will fail, ie, Time To Failure (TTF). For example, the circuit 42 'can calculate the difference between the two most recently calculated mismatches for each isolation diaphragm. If the most recent mismatch (Di) is greater than the previous mismatch (Di-1), but both are less than the preset threshold THRESH, (in units of the time interval between successful fluid loss tests) The (measured) TTF can be calculated immediately and transmitted to the control unit.

TTF = (THRESH-Di) / (Di-Di-1) The circuit 42 'may, of course, use more sophisticated known data manipulation techniques, such as fuzzy logic, to determine the tendency to predict TTF. Alternatively, techniques such as the least squares method of previously accumulated discrepancy values can be used. If desired, the calculated TTF is stored in the transmitter's memory and can be transmitted to the control unit remote from the transmitter only after interrogation by the control unit.

The unique effect of using ultrasonic technology to measure the position of the isolation diaphragm is apparent from a comparison of FIGS. That is, this technique can be applied to existing transmitters with only minor modifications to the hardware and without physical changes in the structure of the container containing the fill fluid or in the pressure sensor. it can.

FIG. 4 shows a transmitter 60 similar to that of FIG. However, the volumetric technique for measuring the position of the isolation diaphragms 18L, 18H can be substituted for the ultrasonic technique. The plugs 62L, 62H are hermetically supported in holes in the base 12 of the transmitter as shown. Each plug has a small conductive patch or electrode 64L, 64H on the side facing the respective isolation diaphragm, in contact with the fill fluid. Preferably, the electrode surface conforms to the surrounding backplate surface. The plug 62 is configured such that the electrodes 64L, 64H are electrically insulated from the transmitter base and the isolation diaphragm welded thereto. As the isolation diaphragm moves away from or near the plug opposite it, the capacitance between the isolation diaphragm and the electrodes increases or decreases. The electrodes are preferably sized so that the capacity range of each electrode / isolation diaphragm pair is within about one-tenth of the capacity range of pressure sensor 24. In this way, the same circuit used to measure the capacitance of the pressure sensor can be used to measure the capacitance of the electrode / isolation diaphragm pair. In the preferred embodiment, transmitter 60 includes a switch 66 controlled by circuit 42 '. This switch intermittently couples the wire 68 to the capacitance measurement circuit 38. At all other times, switch 66 couples pressure sensor lead 69 to circuit 38.

If desired, switch 66 can be omitted and other circuitry can be added to measure the capacitance of the electrode / isolation diaphragm pair. Such a device can be used where the pressure sensor 24 uses a sensing technology other than capacitance, such as strain gauge technology. The reduced number of circuit elements required, reduced cost, and improved reliability can be obtained from the use of switches 66, the use of the same sensing technology for pressure measurement and measurement of isolation diaphragm position. it can.

Except for the technique of measuring the isolation diaphragm position, the transmitter 60 of FIG. 4 is identical in all other respects to the transmitter 50 of FIG. In particular, alarms, predicted capacity, and other features have been used as is.

FIG. 5 shows another embodiment of the pressure sensor and the isolation diaphragm device modified according to the present invention. The pressure sensing location of the pressure cell 70 is welded around one half 74 of the metal cell and the other half 74 and a single sensing diaphragm 72 sandwiched between substantially identical sensing electrodes 76H, 76L. Contains. The detection electrodes 76H and 76L are formed on the concave surface of the glass piece 78. Conductors 80H, 80L extending rearward to fill the inner chamber on one side of the detection diaphragm with a fill fluid connect the detection electrodes 76H, 76L to a capacitance measurement circuit not shown in FIG. ing.

The processing fluid in contact with the isolation diaphragms 82H, 82L exerts pressures PH and PL on the isolation diaphragms, as shown, which pressures the isolation diaphragms 82H, 82L and the stop surfaces 84H, The fluid is transmitted to the detection diaphragm 72 through the space between the detection diaphragm 72 and the space between the detection diaphragm 72 and the detection electrodes 76H and 76L and the two filled fluids filling the connection passages 85H and 85L. According to the invention, the pressure cell 70 includes conductors 86H, 86L provided within a piece of glass 78 that is electrically insulating. Each conductor 86H, 86L terminates at a back plate 84H, 84L, respectively. The capacitance between each isolation diaphragm and its adjacent leads 86H, 86L provides a measure of the position of each isolation diaphragm. If desired, electrodes similar to the electrodes 64L, 64H can be attached to the backplate of the pressure cell 70 to increase the capacitance between the isolation diaphragm and the adjacent conductors 86H, 86L. it can.

The embodiment of FIG. 5 operates in a manner similar to that of FIG. The advantages described in connection with the transmitter of FIG. 4 are equally valid in the embodiment of FIG.

The pressure cell 70 of FIG. 5 can be easily deformed to measure the isolation diaphragm position ultrasonically, rather than capacitively. An ultrasonic transducer may be fixed at the end of each lead 86H, 86L and positioned to launch an ultrasonic pulse through the fill fluid. The time taken for the wave reflected from the surface of the isolation diaphragm to reach the transducer indicates the position of the isolation diaphragm.

FIGS. 6 and 7 show an embodiment of the present invention that uses a contact switch different from the non-contact technology described above. 6 and 7, half of the basic pressure cell described in FIG. 5 is shown for simplicity. Here, the symbols “H” and “L” have been deleted from the reference symbols. In the embodiments of both figures, means are provided so that each isolation diaphragm makes electrical contact with each opposing conductor 86 if a sufficient volume of the fill fluid leaks out of the fill fluid reservoir. . Lead 86 is coupled to a circuit that provides an alarm indication to the control unit when the contact is made. In FIG. 6, the spring contact 88 is welded to the end of the conductor 86. Contact is made between the top of the spring contact 88 and the underside of the isolation diaphragm. In FIG. 7, a recess 89 is formed in the isolation diaphragm 82 by a fast moving blunt tool with no cutting edge. Here, contact is made between the depression 89 and the end of the conductor 86.

[0028] The transmitter usually operates such that at the maximum estimated pressure, the isolation diaphragm exposed to the high pressure side moves about half way from its equilibrium position (ΔP = 0) towards the backplate. , Designed. For a transmitter with the isolating diaphragm equilibrium position (center) 0.010 inches (0.254 mm) from the backplate, the isolating diaphragm will be approximately 0.005 inches (0,5) at maximum applied pressure and minimum estimated temperature. .127 mm). Losses in the fill fluid act to move the isolation diaphragm closer to the backplate under the same conditions. Thus, setting the height of the spring contact 88 or the depth of the recess 89 to about 0.005 inch (0.127 mm) or slightly less will result in a small loss of fill fluid at high pressure (initial It enables the detection of step loss) and the detection of a larger loss of fluid at low pressure.

FIGS. 8 and 9 show another standard pressure transmitter circuit 90 that receives energy from an external power supply 92 and is a function of the detected differential pressure. As a result, the current i flowing through the wire pair 44 is adjusted, and is modified by adding the contact type switch elements 94L and 94H as shown in FIGS. One switch is used for each isolation diaphragm. Reference numeral 96 indicates a pressure detector of the pressure cell shown in the vertically long inset.

When there is no loss in the filled fluid in the transmitter circuit connected as shown in FIG. 8, the transmitter output current i as a function of the fluid differential pressure ΔP is represented by the curve 98 a in FIG. Indicated by Curve 98a is substantially linear, varying from 4 mA in the lowest range range ("LRL") to 20 mA in the highest range range ("URL"). This operation is the same as the operation of the conventional transmitter having no fluid loss detecting means. However, if some fill fluid leaks from one side of the pressure cell, its operation will jump to that of curve 98b. For small pressures, the only change is the offset due to the reduced pressure created on the sensing diaphragm, depending on the amount of fill fluid that has leaked. However, for pressures large enough to cause the switching elements to approach, the transmitter circuit will force the current to be about 2 mA off-scale. This "hard" failure occurs before the response time of the transmitter falls into excessive fill fluid loss and immediately informs the operator that a problem has occurred. If more fill fluid is lost, curve 98c shows that at low pressure the DC offset increases and off-scale output levels are achieved at low pressure. Of course, if a sufficient amount of fill fluid leaks, the transmitter output will remain at 2 mA off-scale over the entire pressure range. Curves 98b and 98c correspond to the case where fill fluid is lost from the high pressure side "H" of the pressure cell. Conversely, if fluid leaks from the low side "L", the DC offset from curve 98a will be on the opposite side, although closing the switch element will drive its output to an off-scale value of 2 mA.

For convenience, “H” and “L” have been removed from FIGS. 11 and 12. The purpose of the exploded perspective view of FIG. 11 of a known pressure transmitter is to illustrate the known use of the side flange 100 with the pressure cell 102. The pressure cell 102 is the same as the pressure cell of FIG. 4 except that the cell 102 has no means for detecting fill fluid loss. The side flange 100 serves to carry process fluid from the threaded port 104 to the isolation diaphragm of the pressure cell 102.

FIG. 12 shows two devices for measuring the position of the isolation diaphragm from a location that is on the processing side of the isolation diaphragm rather than the fill fluid side. The pressure cell 102 is shown in outline, with the isolation diaphragm 82 and the back plate 84 therein depicted only in dashed lines for simplicity. The side flange 100 is likewise shown in simplified form in a cross-sectional view, omitting the structure or details of the small parts. In a first embodiment, the operator couples the ultrasonic probe 106 to the outer surface of the side flange 100 to determine the position of the isolation diaphragm in a manner similar to that described with respect to FIG. During the test, the inner chamber 108 of the flange 100 should be filled with a processing fluid to allow transmission of the ultrasonic waves 110 across the chamber 108. The drive / detection / readout device 112 controls the signal received outside the predetermined time window so that reflections from surfaces such as the surface 114 of the flange 100 do not interfere with the measurement of the isolation diaphragm position with respect to the backplate. It is made in a shape that rejects. By knowing the output of the pressure transmitter and preferably the local temperature, the operator can compare the position of the target isolation diaphragm from a simple chart or look-up table with the measured value. Mismatch indicates loss of fill fluid. This first embodiment can be compatible with existing transmitters without changing the configuration or circuit of the transmitter.

In the second embodiment shown in FIG. 12, only the side flange of the transmitter needs to be modified. A plug 116 with a piezoelectric tip 118 embedded therein is provided in the wall of each flange. Tip 118 generates and detects ultrasonic waves. When the operator couples the plug 116 to the drive / detection / readout device 112 using a standard connector 119, he measures the position of the isolation diaphragm in the same manner as in the first embodiment described in the previous paragraph. And thereby monitor for leakage of the fill fluid.

In a third embodiment, not shown, a rod-shaped probe can be inserted into the chamber 108. The probe coupled to the drive / detection / readout device 112 includes a piezo electric chip 118 and can launch and detect ultrasonic vibrations as described in the previous two embodiments.

Although the present invention has been described with reference to preferred embodiments, it should be recognized that a skilled technician could vary without departing from the spirit or scope of the invention. For example, the invention can be applied not only to pressure transmitters, but also to fluid transmitters and other transmitters that utilize a pressure sensor combined with a fill fluid. In addition, the transmitter can have one, two, or more than one isolation diaphragm. The invention is also applicable to transmitters where the isolation diaphragm or diaphragm is substantially stationary with respect to the backplate as a function of the applied pressure. The skilled technician will recognize that inductive, magnetic, optical and other known position measurement techniques can be used in place of a volumetric or ultrasonic device to determine the position of the isolation diaphragm.

[Brief description of the drawings]

FIG. 1 illustrates a conventional ART pressure transmitter, partially shown in cross-section and partially in block diagram.

FIG. 2 is a diagram partially illustrating a cross-sectional view and partially illustrating a block diagram according to an embodiment of the present invention.

FIG. 3 is a flowchart of a method for detecting a loss of a filling fluid according to the present invention.

FIG. 4 is a view similar to FIGS. 1 and 2 of another embodiment of the present invention.

FIG. 5 is a sectional perspective view of another embodiment of the present invention.

FIG. 6 is a partial cross-sectional view of another embodiment of the present invention.

FIG. 7 is a partial cross-sectional view of another embodiment of the present invention.

FIG. 8 is a circuit diagram of a conventional capacitance measuring circuit modified for use in the embodiments of FIGS. 6 and 7;

FIG. 9 is a circuit diagram illustrating the operation of the output of the circuit of FIG. 8 as a function of process fluid pressure.

FIG. 10 is a diagram showing a relationship between a fluid differential pressure ΔP and a transmitter output current i.

FIG. 11 is an exploded perspective view of a conventional ART pressure transmitter using a flange.

FIG. 12 is a schematic sectional view and a block diagram of still another embodiment of the present invention.

[Explanation of symbols]

10: pressure transmitter, 12: base, 14: partition member,
16 ... cover, 18L, 18H ... isolation diaphragm, 2
2L, 22H: filled fluid, 24: pressure sensor, 36: lead wire, 38: spare circuit board, 42: transmitter circuit, 48
... Platinum resistance thermometer, 52L, 52H ... Ultrasonic transducer, 66 ... Switch.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Petrich, William E. United States 55422 Perry Avenue North, Goldbury, Minnesota 2134 (72) Inventor Lawy, Bennett El. United States 55441 Union Terrace, Plymouth, Minnesota Rain North 211 (72) Inventor Cruz, Terrance F. USA 55347 Stanley Trail 8722, Eden Prairie, Minnesota

Claims (9)

[Utility model registration claims] 1. A pressure sensor fluidly coupled to at least one isolation diaphragm by a fill fluid and utilizing a first measurement technique, a measurement circuit, and a monitor for loss of the fill fluid. A pressure transmitter, comprising: an apparatus that performs measurement using the second measurement technique, wherein the first and second measurement techniques are substantially the same. 2. The pressure transmitter of claim 1 including a switch for selectively coupling said measurement circuit to said pressure sensor and said device. 3. A switch for monitoring a loss of a fill fluid coupled to the switch, the switch selecting the measurement circuit between the pressure sensor, the device, and the second device. 3. The pressure transmitter according to claim 2, wherein the pressure transmitter is combined. 4. The apparatus for monitoring the loss of a fill fluid comprises a non-contact position sensor having electrodes whose capacity with the isolation diaphragm varies as a function of the position of the isolation diaphragm. The pressure transmitter according to any one of claims 1 to 3, characterized in that: 5. The method according to claim 1, wherein the device for monitoring the loss of the filling fluid comprises a non-contact type position sensor having an ultrasonic transducer. The pressure transmitter as described. 6. The device for monitoring the loss of a fill fluid comprises a contact switch configured to be contacted by the isolation diaphragm if a portion of the fluid is lost. The pressure transmitter according to any one of claims 1 to 3, wherein the pressure transmitter is provided. 7. A transmitter having a pressure sensor coupled to an isolation diaphragm by a fluid, the transmitter including circuitry for detecting loss of fluid, means for monitoring the position of the isolation diaphragm, and the isolation. Means for measuring a temperature representative of the temperature of the diaphragm; means for determining an expected position of the isolation diaphragm as a function of the measured temperature; and means for comparing the monitored position of the isolation diaphragm to the expected position. And a pressure transmitter. 8. The pressure transmitter of claim 7, wherein said means for monitoring includes means for monitoring a capacitance between an electrode and said isolation diaphragm. 9. The system of claim 7, further comprising means for measuring the output of the pressure sensor, wherein the means for determining is responsive to the measured output of the pressure sensor.
Or 8 pressure transmitters.