Capacitive pressure sensor having encapsulated resonating components

[0002] This application is a continuation in part of U.S. patent application Ser. No. 10/391,064, filed on Mar. 18, 2003, which is a continuation in part of U.S. patent application Ser. No. 09/369,573, filed on Aug. 6, 1999 now U.S. Pat. No. 6,532,834. Both of said prior applications are incorporated herein by reference.

[0003] The present invention relates to a pressure sensor, and more particularly, a pressure sensor which relies on changes in capacitance to indicate pressure fluctuations.

[0004] Capacitive pressure sensors are well known in the prior art. Such sensors typically include a fixed element having a rigid, planar conductive surface forming one plate of a substantially parallel plate capacitor. A displaceable (relative to the fixed element) conductive member, such as a metal diaphragm, or a plated non-conductive member, such as a metalized ceramic diaphragm, forms the other plate of the capacitor. Generally, the diaphragm is edge-supported so that a central portion is substantially parallel to and opposite the fixed plate. Because the sensor generally has the form of a parallel plate capacitor, the characteristic capacitance C of the sensor may be approximated by the equation: $\begin{array}{cc}C=\frac{ϵA}{d}& \left(1\right)\end{array}$

[0005] where ε is the permittivity of the material between the parallel plates, A is the surface area of the parallel plate and d represents the gap between the plates. The characteristic capacitance is inversely proportional to the gap between a central portion of the diaphragm and the conductive surface of the fixed element. In order to permit a pressure differential to develop across the diaphragm, the region on one side of the diaphragm is sealed from the region on the opposite side.

[0006] In practice, the diaphragm elasticity is selected so that pressure differentials across the diaphragm in a particular range of the interest cause displacements of the central portion of the diaphragm. These pressure differential-induced displacements result in corresponding variations in the gap, d, between the two capacitor plates, and thus in capacitance variations produced by the sensor capacitor. For relatively high sensitivity, such sensors require large changes of capacitance in response to relatively small gap changes. Regarding equation (1), if ε and A are held constant, the greatest slope of the d verses C plot occurs when d is small. Thus, for the greatest sensitivity, the gap is made as small as possible when the device is in equilibrium and the sensor is designed so that the gap d changes as pressure is applied. The multiplicative effect of ε and A increases the sensitivity of the d to C relationship, so ε and A are maximized to achieve the highest possible sensitivity.

[0007] In a typical prior art embodiment, the sensor capacitor formed by the fixed conductive surface and the diaphragm is electrically coupled via conductors to an oscillator circuit. The oscillator circuit typically includes an inductor that forms a tank circuit with the remotely located sensor capacitor. This LC tank circuit provides a frequency reference for the oscillator circuit; the output frequency of which is a direct function of the resonant frequency of the tank circuit. The resonant frequency of the tank circuit is in turn a direct function of the inductance L of the inductor and the capacitance C of the sensor capacitor. It is well known to those in the art that the resonant frequency ω₀of a simple LC tank circuit is given by ${\omega }_0=\frac{1}{\sqrt{\mathrm{LC}}}.$

[0008] As long as the values of the inductor and the capacitor both remain fixed, the output frequency of the oscillator circuit remains constant. However, since the capacitance of the sensor capacitor varies as a function of the pressure applied to the diaphragm, the output frequency of the of the oscillator circuit also varies as a direct function of the applied pressure.

[0009] Such a configuration produces a signal whose frequency is indicative of the pressure applied to the remote sensor. One disadvantage to this configuration is that having the capacitive sensor located remotely can introduce environmentally induced errors in the expected resonant frequency of the tank circuit. For example, it is well known to those in the art that the inductance value L of an inductor and the capacitance value C of a capacitor are each temperature dependent to some extent, depending upon the design of each particular physical component. The effect of the temperature on the capacitance or inductance of a particular component is often quantified as the “temperature coefficient” associated with that component. It is possible to design a component so as to minimize the temperature coefficient, thus rendering the value of the device relatively insensitive to temperature, but commercially available components typically do have a measurable temperature coefficient which affects the component performance. It is also possible to choose components whose temperature coefficients are complementary, such that the net effect of a temperature change to the components together is nominally zero. However, when two components are not located together, such as the capacitive sensor and the inductor in the oscillator circuit, the ambient temperatures are often different, and complementary temperature coefficients do not produce a nominally zero sensitivity to temperature changes.

[0010] Another disadvantage to having a remotely located capacitive sensor is that the conductors used to electrically couple the sensor to the oscillator circuit introduce stray capacitances and inductances to the basic LC tank circuit. This disadvantage could be mitigated and thus acceptable if the stray values remained constant, but the stray values can change with environmental factors, physical movement of the conductors, etc.

[0011] It is an object of the present invention to substantially overcome the above-identified disadvantages and drawbacks of the prior art.

[0012] The foregoing and other objects are achieved by the invention which in one aspect comprises a capacitive sensor for measuring a pressure differential across a conductive, elastic member, or a plated non-conductive elastic member, having at least a first substantially planar surface and being supported on at least one edge. The elastic member extends about a central axis. The sensor includes a support member or a housing extending about the central axis for supporting the elastic member by its edge, thereby forming (i) a first pressure region disposed on the side of the elastic member corresponding to the first planar surface, and a second pressure region disposed on the side of the elastic member opposite said first side. The first pressure region and the second pressure region generate the pressure differential across the elastic member. The sensor also includes a capacitive plate disposed substantially adjacent to the elastic member so as to define a gap between the first planar surface and a corresponding planar surface of the capacitive plate. The gap, capacitive plate and elastic member together define a capacitor having a characteristic capacitance. The sensor further includes an elongated electrical conductor characterized by an associated inductance value. The conductor is fixedly attached to and electrically coupled with the capacitive plate. The gap between the capacitive plate and the elastic member varies as a predetermined function of the pressure differential across the elastic member so as to vary the characteristic capacitance. The capacitor and the electrical conductor together form a tank circuit having a characteristic resonant frequency; varying the capacitance of this tank circuit varies the resonant frequency of the tank circuit. Thus, the resonant frequency of the tank circuit is indicative of the pressure differential across the elastic member.

[0013] In another embodiment of the invention, the pressure applied to the elastic member is generated by a pressure differential across (i) the first planar surface of the elastic member and (ii) a second planar surface of the elastic member disposed substantially parallel to the planar surface. In one embodiment, this pressure differential is the result of a constant, controlled environment being in contact with the first planar surface, along with a fluid under pressure being in contact with the second planar surface of the elastic member. In an alternative embodiment, the two planar surfaces of the elastic member can be respectively connected to two fluid mediums, and the pressure sensor can be used to measure the pressure difference between the two fluid mediums.

[0014] In another embodiment, the electrical conductor is disposed in a spiral configuration within a plane substantially parallel to the capacitive plate.

[0015] In a further embodiment, the sensor further includes an insulator disposed between the capacitor plate and the electrical conductor. The insulator may be fixedly attached to either the capacitor plate, the electrical conductor, or both.

[0016] In another embodiment, the sensor further includes a stiffening element fixedly attached to the capacitive plate and the conductive element.

[0017] The foregoing and other objects of this invention, the various features thereof, as well as the invention itself, may be more fully understood from the following description, when read together with the accompanying drawings in which:

[0018] FIG. 1 shows a sectional view of one preferred embodiment of a capacitive pressure sensor;

[0019] FIG. 2A shows the capacitive sensor of FIG. 1 with an elastic member having a neutral displacement;

[0020] FIG. 2B shows the capacitive sensor of FIG. 1 with a higher pressure in the second pressure region than the first pressure region;

[0021] FIG. 3A shows a bottom view of the capacitor plate;

[0022] FIG. 3B shows a top view of the inductor coil;

[0023] FIG. 4A shows the capacitor and the inductor coil connected as a series resonant tank circuit;

[0024] FIG. 4B shows the capacitor and the inductor coil connected as a parallel resonant tank circuit;

[0025] FIG. 5 shows the tank circuit of FIG. 4A connected to an oscillator circuit;

[0026] FIG. 6 shows a closing-gap embodiment of the pressure sensor in accordance with the present invention;

[0027] FIG. 6A shows an alternative embodiment of the pressure sensor of FIG. 6;

[0028] FIG. 7 shows a sensor including a stiffening element attached to the electrode assembly in accordance with one embodiment of present invention;

[0029] FIG. 8 shows an alternate, multiple layer embodiment of the inductor coil from the sensor in accordance with one embodiment of the present invention;

[0030] FIG. 9 shows another view of the multiple layer inductor coil shown in FIG. 8;

[0031] FIG. 10 shows another embodiment of the sensor in accordance with the present invention;

[0032] FIG. 10A shows an alternative embodiment of the pressure sensor of FIG. 10; and

[0033] FIG. 11 is a schematic view of one embodiment of the present invention showing the pressure sensor being used to measure the pressure difference between two fluid mediums.

[0034] FIG. 1 shows a sectional view of one preferred embodiment of a capacitive pressure sensor 100 constructed in accordance with the present invention, which produces a characteristic capacitance proportional to a pressure (e.g., pressure via a fluid medium) applied to the sensor 100. Sensor 100 includes a first body member 103A, a second body member 103B, and an electrically conductive, elastic member 102 that forms a physical boundary between a first pressure region 106 and a second pressure region 104 defined by the two body members 103A and 103B. The elastic member 102, the first body member 103A, and the second body member 103B extend about a central axis X.

[0035] The elastic member 102 is supported at its periphery 108 by a support member 110. The support member 110 may include, or be integral with, the pressure sensor 100 housing, as is disclosed and described in detail in U.S. Pat. No. 5,442,962, assigned to the assignee of the subject invention and is hereby incorporated by reference.

[0036] In this embodiment, the planar surface of the elastic member 102 is substantially circular, although alternate embodiments may incorporate other shapes. A connection post 112 extending along the central axis X for supporting an electrode assembly 114 is fixedly attached to the elastic member 102. The connection post 112 may be attached to the elastic member 102 by brazing, soldering, welding, gluing, press fit, stud mount, or by other securing methods known to those in the art. The cross section of the elastic member 102 (shown in FIG. 1) is somewhat greater (i.e., thicker) at the center, as compared to the perimeter, to provide a foundation for attaching the connection post 112. Other elastic member 102 cross sections may be used to provide similar results. Similarly, the electrode assembly 114 may be attached to the connection post 112 by brazing, soldering, gluing, press fit, stud mount, or by other methods of securing components known to those in the art.

[0037] The electrode assembly 114 includes a capacitor plate 116, an insulator 118 and a planar inductor coil 120. The capacitor plate 116, a bottom view of which is shown in FIG. 3A, is shaped, sized and contoured to substantially match the planar surface of the electrically conductive elastic member 102. In a preferred embodiment, the capacitor plate 116 includes a sheet of copper, silver or gold bonded to an insulating base 117 such as fiberglass, polyimide, glass, or ceramic, although other electrically conductive materials and other insulating materials known to those in the art may be used to form the capacitor plate 116 and the insulating base 117, respectively. Alternately, the capacitor plate 116 may be etched from a copper-clad substrate, or screened and fired using thick-film techniques, using procedures well known for the fabrication of printed circuits.

[0038] The insulator 118 may include a separate piece of insulating material bonded to and contiguous with the capacitor plate 116 and the inductor coil 120, or it may include an extension of the insulating base from the capacitor plate 116. The insulator 118 may include fiberglass, polyimide, ceramic, or other insulating materials known to those in the art.

[0039] A preferred embodiment of the inductor coil 120, a top view of which is shown in FIG. 3B, includes an elongated electrical conductor wound in a spiral form within a plane that is substantially parallel to the capacitor plate 116. As with the capacitive plate 116, the inductor coil 120 may be etched from a sheet of conductive foil bonded to an insulator 118, using printed circuit board techniques well known to those in the art. Alternatively, the coil may be screened and fired using thick-film techniques well known to those in the art. In other embodiments, the coil 120 may include a single long conductor, wound in the shape shown in FIG. 3B and bonded to an insulator 118. Other methods of fabricating the coil 120 known to those in the art (e.g., vapor deposition, photoetching, etc.) may also be used, as long as the resulting coil 120 provides the inductive properties described herein. The end of the coil 120 shown in FIG. 3B is electrically coupled to a plated through-hole 128 that passes through the insulator 118. The plated through-hole 128 is also electrically coupled to the capacitor plate 116; the coil 120 is thus electrically coupled to the capacitor plate 116. In alternate embodiments, this electrical coupling between the coil 120 and the capacitive plate 116 may be accomplished by an electrical conductor passing through the insulator 118, by a conductor wrapping around the side of the insulator 118, or by other methods known to those in the art.

[0040] The capacitive plate 116, the conductive elastic member 102 and the gap 126 formed between the capacitive plate 116 and the elastic member 102 form a capacitor 130 having a characteristic capacitance. In general, the characteristic capacitance of such a structure is directly proportional to the areas of the capacitive plate 116 and the elastic member 102, and inversely proportional to the distance between the capacitive plate 116 and the elastic member 102.

[0041] In a preferred embodiment of the invention, the pressure sensor 100 senses a pressure differential generated by the first pressure region 106 and the second pressure region 104 across the elastic member 102. The pressure in the first pressure region 106 may be ambient atmospheric pressure (i.e., simply exposed to the “open air”), or it may be more precisely controlled with respect to a constant pressure reference. In another preferred form, the first pressure region 106 is a vacuum chamber, and the pressure in the first pressure region 106 substantially equals zero.

[0042] In one preferred embodiment, the device 100 can be used to measure the pressure of a fluid medium. In this embodiment, the second pressure region 104 is preferably constructed as a chamber having a port 105 for receiving air or another fluid whose pressure is to be measured. The first pressure region 106 can be a sealed chamber or an opened chamber having a port 107, and the pressure in the first pressure region 106 is preferably maintained constant.

[0043] In another preferred embodiment, the device can be used to measure the pressure difference between two fluid mediums that are respectively introduced into the first pressure region 106 and the second pressure region 104. In this embodiment, both pressure regions 104 and 106 have ports 105 and 107 for connecting with two fluid mediums, between which, the pressure difference is to be measured. A difference in pressure across the two regions 104 and 106 produces a net differential pressure 124 on the elastic member 102. When the second pressure region 104 is greater than the first pressure region 106, the direction of the elastic member displacement (along the central axis X) is from the second pressure region 104 to the first pressure region 106, as shown in FIG. 2B. A change of ambient pressure in any of the pressure regions 104 and 106 produces a corresponding change in the amount of displacement of the elastic member 102. FIG. 2A shows the elastic member 102 in a neutral displacement position; i.e., when the differential pressure across the elastic member 102 is substantially zero. In the neutral displacement position, a substantially uniform gap 126 exists between the capacitive plate 116 and the elastic member 102. FIG. 2B shows the elastic member 102 displaced (along the central axis X) toward the first pressure region 106, such that the elastic member 102 presents a convex surface in the first pressure region 106. In this convex displacement position, a non-uniform gap 126 exists between the capacitive plate 116 and the elastic member 102. The width of the non-uniform gap 126 near the connection post 112 is substantially the same as the uniform gap 126 in the neutral displacement position, and the width of the non-uniform gap 126 increases as the distance from the post 112 increases. The increase in the gap 126 distance as the elastic member 102 displaces toward the first pressure region 106 produces a decrease in the characteristic capacitance. Thus, the characteristic capacitance of the capacitor 130 formed by the capacitive plate 116, the conductive elastic member 102 and the gap between them is inversely proportional to the magnitude of the differential pressure 124 across the elastic member 102.

[0044] In one embodiment of the invention, the capacitor 130 is electrically coupled in series to the inductive coil 120 so as to form a series resonant tank circuit 132 having a resonant frequency ${\omega }_0=\frac{1}{\sqrt{\mathrm{LC}}}$

[0045] as shown schematically in FIG. 4A. Alternately, the capacitor 130 may be electrically coupled in parallel to the inductive coil 120 so as to form a parallel resonant tank circuit 132 having a resonant frequency ${\omega }_0=\frac{1}{\sqrt{\mathrm{LC}}}$

[0046] as shown schematically in FIG. 4B. In either case, the tank circuit (132 or 134) is electrically coupled to an oscillator circuit 136 that uses the tank circuit 132 as a frequency reference, as shown in FIG. 5 for a series resonant tank circuit 132. The oscillator circuit 136 is electrically coupled to the tank circuit 132 via conductors electrically coupled to inductor terminal 129, which is insulated from the first body member 103A by an insulator 129A, and capacitor terminal 131. The output of the oscillator circuit is a signal S_OUThaving a frequency of ${\omega }_{\mathrm{OUT}}=\frac{1}{\sqrt{\mathrm{LC}}},$

[0047] thus the capacitance C is a function of the frequency; i.e., $C=\frac{1}{{\omega }_{\mathrm{out}}^{2}L}.$

[0048] Since the characteristic capacitance of the capacitor 130 is directly proportional to the magnitude of the differential pressure 124 across the elastic member 102, the frequency ω_OUTof the output signal S_OUTis also a function of the magnitude of the differential pressure 124. The close mutual proximity of the inductive coil 120 and the capacitor 130 ensures similar environmental conditions for both components of the tank circuit 132A closing-gap embodiment of a pressure sensor 200, shown in FIG. 6, includes an electrically conductive elastic member 202 extending about the central axis X and secured about its perimeter 208 by a housing 210. In this form of the invention, the housing 210 includes an upper portion 210aand a lower portion 210b, and the elastic member 202 is secured between the two portions at its perimeter 208. The elastic member may be secured by a bonding technique known in the art such as brazing, welding, gluing, etc., or the elastic member may be secured by pressure (i.e., clamping) between the upper portion 210aand the lower portion 210bof the housing 210. As with the embodiment shown in FIG. 1, the elastic member 202 forms a physical boundary between a first pressure region 206 and a second pressure region 204. In the closing-gap embodiment, however, the electrode assembly 214 is not mechanically coupled to the elastic member 202 via a connection post. Rather, the electrode assembly 214 is suspended from the housing 210 by a suspension post 212, such that the electrode assembly 214 is disposed substantially adjacent to the elastic member 202. Because the electrode assembly 214 is not attached to the elastic member 202 in this embodiment, the cross section of the elastic member 202 can be relatively uniform as shown in FIG. 6, as opposed to the non-uniform cross section (i.e., thicker at the center and tapering out toward the perimeter) of the elastic member 102 shown in FIG. 1.

[0049] The construction of the electrode assembly 214 in this embodiment is essentially the same as for the form of the invention shown in FIG. 1; the electrode assembly 214 includes a capacitor plate 216, an insulator 218 and a planar inductor coil 220. The inductor coil 220 and the capacitor plate 216 are electrically coupled via the plated through-hole 228. A capacitor 230 having a characteristic capacitance C is formed by the capacitor plate 216, the conductive elastic member 202 and the variable gap 226 formed between the plate 216 and the member 202. Since the areas of the capacitive plate 216 and the elastic member 202 do not vary, the characteristic capacitance C varies only as a function of the gap 226. As a differential pressure 224 is applied across the elastic member 202 in a direction from the second pressure region 204 toward the first pressure region 206, the elastic member deflects toward the electrode assembly 214, so as to be substantially convex in the first pressure region 206. This pressure induced deflection toward the electrode assembly closes the variable gap 226, thereby increasing the characteristic capacitance C. The characteristic capacitance C is thus directly proportional to the magnitude of the differential pressure 224 across the elastic member 202 for this embodiment of the invention. Electrical access to the capacitor 230 is gained by a first electrical terminal 229 and a second electrical terminal 231. In one preferred embodiment, the first electrical terminal 229 is electrically coupled to the inductor coil 220 through an electrically conductive suspension post 212, and the second electrical terminal 231 is electrically coupled to the elastic member 202 at its perimeter 208. The second pressure region 204 preferably includes a port 205 for receiving fluid medium whose pressure is to be measured. The first pressure region can be a sealed chamber, as shown in FIG. 6. In a preferred embodiment, the sealed chamber 206 is vacuum. In another preferred embodiment, as shown in FIG. 6A, the first pressure region 206 also includes a port 207 for receiving another fluid medium. The port 207 can be connected to a fluid medium with a constant pressure to maintain a constant pressure in the first pressure region 206. Also, the port 207 can be connected to a second fluid medium and the device can measure the pressure difference between the first fluid medium that is introduced into the second pressure region 204 and the second fluid medium in the first pressure region 206. FIG. 11 schematically shows that the device 100, 200, or 300 is connected to two fluid mediums to measure the pressure difference between the two mediums having pressures P1 and P2. The first pressure region and the second pressure region each may have more than one ports for connecting with pressurized mediums.

[0050] In one embodiment, the electrode assembly 214 includes a stiffening element 140 as shown in FIG. 7. The stiffening element 140 prevents flexure of the overall electrode assembly, which in turn maintains the capacitor plate 116 within its nominal plane 142. The stability of capacitor 130 of FIG. 1, formed in part by the variable gap 126, is dependant upon the capacitor plate 116 being substantially planar. Flexure of the plate 116 due to temperature variations or other environmental forces (such as vibration and shock) may corrupt the measured value of the characteristic capacitance of the capacitor 130. Any corruption of the characteristic capacitance translates directly to a corruption of the resonant frequency ω₀of the tank circuit 132 and thus to a corruption of the measurement of the differential pressure 124. The stiffening element 140 may include ceramics or other materials that are known to exhibit small amounts of expansion or contraction with respect to ambient temperature variations.

[0051] In another embodiment of the invention, the inductor coil 120 of FIG. 1 may include a multi-layer inductive coil. The coil 150 shown in FIG. 8 includes two layers of electrical conductor electrically coupled in series via a plated through-hole 152, although alternate embodiments may include any number of layers. The two layers of electrical conductor are bonded to opposite sides of an insulating layer 154, similar to the construction of a multi-layered printed circuit board. One utility of a multiple layer inductive coil 150 is a higher characteristic inductance value due to the increase in the length of the conductor. Another utility of the multiple layer inductive coil 150 is the ability to compensate a variation of the coil's characteristic inductance with respect to temperature variations. It is well known to those in the art that as a planar spiral coil 150 expands in its spiraling plane and the distance d1 between adjacent turns of a single coil increases, the characteristic inductance L of the coil increases (see FIG. 9). It is also well known that as the distance d2 between two coils increases, the characteristic inductance L of the coils decreases. An expansion of the insulating layer due to a temperature change results in a corresponding increase in both d1 and d2. By choosing the appropriate initial dimensions d1 and d2, and by choosing a material for the insulating layer 154 having an appropriate expansion coefficient (with respect to temperature), the changes in characteristic inductance of the coil 150 due to the changes in d1 and d2 can be made to cancel.

[0052] In yet another form of the invention, as shown in FIG. 10, the capacitor 330 portion of the electrode assembly 314 is located within the housing 310, formed by upper portion 310aand lower portion 310b, while the insulator 318 and the inductor 320 portions are disposed outside of the housing 310. An electrically conductive post 312 extends through the upper portion 310aof the housing 310, and is secured in place by a non-conductive sleeve 322. This sleeve 322 electrically isolates the conductive post from the housing 310. Electrical access to the resonator formed by the inductor 320 and the capacitor 330 is gained via a first terminal 329 and a second terminal 331. The first terminal 329 is electrically coupled to the diaphragm 302 at the perimeter 308. The second terminal 331 is electrically coupled to a first end of the inductor 320. The second end of the inductor 320 is electrically coupled to the conductive post 312, through which to the capacitive plate 316. Thus, the conductive post serves not only to support the capacitive plate 316 and the inductor 320, but also to electrically couple the inductor 320 to the capacitor 330. The second pressure region 304 includes a port 305 for receiving fluid medium whose pressure is to be measured. Similar to previously described embodiments, the first pressure region 306 can be a sealed chamber, as shown in FIG. 10, and also can be an opened chamber having a port 307, as shown in FIG. 10A.

[0053] The present invention may be used as different types of pressure sensors, including “gauge” pressure sensor, “vacuum gauge” pressure sensor, “compound gauge” pressure sensor, “scaled gauge” pressure sensor, “absolute” pressure sensor, and “vacuum” pressure sensor.

[0054] A “gauge” pressure sensor is defined when the pressure P1 in the first pressure region is allowed to equal atmospheric pressure by virtue of having the first pressure region vented to the atmosphere and the measured pressure P2 in the second pressure region is typically greater than atmospheric pressure.

[0055] A “vacuum gauge” pressure sensor is defined when the pressure P1 in the first pressure region is allowed to equal atmospheric pressure by virtue of having the first pressure region vented to atmosphere and the applied pressure P2 in the second pressure region is typically lower than atmospheric pressure.

[0056] A “compound gauge” pressure sensor is defined when the pressure P1 in the first pressure region is allowed to equal atmospheric pressure by virtue of having the first pressure region vented to atmosphere and the applied pressure P2 is allowed to be either higher or lower than atmospheric pressure.

[0057] A “sealed gauge” pressure sensor is defined when the pressure P1 contains a sealed finite reference pressure where the reference pressure may be subject to change in response to changes in temperature if the sealed media approximates an ideal gas.

[0058] An “absolute” pressure sensor is defined when the first pressure region is substantially evacuated and sealed, causing pressure P1 in the first pressure region to approach 0 psia.

[0059] A “vacuum” pressure sensor is fabricated substantially the same way as an “absolute” pressure sensor, but the full-scale pressure ranges are typically lower than atmospheric pressure.

[0060] The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of the equivalency of the claims are therefore intended to be embraced therein.