Differential pressure transmitter with complimentary dual absolute pressure sensors

A process variable transmitter for measuring a pressure of a process fluid includes a process coupling having a first port configured to couple to a first process pressure and a second port configured to couple to a second process pressure. A differential pressure sensor is coupled to the first and second ports and provides an output related to a differential pressure between the first pressure and the second pressure. First and second pressure sensors couple to the respective first and second ports and provide outputs related to the first and second pressures. Transmitter circuitry is configured to provide a transmitter output based upon the output from the differential pressure sensor and/or the first and/or second pressure sensors. Additional functionality is provided by the transmitter using the sensed first and/or second pressures.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is related to co-pending application Ser. No. 12/834,393, filed on Jul. 12, 2010, and entitled TRANSMITTER OUTPUT WITH SCALABLE RANGEABILITY, the content of which is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to measurement of pressures of process fluids in industrial processes. More specifically, the present invention relates to measuring line pressure in a differential pressure transmitter.

Industrial processes are used in the manufacturing and transport of many types of materials. In such systems, it is often required to measure different types of pressure within the process. One type of pressure which is frequently measured is a differential pressure. This is the pressure difference between one point in the process and another point in the process. For example, the differential pressure across an orifice plate in a pipe containing a flow of process fluid is related to the flow rate of the fluid. Differential pressures can also be used, for example, to measure height of a process fluid in a tank or other container.

In industrial processes, the pressure sensors are typically contained in, or coupled to, a pressure transmitter which is located at a remote location and transmits pressure related information back to a centralized location such as a control room. The transmission is frequently over a process control loop. For example, a two-wire process control loop is often used in which two wires are used to carry both information as well as power to the transmitter. Wireless communication techniques may also be used.

Advancements in transmitter technology have increased the amount of information that can be produced by a transmitter. In particular, transmitters can be equipped with multiple sensors to measure multiple process variable inputs or a wider range of a single process variable. For example, transmitters can be provided with multiple pressure sensors as described in U.S. Pat. No. 5,495,769 to Braden et al., U.S. Pat. No. 6,047,244 to Rud, Jr. and U.S. Pat. No. 7,467,555 to Schulte et al., all of which are assigned to Rosemount Inc., Eden Prairie, Minn.

In many process installations, in addition to measuring a differential pressure, it is also desirable to measure an absolute or gauge pressure (also referred to as “line pressure”) of the process. This information can be used, for example, to provide more accurate flow measurements by including changes in density of the process fluid in the flow calculations. The additional pressure measurements can be made using a separate pressure sensor coupled to the process fluid.

SUMMARY OF THE INVENTION

A process variable transmitter for measuring a pressure of a process fluid includes a process coupling having a first port configured to couple to a first process pressure and a second port configured to couple to a second process pressure. A differential pressure sensor is coupled to the first and second ports and provides an output related to a differential pressure between the first pressure and the second pressure. At least a first pressure sensor couples to the first port and provides an output related to the first pressure. Transmitter circuitry is configured to provide a transmitter output based upon the output from the differential pressure. Additional functionality is provided by the transmitter using the sensed first pressure and second sensed pressure.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In one embodiment, the present invention provides an apparatus and method for providing additional functionality to a differential pressure measurement transmitter using dual line pressure sensors. More specifically, in one aspect, the present invention includes line pressure sensors which are coupled opposite sides of a differential pressure sensor for use in performing diagnostics or providing other functionality.

FIG. 1shows generally the environment of a process measurement system32.FIG. 1shows process piping30containing a fluid under pressure coupled to the process measurement system32for measuring a process pressure. The process measurement system32includes impulse piping34connected to the piping30. The impulse piping34is connected to a process pressure transmitter36. A primary element33, such as an orifice plate, venturi tube, flow nozzle, and so on, contacts the process fluid at a location in the process piping30between the pipes of the impulse piping34. The primary element33causes a pressure change in the fluid as it flows past the primary element33.

Transmitter36is a process measurement device that receives process pressures through the impulse piping34. The transmitter36senses a differential process pressure and converts it to a standardized transmission signal that is a function of the process pressure.

A process loop38provides both a power signal to the transmitter36from control room40and bi-directional communication, and can be constructed in accordance with a number of process communication protocols. In the illustrated example, the process loop38is a two-wire loop. The two-wire loop is used to transmit all power to and all communications to and from the transmitter36during normal operations with a 4-20 mA signal. A control room40includes a voltage power supply46that powers the transmitter36and a series resistance44. In another example configuration, loop38is a wireless connection in which data may be transmitted or received wirelessly either in a point-to-point configuration, a mesh network, or other configuration.

FIG. 2is a simplified block diagram of pressure transmitter36. Pressure transmitter36includes a sensor module52and an electronics board72coupled together through a databus66. Sensor module electronics60couples to differential pressure sensor56which received an applied differential pressure54. The data connection58couples sensor56to an analog to digital converter62. An optional temperature sensor63is also illustrated along with sensor module memory64. The electronics board72includes a microcomputer system74(or microprocessor), electronics memory module76, digital to analog signal conversion78and digital communication block80.

Also illustrated inFIG. 2are capillary or “fill” tubes93and94which are used to couple the differential pressure sensor56to the process fluid54. Isolation diaphragms90receive pressures from the process fluid54which is responsibly applied to a fill fluid carried in capillary tubes93and94. Through this fill fluid, the pressures of the industrial process are applied to the differential pressure sensor56.

In accordance with the illustrated embodiment, a line pressure sensors97and98couple to capillary tubes93and94, respectively, and are arranged to monitor the pressures in capillary tubes93and94. Line pressure sensors97and98couple to pressure measurement circuitry99. Circuitry99can comprise, for example, circuitry which is responsive to electrical parameters of the sensor97and99which change as functions of the applied link pressure. For example, pressure sensors97and98may operate in accordance with known techniques including pressure measurement techniques in which an electrical capacitance of the sensors97and98changes, an electrical resistance of the sensors97and98changes, a resonant frequency of the sensors changes, etc. One specific configuration is discussed below in more detail. Circuitry99can be stand alone circuitry or, in some configurations, may be embodied in other circuitry used to measure the differential pressure. For example, some or all of the components used to monitor the various sensors may be shared components.

Sensor board156and sensor56are mounted within housing152of sensor module52. Housing152of sensor module52connects to housing164of electronics module136through, for example, a threaded connection. Similarly, covers166and168are connected to housing164through threaded connections that form flame-quenching seals, as are known in the art, to prevent escape of flames from within housing164. Output interface170and transmitter circuitry172are mounted to housing164within transmitter electronics module136and form electronics board72shown inFIG. 2.

In the illustrated embodiment, sensor56is a capacitance-based differential pressure cell having a sensing diaphragm disposed between a pair of electrode plates. In one embodiment, sensor56is a metal-diaphragm type sensor as is described in U.S. Pat. No. 6,295,875 to Frick et al., which is assigned to Rosemount Inc., Eden Prairie, Minn. Sensor56is connected to base54with isolation tubes93and94, in which is disposed a hydraulic fill fluid. Isolation diaphragms90separate the fill fluid within isolation tubes93and94from pressures P1and P2of process fluid. Changes in pressures P1and P2of process fluid are sensed as differential pressure ΔP by sensor56through isolation diaphragms90within base54and the hydraulic fill fluid within isolation tubes93and94. However, the invention is not limited to this differential pressure measurement configuration.

In the described embodiment, line pressure sensors97and98are capacitance-based absolute pressure sensors. In one configuration, sensors97and98are capacitive pressure sensors as are described in U.S. Pat. No. 6,484,585 to Sittler et al. and the related series of applications, all of which are assigned to Rosemount Inc., Eden Prairie, Minn. Such sensors include technology that uses brittle sensing materials having high compression strength. For example, a brittle material such as sapphire. Sensors97and98sense line pressures P1and P2of the fill fluid in isolation tubes93and94, respectively. The pressures sensed by each of pressure sensors97and98can be compared to one another to produce a signal representative of the differential pressure ΔP that can be used in place of, or for comparison to, the differential pressure ΔP sensed by sensor56.

Sensors56,97and98together form a sensor system that includes a differential sensor apparatus having a single sensor device, and an absolute sensor apparatus having two sensor devices. Differential pressure sensor56and absolute pressure sensors97and98are in electronic communication with sensor board156. Sensor board156is in electronic communication with transmitter circuitry172through sensor cable176. Transmitter circuitry172is in electronic communication with output interface70through electronics cable178. Output interface170connects transmitter36to wiring of control loop38. In other embodiments of the invention, performance of the functions of transmitter circuitry172, sensor board156and output interface170are distributed and implement differently as desired.

Sensor56accurately measures small differential pressures. Further, sensors97and98accurately measure absolute pressures. Sensor56directly measures the difference between line pressures P1and P2. Sensors97and98directly measure pressures P1and P2, respectively. The difference between the outputs from sensors97and98can be used to determine large differential pressures, perform diagnostics, provide calibration to sensor56, etc. or provide other functionality. Sensors56,97and98can thus be used in a wide variety of scenarios to sense line pressures and differential pressures.

FIG. 4is a cross sectional view of sensor module52shown in greater detail. As illustrated inFIG. 4, tube93includes a bent portion202which couples to the differential pressure sensor56. Further, tube93includes an extension portion200which couples to line pressure sensor97. In a similar manner, tube94includes a bent portion206which connects to differential pressure sensor56and an extension portion204which couples to line pressure sensor98.

FIG. 5is a side cross sectional view of one example of line pressure sensor97. In the example ofFIG. 5, line pressure sensor97is formed of two sapphire substrates220and222which are bonded together and form a vacuum cavity224there between. Vacuum cavity224includes two capacitor plates (not shown) which couple to electrical connection leads226. Electrical connection leads226connect to circuitry in sensor board156. A braze band230is provided and is used for coupling the sensor97to the tube93.

FIG. 6is a cross sectional view of line pressure sensor97mounted in the extension portion200of tube93. The tube93can be formed of nickel and filled with a substantially incompressible fluid such as oil. The extension portion200of the tube93includes an opening formed therein which supports pressure sensor97. Pressure97is sealed to the extension portion200using, for example, to a braze232.

Referring back toFIG. 2, the sensor signals from pressure sensors56,97and98are received by circuitry in transmitter36. This circuitry includes, for example, circuitry in the sensor module52or on the electronics board72. For example, microprocessor circuitry74can process the line pressure signals to provide enhanced functionality to the transmitter36. Such enhanced functionality includes diagnostics, extended measurement range, redundant sensor measurements, calibration, calculation of additional process variables such as mass flow rate, etc.

The use of additional sensors can be used to extend the range over which the device is capable of sensing pressure. For example, the line pressure sensors97and98can be used to sense the differential pressure of the process fluid in instances when the pressure exceeds the upper range limit of pressure sensor56. Although such a configuration may reduce the accuracy of the pressure measurements, in some instances this tradeoff may be acceptable in order to provide the increase measurement range. This can be useful both for measuring flow rates as well as differential pressure.

The additional sensors97,98can be used to enable redundant differential pressure measurements for use in providing sensor diagnostics. For example, at pressures less than 250 inches H2O, the sensors97,98can be used to measure the differential pressure and provide a redundant differential pressure measurement. The differential pressure measured using line sensors97and98can be compared with differential pressure measured using differential pressure sensor56. The microprocessor74can use any differences between these two measurements to identify a failing sensor.

In one configuration, the additional absolute pressure sensors97and98are used to provide differential pressure measurements if the differential pressure sensor56has failed or is providing an inaccurate measurement. This configuration allows the transmitter36to operate in a limited (or “limp”) mode having a reduced accuracy until the failed equipment can be repair or replaced. If microprocessor system74detects that sensor56has failed, for example, by providing no signal output, the microprocessor74can calculate differential pressure based upon the outputs from sensors97and98. This calculated differential pressure can be transmitted to a remote location. The transmission may optionally include diagnostic information such as information which indicates that the transmitted process variable has a reduced accuracy because the transmitter is operating in a “limp” mode. This configuration allows the industrial process to continue operation, perhaps at a reduced capacity, until repairs can be implemented.

In another configuration, diagnostics are performed by microprocessor system74on the absolute pressure sensors97,98based upon the differential pressure measured by differential pressure sensor56. During normal operation, the pressure measured by one of the line pressure sensors97,98should be substantially equal to a difference between the pressure measured by the other line pressure sensor97,98and the differential pressure measured by differential pressure sensor56. Similarly, the additional line sensors97,98can be used to identify a plugging of impulse piping or a failing primary element.

In the embodiments described, the use of two different types of sensors can be used to provide sensors having different frequency responses. For example, the metal diaphragm used in differential pressure sensor56described herein has an effective low pass filter that tends to filter out higher frequency process noise. On the other hand, the sapphire based line pressure sensors97,98described herein have a higher frequency response and are capable of providing measurements with a faster response time. This high frequency response can be used to measure noise signals on either side of the differential pressure sensor56. This can be used to provide enhanced process statistics or diagnostics, such as identifying a plugged line or other failing component in the process. The line pressure signals can also be used to calibrate the differential pressure sensor56as well as be used to compensate the differential pressure measurement for any changes due to high line pressure. For example, the configuration of pressure sensors97and98described above offers relatively stable measurements over an extended time period. As the sensors97and98are relatively stable, their measurements can be used to calibrate drift in the measurements provided by pressure sensor56. Thus, calibration can be performed by microprocessor74. In another example, the additional pressure measurements provided by line pressure sensors97and98can be used to provide accurate span line pressure compensation by microprocessor74to differential pressure sensor56pressure measurements. In one configuration, the use of two absolute pressure sensor measurements can be used to more accurately compensate for variations of the differential pressure measurements. Compensation algorithms can be implemented in microprocessor74.

In one configuration, the line pressure sensors97and98have an upper range limit of about 5,000 psi. The sensors97,98can be placed anywhere in the device, however the location described above in association with the fill tubes93,94offer a convenient position. As the line pressure sensors97,98described herein operate based upon capacitance variation, various operations and components of the measurement system may be shared with the differential pressure sensor56, such as temperature sensor63shown inFIG. 2, which also operates based upon capacitance variations. In one embodiment, a temperature sensor is provided within sensors97and/or98. This can be used to compensate for temperature variations in their measurements. Further, a reference capacitor (not shown) can be implemented in sensor97and/or98to further enhance accuracy of absolute pressure measurements. The dual absolute line pressure sensors96,98may also be used in other process variable measurement configurations such as an inline unit in which the sensors can be positioned in parallel with a strain gauge.

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. The line pressures sensors97and98can be coupled to P1and P2in any appropriate manner and are not limited to the tube configuration shown herein. The various functions described above can be implemented using any appropriate circuitry and the implementation of such functionality can be shared between components and implemented using the same or separate circuitry. As used herein “transmitter circuitry” refers to any circuitry within transmitter36. As used herein, “enhance functionality” includes system diagnostics, component, diagnostics, process diagnostics, extended operation range, calibration of components, statistical process measurements and limited device operation in the event of a component failure.