High output expansion range type strain gauge pressure sensor

Abstract

A pressure sensor includes a first wafer having a first deflectable diaphragm and a second deflectable diaphragm. The first deflectable diaphragm has a first relationship between deflection and applied pressure, and the second deflectable diaphragm has a second relationship between deflection and applied pressure. The first relationship and the second relationship are different from each other. The second wafer is attached to the first wafer. At least one tensile strain gauge is coupled to the first deflectable diaphragm and at least one compressive strain gauge is coupled to the second deflectable diaphragm. An overvoltage feature is mounted relative to one of the first wafer and the second wafer. The overpressure feature is configured to contact the other of the first wafer and the second wafer during an overpressure condition exceeding a maximum measured pressure of the first deflectable diaphragm and a maximum measured pressure of the second deflectable diaphragm.

Description

Technical Field

[0001] This invention relates to a pressure sensor, particularly a high-output extended-range strain gauge pressure sensor. Background Technology

[0002] In some process control system devices, pressure transmitters are used to monitor the pressure of process fluids in conduits or tanks. A pressure transmitter includes circuitry that measures or otherwise acquires an electrical indication from a pressure sensor hydraulically coupled to a remote location of the pressure being monitored. The strength of the pressure sensor signal represents the pressure of the process fluid.

[0003] In many pressure sensors, a flexible diaphragm moves relative to its base in response to pressure applied to the top of the diaphragm. The diaphragm typically includes one or more electrical structures, such as electrodes or traces, that have electrical properties, such as resistance or capacitance, that change with the deflection of the sensing diaphragm. Diaphragms that provide repeatable monotonic movement in response to applied pressure are preferred. As a result, crystalline diaphragms, such as those made of crystalline silicon, have been widely used because crystalline silicon provides monotonic movement in response to applied pressure and is generally free of hysteresis.

[0004] A strain gauge pressure sensor is a specific type of pressure sensor that includes one or more conductive elements located on a deformable structure of the pressure sensor. When the deformable structure deforms in response to applied pressure, the resistance of the conductive elements changes. Therefore, measuring the resistance of the strain gauge pressure sensor provides an indication of the applied pressure.

[0005] Pressure sensors are often specified to have an operating range for the pressure at which the sensor will be exposed. End users often specify the range they need when ordering pressure sensors. While offering pressure sensors with extended operating ranges simplifies ordering and inventory requirements, it can present challenges from a sensor design perspective. Summary of the Invention

[0006] A pressure sensor includes a first wafer having a first deflectable diaphragm and a second deflectable diaphragm. The first deflectable diaphragm has a first relationship between applied pressure and deflection of the first deflectable diaphragm, and the second deflectable diaphragm has a second relationship between applied pressure and deflection of the second deflectable diaphragm. The first and second relationships are different from each other. A second wafer is attached to the first wafer. At least one tensile strain gauge is coupled to the first deflectable diaphragm, and at least one compressive strain gauge is coupled to the second deflectable diaphragm. An overpressure feature is mounted relative to one of the first and second wafers. The overpressure feature is configured to contact the other of the first and second wafers during an overpressure condition exceeding the maximum measuring pressure of the first and second deflectable diaphragms. A pressure transmitter using a pressure sensor is also disclosed.

[0007] This "Summary" is provided to introduce a selection of concepts in a simplified form, which will be further described in the "Detailed Description" section below. This "Summary" is not intended to identify key or essential features of the claimed subject matter, nor is it intended to help determine the scope of the claimed subject matter. The claimed subject matter is not limited to embodiments that address any or all the shortcomings pointed out in the "Background Art". Attached Figure Description

[0008] Figure 1 This is a schematic diagram showing various tensile and compressive strain gauges on a square deflectable diaphragm of a silicon pressure sensor.

[0009] Figure 2 This is a schematic cross-sectional view of a pressure sensor with overpressure capability.

[0010] Figure 3 The diagram is a finite element analysis (FEA) diagram that compares existing narrow-range pressure sensors with pressure sensors that incorporate overpressure protection and extend beyond the 4KSI range limit.

[0011] Figure 4 This is a schematic cross-sectional view of a high-output extended-range pressure sensor according to an embodiment of the present invention.

[0012] Figure 5 This is a bottom plan view of the device wafer of a pressure sensor according to an embodiment of the present invention.

[0013] Figure 6 It is a FEA diagram that compares the strain at the strain gauge of a pressure sensor with and without a rectangular section according to the embodiments described herein.

[0014] Figure 7 This is a strain distribution diagram of a pressure sensor according to an embodiment of the present invention.

[0015] Figure 8 This is a schematic diagram of the maximum principal stress of a pressure sensor according to an embodiment described herein.

[0016] Figure 9 A schematic diagram of the maximum principal stress concentration at the corner of the overpressure boss of the pressure sensor according to an embodiment of the present invention.

[0017] Figure 10A It is a graph of the measurement factor with respect to pressure, comparing the response of a known pressure sensor with the response of a pressure sensor according to an embodiment of the present invention.

[0018] Figure 10B It is an enlarged portion of the graph of the measurement factor versus pressure, comparing the response of a known pressure sensor with the response of a pressure sensor according to an embodiment of the present invention.

[0019] Figure 11 This is a graph showing the sensor sensitivity versus pressure, comparing the sensitivity of known pressure sensors with that of a pressure sensor according to an embodiment of the present invention.

[0020] Figure 12A and Figure 12B These are, respectively, a finite element diagram of the top membrane of the rectangular diaphragm according to an embodiment of the present invention and a graph of the maximum principal stress versus the applied pressure during an overpressure event.

[0021] Figure 13A and Figure 13B The figures are a finite element diagram of the unsupported bottom web of the rectangular diaphragm according to an embodiment of the present invention during an overpressure event, and a graph of the maximum principal stress versus the applied pressure.

[0022] Figure 14 This is a bottom plan view of a device wafer of a strain gauge-based pressure sensor according to another embodiment of the present invention.

[0023] Figure 15 This is a block diagram of a strain gauge-based pressure sensing system according to an embodiment of the present invention.

[0024] Figure 16 This is a schematic diagram of a pressure sensing system that is particularly useful for the embodiments described herein. Detailed Implementation

[0025] The embodiments described herein typically provide multi-range inline pressure sensors. As used herein, an inline pressure sensor is a device that is directly connected or otherwise coupled to a process. In some embodiments, a pressure sensor is provided that can serve multiple pressure ranges previously served by multiple existing sensors. In some embodiments, the pressure sensor includes overpressure protection that engages when the pressure exceeds the entire extended measurement range. Additionally, some embodiments described herein include a rectangular stress region within a sensing diaphragm configured to amplify the compressive strain of the sensor, thereby increasing the sensor's output.

[0026] The embodiments described herein are particularly applicable to strain gauge-based pressure sensors using crystalline deformable diaphragms. While embodiments will be described with reference to silicon structures, it is explicitly envisioned that other forms of crystalline structures may be used, including deformable diaphragms formed from other types of crystalline or brittle materials such as alumina, sapphire, glass, and borosilicate glass.

[0027] Figure 1 This is a schematic diagram illustrating various tensile and compressive strain gauges on a square deflectable diaphragm of a silicon pressure sensor. When designing such pressure sensors, there is a trade-off between overpressure capability and sensor sensitivity. For simple silicon pressure sensors, such as... Figure 1 The sensor 100 shown typically has strain gauges 102 and 104 located on the top side 106 of a diaphragm 108. Strain gauges 102 and 104 are tensile strain gauges located on the edges 110 and 112 of the diaphragm 108, respectively. Compressive strain gauges 114 and 116 are located at the center of the top side 106 of the diaphragm 108. The electrical output of the pressure sensor 100 is proportional to the strain difference between the tensile strain gauges 102 and 104 and the compressive strain gauges 114 and 116. Typically, to increase the electrical sensitivity of the pressure sensor, the diaphragm 108 is made more flexible. This is achieved by increasing the size, decreasing the thickness, or a combination of both. This increases both tensile and compressive strain. However, as the diaphragm 108 becomes more flexible, it also becomes more brittle. This is because silicon is a brittle material that fractures under high tensile stress.

[0028] Figure 2This is a schematic cross-sectional view of a pressure sensor with overpressure capability. The pressure sensor 120 is formed from a device wafer 122 attached to a backing wafer 124 using a glass frit 126. The pressure sensor 120 provides good sensor sensitivity and high overpressure capability for a relatively narrow measurement range. The overpressure feature is in the form of a cooperative interaction between a mesa 128 on the backing wafer 124 and an overpressure boss 130 on the device wafer 122. Shortly after the pressure exceeds the upper limit of the range, the overpressure boss 130 contacts the mesa 128, limiting any further deformation and preventing the device wafer 122 from breaking. Although... Figure 2 The arrangement shown is useful for narrower ranges, but it is limited in extended range applications because overpressure contacts are required to make contact at low pressure to prevent diaphragm rupture.

[0029] Figure 3 It is a finite element analysis diagram that compares existing narrow-range pressure sensors with pressure sensors that combine overpressure protection with an extended range exceeding 4KSI. Figure 3 The first sensor is shown, which is composed of Figure 3 The first sensor, indicated by lines of varying shapes, has an overpressure contact point 130, where one or more overpressure stop elements engage and further pressure changes cannot be reliably detected. The second sensor, indicated by lines with multiple ●s, has an overpressure contact point 132, which, while higher than the first sensor's overpressure contact point, is still insufficient for a 10 kSI sensor. It can be seen that if a 10 kSI overpressure sensor is required, the stress level must be reduced by making the diaphragm more rigid or introducing overpressure stop elements. However, this results in reduced sensor sensitivity (for a more rigid diaphragm) or a smaller useful range (if overpressure stop elements are added). Therefore, there is a trade-off between high overpressure capability, a large useful range, and sufficient sensor sensitivity.

[0030] Figure 4 This is a schematic cross-sectional view of a high-output extended-range pressure sensor according to an embodiment of the present invention. The pressure sensor 200 includes a device wafer 202 bonded to a backing wafer 204 using a glass frit 206. The glass frit 206 may be disposed in a recess in the device wafer 202, the backing wafer 204, or both, to ensure that the distance between the distal surface 224 and the contact surface 226 is set by direct contact between the wafers, rather than by the thickness and / or deformation of the glass frit 206 during the bonding process. The pressure sensor includes a primary diaphragm 208 and a secondary diaphragm 210.

[0031] The backing wafer 204 may include a chamber 225 that can be formed by etching. During the fabrication of the pressure sensor 200, the chamber 225 may be sealed and subjected to a vacuum so that the pressure sensor 200 becomes an absolute pressure sensor. In other embodiments, the backing wafer 204 may include a hole 227 that allows the chamber 225 to be at a reference pressure (such as atmospheric pressure) so that the pressure sensor 200 becomes a gauge pressure sensor.

[0032] The primary diaphragm 208 has a different size and thickness than the secondary diaphragm 210, and therefore the diaphragms 208 and 210 respond differently to pressure applied to the surface 212 of the device wafer 202. For example... Figure 4 As shown, a pair of tensile strain gauges 214 and 216 are positioned near the edge of the main diaphragm 208 (the outer edge in the illustrated example). Additionally, a pair of compressive strain gauges 218 and 220 are positioned near the center of the secondary diaphragm 210. The combined signals from the tensile strain gauges 214 and 216 and the compressive strain gauges 218 and 220 provide pressure information over an extended operating pressure range.

[0033] The pressure sensor 200 includes an overpressure protection feature in the form of an overpressure bump 222. In the illustrated embodiment, although the overpressure bump 222 extends from the device wafer 202 toward the backing wafer 204, the overpressure bump 222 has a distal surface 224 that is spaced precisely from the surface 226 of the backing wafer 204 by a gap 228. Under overpressure conditions, such as 10% higher than the maximum operating pressure, the device wafer 202 will sufficiently flex to allow the distal surface 224 to contact the surface 226 of the backing wafer, thus preventing further deflection.

[0034] Figure 5 This is a bottom plan view of the device wafer of a pressure sensor according to an embodiment of the present invention. It can be seen that the main diaphragm 208 is substantially square in shape and surrounds the sub-diaphragm 210. Each of the diaphragms 208 and 210 is preferably formed by etching. Overpressure bosses 222 are disposed within the periphery of the main diaphragm 208 on either side of the sub-diaphragm 210.

[0035] In operation, sensor sensitivity is directly related to the difference between tensile and compressive strain at the strain gauge. However, as described above, sensor robustness decreases with increasing tensile strain. Pressure sensors according to embodiments described herein typically achieve high sensitivity and robustness by incorporating a thin rectangular segment within the diaphragm that amplifies only compressive strain. This allows for increased diaphragm thickness, enabling the diaphragm to withstand significantly higher overpressures without sacrificing sensor sensitivity.

[0036] Figure 6This is a FEA plot comparing the strain at the strain gauge of a pressure sensor with and without a rectangular section according to the embodiments described herein. It can be seen that the thin rectangular section increases the compressive strain by 62%.

[0037] Figure 7 This is a strain distribution diagram of a pressure sensor according to an embodiment of the present invention. The rectangular shape of the sub-diaphragm 210 is selected for various reasons (such as...). Figure 4 (As shown). First, for strain gauges 214, 216, 218, and 220, the rectangular shape helps to maximize the strain area. The rectangular shape increases the area of ​​maximum strain, thus increasing the average strain of the entire strain gauge. Figure 7 The positions of strain gauges 214, 216, 218 and 220 are shown.

[0038] Figure 8 This is a schematic diagram of the maximum principal stress of a pressure sensor according to an embodiment described herein. Figure 8 The corner of the secondary diaphragm 210 is shown, where stress concentration occurs. These stresses are significantly minimized by moving the corner away from the region of higher tensile stress in the main diaphragm 208. Furthermore, making the secondary diaphragm rectangular minimizes bending of the unsupported sections, thereby further increasing overpressure capacity.

[0039] Figure 9 A schematic diagram illustrating the maximum principal stress concentration at the corner of the overpressure boss of a pressure sensor according to an embodiment of the present invention. The design of the overpressure boss shown helps ensure that the main diaphragm is fully supported during an overpressure event. Additionally, it is important to ensure that the stress concentration at the corner of the overpressure stop is kept outside the region of high tensile stress, thus reducing the maximum stress intensity.

[0040] Figure 10A It is a graph of the measurement factor with respect to pressure, comparing the response of a known pressure sensor with the response of a pressure sensor according to an embodiment of the present invention. Figure 10B It is box 250 of the graph of the measurement factors with respect to pressure (in Figure 10A The magnified portion (shown in the image) compares the response of a known pressure sensor with that of a pressure sensor according to an embodiment of the present invention. Figure 10A As can be seen, within the pressure range of 0 to 800 psi, the lines of the known pressure sensors (identified by the lowercase letter 'o' on the lines) substantially overlap with the lines of the pressure sensor according to an embodiment of the present invention (labeled "multi-range sensor" and identified by the lowercase letter 'x' on the lines). This overlap is... Figure 10B The diagram is shown in more detail. However, as the pressure increases from 800 psi to 4 kSI, the "multi-range sensor" line continues essentially linearly, while the known sensor line becomes horizontal upon engagement of the overpressure stop.

[0041] It should be believed that at least some of the embodiments described herein can be used as direct replacements for current narrower range pressure sensors due to their similar output characteristics. However, since the overpressure characteristic of the "multi-range sensor" is only activated when the pressure is greater than 4 kSI, this sensor can also be used as a higher range sensor, such as one with a sensor upper limit of 4000 psi. The sensor's ability to work as well as both lower range sensors (0-800 psi) and higher range sensors (0-4000 psi) is that it… Figure 10A and Figure 10B The reason why it is shown as a "multi-range sensor".

[0042] Figure 11 This is a graph showing the sensor sensitivity versus pressure, comparing the sensitivity of known pressure sensors with that of a pressure sensor according to an embodiment of the present invention. The line indicated by the lowercase letter "o" depicts the relationship between the sensitivity and pressure of a known 0-4000 psi range pressure sensor. The line indicated by the lowercase letter "x" depicts the relationship between the sensitivity and pressure of a pressure sensor according to an embodiment of the present invention. It can be seen that the sensitivity of the new pressure sensor is approximately 60% higher at 4000 psi than at 100 psi, making this new pressure sensor an excellent choice for high-pressure applications. The output of the new sensor is relatively smooth and without abrupt changes, meaning it can be accurately characterized and fitted with a suitable polynomial curve, such as polynomial curve fitting.

[0043] Figure 12A and Figure 12B These are, respectively, a finite element diagram of the top membrane of the rectangular diaphragm according to an embodiment of the present invention and a graph of the maximum principal stress versus the applied pressure during an overpressure event. Figure 13A and Figure 13B The figures show a finite element diagram of the unsupported bottom web of the rectangular diaphragm according to an embodiment of the invention during an overpressure event, and a graph of the maximum principal stress versus the applied pressure. It should be noted that the peak stress occurs on the etched side of the thin rectangular segment. This is desirable because the failure mode is expected to be sudden and catastrophic, rather than exhibiting a drift that could be mistaken for a process drift prior to sensor breakage.

[0044] Figure 14 This is a bottom plan view of a device wafer of a strain gauge-based pressure sensor according to another embodiment of the present invention. The device wafer 300 is preferably formed of silicon and bonded to a backing wafer, such as backing wafer 204, using a glass frit. Figure 4 As shown). Device chip 300 and device chip 202 (as shown). Figure 4The components shown have some similarities, and similar parts are similarly numbered. Device wafer 300 includes a main diaphragm 308, shown as having a square shape surrounding a single overpressure boss 322. Additionally, both the overpressure boss 322 and the main diaphragm 308 surround a square sub-diaphragm 310. The overpressure boss 322 may include a tapered corner 304 as shown. Furthermore, the transition section 306 from the overpressure boss 322 to the sub-diaphragm 310 may also be tapered. Figure 14 The cone shape shown helps reduce stress concentration.

[0045] Figure 15 This is a block diagram of a pressure sensing system that is particularly useful for the embodiments described herein. Transmitter electronics 400 includes a controller 402, a communication module 408, a measurement circuit 404, and a power module 406. (As...) Figure 15 As shown, the measuring circuit 404 is connected to a strain gauge on the pressure sensor 410, which can be a pressure sensor 200 (e.g., Figure 4 (as shown) or a pressure sensor using a device chip 300 (such as Figure 14 (As shown).

[0046] The controller 402 can be any suitable circuit capable of performing multiple programmed steps or functions to communicate with external devices using the communication module 408. The controller 402 can be an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), a microcontroller, or a microprocessor.

[0047] The communication module 408 is configured to interact with the controller 200 and communicate according to one or more standard protocols. The standard protocols can be wired communication protocols, such as HART, 4-20mA, FOUNDATION™ Fieldbus, Profibus, Modbus, Ethernet, and Ethernet-APL. The standard protocols can also be wireless communication protocols. Examples of wireless communication protocols include, but are not limited to, WirelessHART (IEC 62591), Cellular (NB-IoT, LTE-M), WiFi, LoRaWAN, and Bluetooth Low Energy.

[0048] The transmitter electronics 400 includes a power management circuit 406 that provides regulated power to the components of the transmitter electronics 400. Additionally, the power management circuit 406 can also provide voltage monitoring for battery-operated components.

[0049] like Figure 15As shown, the transmitter electronics 400 includes a measurement circuit 404 coupled to a controller 402. The measurement circuit 404 includes suitable circuitry for measuring the analog electrical characteristics (e.g., resistance) of one or more strain gauges on the pressure sensor 410 and providing a digital indication of the measured analog electrical characteristics to the controller 402. Suitable examples of the measurement processing circuitry include one or more analog-to-digital converters, one or more amplifiers, and / or one or more multiplexers or switches.

[0050] Figure 16 This is a schematic diagram of a pressure sensing system that is particularly useful for the embodiments described herein. Figure 16 In the process section 508, the process variable transmitter 500 is installed to the process coupling 504 via the mounting component 516.

[0051] Mounting member 500 includes an aperture 502 extending from process coupling 504 to isolation diaphragm assembly 506. Isolation diaphragm assembly 506 includes an isolation diaphragm that isolates process fluid in tubing section 508 from isolation fluid carried in isolation capillary 510. Isolation capillary 510 is coupled to pressure sensor 512, which takes the form of pressure sensor 100 described above. Pressure sensor 512 is configured to measure absolute pressure (relative to vacuum) or gauge pressure (relative to atmospheric pressure) and provides electrical output 514 to transmitter circuit 400.

[0052] Transmitter circuit 400 communicates with control room 518 to provide control room 518 with one or more process variables, such as absolute pressure and gauge pressure. Transmitter circuit 400 can use various technologies, including wired and wireless communication, to communicate with control room 518. A common wired communication technology uses a known two-wire process control loop 520, in which a single pair of wires is used to carry information and provide power to transmitter 500. One technique for transmitting information is to control the current level through process control loop 520 between 4 mA and 20 mA. Current values ​​in the 4 to 20 mA range can be mapped to corresponding values ​​of process variables.

[0053] Although the invention has been described with reference to preferred embodiments, those skilled in the art will recognize that changes in form and detail may be made without departing from the spirit and scope of the invention.

Claims

1. A pressure sensor, comprising: a first wafer having a first deflectable diaphragm and a second deflectable diaphragm, wherein the first deflectable diaphragm has a first relationship between an applied pressure and a deflection of the first deflectable diaphragm, and the second deflectable diaphragm has a second relationship between an applied pressure and a deflection of the second deflectable diaphragm, wherein the first relationship and the second relationship are different from each other; a second wafer attached to the first wafer; at least one tensile strain gauge coupled to the first deflectable diaphragm; at least one compressive strain gauge coupled to the second deflectable diaphragm; and an overpressure feature mounted relative to one of the first wafer and the second wafer, the overpressure feature configured to contact the other of the first wafer and the second wafer during an overpressure condition that exceeds a maximum measurement pressure of the first deflectable diaphragm and a maximum measurement pressure of the second deflectable diaphragm.

2. The pressure sensor of claim 1, wherein the overpressure feature is mounted to the first wafer.

3. The pressure sensor of claim 1, wherein the overpressure feature includes a plurality of overpressure bosses disposed on opposite sides of the second deflectable diaphragm.

4. The pressure sensor of claim 1, wherein the first wafer and the second wafer are formed of silicon.

5. The pressure sensor of claim 4, wherein the first wafer and the second wafer formed of silicon are attached to each other by a glass frit disposed in a recess.

6. The pressure sensor of claim 1, wherein the first deflectable diaphragm surrounds the overpressure feature.

7. The pressure sensor of claim 1, wherein the first deflectable diaphragm surrounds the second deflectable diaphragm.

8. The pressure sensor of claim 7, wherein the second deflectable diaphragm is rectangular in shape.

9. The pressure sensor of claim 7, wherein the second deflectable diaphragm is square in shape.

10. The pressure sensor of claim 1, wherein the at least one tensile strain gauge includes a plurality of tensile strain gauges.

11. The pressure sensor of claim 10, wherein the at least one compressive strain gauge includes a plurality of compressive strain gauges.

12. The pressure sensor of claim 1, wherein the at least one compressive strain gauge includes a plurality of compressive strain gauges disposed near a center of the second deflectable diaphragm.

13. The pressure sensor of claim 1, wherein the pressure sensor is an absolute pressure sensor.

14. The pressure sensor of claim 1, wherein the pressure sensor is a gage pressure sensor.

15. A pressure transmitter, comprising: a transmitter circuit configured to measure an electrical characteristic of a pressure sensor and provide a process fluid pressure output; and a pressure sensor operably coupled to the transmitter circuit, the pressure sensor comprising: ​ a first wafer having a first deflectable diaphragm and a second deflectable diaphragm, wherein the first deflectable diaphragm has a first relationship between applied pressure and deflection of the first deflectable diaphragm, and the second deflectable diaphragm has a second relationship between applied pressure and deflection of the second deflectable diaphragm, wherein the first relationship and the second relationship are different from each other; a second wafer attached to the first wafer; at least one tensile strain gauge coupled to the first deflectable diaphragm; at least one compressive strain gauge coupled to the second deflectable diaphragm; and an overpressure feature mounted relative to one of the first wafer and the second wafer, the overpressure feature configured to contact the other of the first wafer and the second wafer during an overpressure condition that exceeds a maximum measured pressure of the first deflectable diaphragm and a maximum measured pressure of the second deflectable diaphragm.

16. The pressure transmitter of claim 15, wherein the transmitter circuitry includes a measurement circuit coupled to the pressure sensor, the measurement circuit configured to measure a resistance of the at least one tensile strain gauge and a resistance of the at least one compressive strain gauge.

17. The pressure transmitter of claim 16, wherein the measurement circuit is configured to measure a resistance of a plurality of tensile strain gauges and a plurality of compressive strain gauges.

18. The pressure transmitter of claim 17, wherein the transmitter circuitry is configured to apply a curve fit to the measured resistances to generate a process pressure output.

19. The pressure transmitter of claim 18, wherein the curve fit is a polynomial.

20. The pressure transmitter of claim 15, wherein the pressure sensor has an increased sensitivity as pressure increases.

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