Abstract:
An apparatus for measuring the differential pressure between at least two fluids including (a) first and second pressure enclosures; (b) first and second pressure deformable members where the first deformable member is sealingly connected to the first pressure enclosure and second deformable member is sealingly connected to the second pressure enclosure and the pressure deformable members are rigidly linked so as to move as one unit; (c) a frame for locating the first and second pressure enclosures in fixed relationship to one another; (d) a position sensing device for determining the displacement of the pressure deformable members relative to the frame; (e) a force balancing device including a first portion rigidly connected to the frame and a moveable second portion connected to the pressure deformable members; (f) a transducer for transducing the displacement of the first and second pressure deformable members into a force acting on the moveable second portion of the force balancing device in a direction to decrease the displacement, the first pressure deformable member including a nonflexible circular first plate sealingly connected to at least one flexible first shell of revolution where the first shell is concave on one surface thereof and said second pressure deformable member including a nonflexible circular second plate sealingly connected to at least one flexible second shell of revolution where the second shell is concave on one surface thereof.

Description:
RELATED APPLICATION 
     This application is a Continuation of Ser. No. 09/004,776 filed Jan. 9, 1998, the entire teachings of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a force balancing manometer for measuring differential pressure over a wide range. 
     BACKGROUND OF THE INVENTION 
     There is a fast growing need for accurate measurement of pressures and differential pressures of pure gases as well as mixtures of gases in vacuum systems used in the manufacture of computer chips and similar high technology devices. So-called capacitance manometers because of their cleanliness and sensitivity are widely used for this purpose almost to the exclusion of all other types of devices. 
     In a capacitance manometer, a relatively thin diaphragm separates the gas in pressure chamber 1 from the gas in chamber 2. A pressure differential across the diaphragm causes it to be deflected until the deformation induced force is equal to the pressure induced force. A capacitance probe is used to determine the amount of deflection which may then be related to the pressure difference across the diaphragm. 
     Capacitance manometers suffer from a number of deficiencies. For example: 
     1. In order to measure small differential pressures, the diaphragm must be thin and flat so that the spring rate is very small. However, a thin flat diaphragm is easily damaged by excessive stresses caused by over pressure conditions. Various diaphragm support means are known but costly over pressure accidents still occur. Because the flat diaphragm deforms in a complex shape, effective stops are also relatively expensive to machine. 
     2. Because of the use of a thin flat diaphragm, capacitance manometers have limited pressure range, typically only four decades. Therefore, multiple manometer heads must be installed to provide pressure measurement over the 7 or 8 decades of pressure range typically required. 
     3. Because the diaphragm is purposely deformed during the measuring process, capacitance manometers are extremely sensitive to changes in material properties with temperature or age. 
     4. Capacitance manometers have sensitive electrical circuits exposed on one side of the diaphragm which are readily damaged by process gases. 
     5. Capacitance manometers require costly vacuum compatible electrical feedthroughs to conduct electrical signals through the vacuum wall. 
     6. Capacitance manometers suffer from hysteresis effects. The pressure indication at a given pressure depends on whether the given pressure is approached by increasing or decreasing the pressure. 
     7. Capacitance manometers require extreme care in manufacture to assure the thin diaphragm is flat and uniformly stressed and are therefore very costly to manufacture. 
     Clearly what is required is a manometer which has a wide range, and is immune to overpressure, hysteresis and changes in material properties. 
     Mechanical Deformation Manometers 
     The above cited problems of capacitance manometers are largely caused by the use of the diaphragm to provide a mechanical deformation force to oppose the pressure induced force. The same type of problems are inherent in the design disclosed in Soviet Union Patent RU2010201, wherein the opposing force is provided by dual flexible bellows. Although using a deformed flexible member for the function of providing the opposing force is simple in principle, it causes many serious problems in practice. Similar problems are inherent in the design disclosed in U.S. Pat. No. 5,457,999, wherein the opposing force is provided by deforming an elastic vibrating member in tension. 
     Force Balancing Manometers 
     Manometers are known which do not depend on mechanical deformation of a flexible member for generating a balancing force to oppose the pressure induced force. These so-called force balancing manometers may be divided into two classes; those which measure a single pressure differential and those which measure multiple pressure differentials. 
     An example of the first class is disclosed in U.S. Pat. No. 3,657,630 wherein it is proposed to use a relatively large dc current in a flat diaphragm which is immersed in a magnetic field parallel to the diaphragm to provide an opposing force perpendicular to the diaphragm. This design depends critically on providing and maintaining a uniform current density across the diaphragm. Such a requirement is very difficult to achieve in practice and to our knowledge this invention has not found significant use. 
     An example of the second class is disclosed in U.S. Pat. No. 3,832,618 wherein two pressure differentials with a common pressure are measured and their difference determined. Thus, a third pressure differential is determined and displayed. The common pressure is produced by hydraulic fluid which is throttled and used for damping purposes. This patent for measuring very high pressure differentials does not teach how to accurately measure the very low pressures commonly used in vacuum processing and still achieve wide range. In U.S. Pat. No. 5,457,999 two pressure differentials with a common very low pressure are measured and their difference determined as noted above. However, this design does not employ force balancing and thus suffers from the effects of changes in material properties as noted above under mechanical deformation manometers. 
     Existing force balancing manometers such as are cited above are seldom if ever used in clean low pressure vacuum processing. This is likely because of lack of stability, inadequate sensitivity and the complexity and the high cost of providing force balancing means in vacuum. 
     The prior art for measuring multiple pressure differentials does not teach how to simultaneously avoid overpressure problems and still obtain high sensitivity. The prior art also does not teach how to avoid the serious problems of the influence of material property changes on accuracy. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a force balancing manometer for measuring a differential fluid pressure. The fluid to be measured may be a gas or a liquid, the reference pressure medium may be a like or differing fluid. 
     The differential pressure is applied to a displaceable force sensing assembly and deviations of the force sensing assembly from a null position are sensed. A servosystem controls a force balancing means acting on the force sensing assembly so as to return the force sensing assembly to a null position. The force required to restore the force sensing assembly to the null position is calibrated to correspond to the differential pressure acting on the force sensing assembly. 
     The present invention provides a flexibly suspended circular first plate to isolate a first chamber at pressure P 1  from the atmosphere and a flexibly suspended circular second plate to isolate a second chamber at pressure P 2  from the atmosphere. A rigid link joins the two plates and locates the plates coaxially. The flexibly suspended plates are provided with mechanical stops to limit axial displacement of the plates to a very small range Δx. 
     The first and second circular plates are preferably flat and relatively thick and stiff. First and second flexible suspension members are preferably very thin shells of revolution of a segment of a circle. A thin shell of revolution has a relatively high spring rate when it is clamped at both its inner and outer edges. However, in accordance with the present invention, it has been found that by clamping the outer edge of a thin shell of revolution and joining the inner edge of the shell to the outer edge of a stiff circular plate that the resulting assembly surprisingly has a very low spring rate but can still withstand relatively high pressure differentials without damage. 
     A flexible plate with very low spring rate would suffer from the same overpressure problems as a thin diaphragm in a capacitance manometer if mechanical deformation were relied upon to provide the opposing force. The present invention uses force balancing means well known in the art to provide the opposing force. In operation, the flexibly suspended plates are constrained to move within a range of axial displacement Δx where the manometer is extremely sensitive. When the manometer is not in operation the force balancing assembly rests on a mechanical stop. 
     When the force balancing assembly rests on a stop, the spring rate of the thin shells increases dramatically compared to the spring rate when the assembly is not against a stop. This large change in spring rate is achieved without the need for elaborate accurately shaped stops for the thin shells as are required in capacitance manometers. Only simple mechanical stops are required. When the force balancing assembly rests against a mechanical stop, the thin flexible shells can resist relatively high pressure differentials without damage if the higher pressure is applied to the concave side. When the force balancing means is in operation the thin flexible shells are in effect clamped at their peripheries and can resist relatively high pressure differentials. However, in operation the force balancing assembly in the present invention has a very low spring rate and can thus measure very low pressure differentials as well as relatively high differentials all in the same manometer. The combination of fixed clamping when not in operation plus pseudo clamping by the force balancing means when in operation provides overpressure protection together with high sensitivity. These advantages have not been achieved simultaneously in the prior art. 
     The advantages of the present invention are to enable a force balancing manometer which: 
     1. Has a very wide differential pressure range suitable for clean vacuum processing. 
     2. Is relatively immune to over pressure. 
     3. Has negligible hysteresis. 
     4. Is relatively insensitive to the effects of changes in material properties with temperature and age. 
     5. Has position sensing and restoring force means located external to the vacuum. 
     6. Has no electrical circuits inside the vacuum and therefore no need for vacuum feedthroughs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
     FIG. 1 is a schematic drawing of an illustrative force balancing manometer for measuring a differential fluid pressure in accordance with the present invention. 
     FIG. 2 is a schematic cross-sectional view of a flexible member in accordance with the invention which constitutes a thin shell of revolution where the diameter of the semicircle which is used to generate the shell is perpendicular to the axis of revolution. 
     FIG. 3 is a schematic cross-sectional view of another embodiment of the invention wherein multiple shells are joined on their inner peripheries to short cylindrical spacers for measuring lower pressure differentials. 
     FIG. 4 is a schematic illustration of a further embodiment of the invention wherein two electro-magnets are employed to facilitate location of capacitor plates optimally on axis and midway within the manometer. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings, like elements are designated by like numbers. In FIG. 1 there is shown a schematic drawing of a force balancing manometer  10  for measuring a differential fluid pressure. The fluid to be measured may be a gas or a liquid and is typically present in chamber  20 . The reference pressure medium may be a like or differing fluid and is typically within chamber  20 ′ although the fluid to be measured may be in chamber  20 ′ while the reference fluid may be in chamber  20  where the term chamber may include any container of any type. The net force produced by the differential pressure is applied to a displaceable force sensing assembly  11 . 
     The displaceable force sensing assembly  11  is comprised of a first plate  12  attached leak free to the inner periphery of flexible first member  14 , a second plate  12 ′ attached leak free to the inner periphery of flexible second member  14 ′, a permanent magnet  40  and rigid links  30  and  30 ′. Rigid link  30  is rigidly attached to first plate  12  on one end and on the other end to permanent magnet  40 . Rigid link  30 ′ is rigidly attached to second plate  12 ′ on one end and on the other end to permanent magnet  40 . 
     Plates  12  and  12 ′ are rigid non-deformable circular metal plates and are mounted coaxially where the thickness of the plates is such as to insure substantial rigidity thereof. Flexible members  14  and  14 ′ are shells of revolution of a segment of a circle wherein the segment and the axis of revolution lie substantially in the same plane as illustrated in FIG.  2 . Plate  12  together with flexible member  14  comprise a first pressure deformable member while plate  12 ′ and flexible member  14 ′ comprise a second pressure deformable member. The thickness of said shells  14  and  14 ′ is preferably made about 0.0004 inches to about 0.002 inches and may range from about 0.0003 inches to about 0.010 inches. 
     The outer periphery of flexible member  14  is attached leak free to first pressure enclosure  16 , the diameter of enclosure  16  typically being about 1½ inches, it being understood all dimensional values mentioned in this specification are for purposes of illustration and not limitation on the claims of this application. The outer periphery of flexible member  14 ′ is attached leak free to second pressure enclosure  16 ′. Enclosures  16  and  16 ′ are rigidly connected to a frame or locating means  32  such that the distance between the enclosures  16  and  16 ′ might typically be about two inches. Face  18  of first plate  12  is exposed to the fluid in first chamber  20  via first fluid connection  22 . Face  18 ′ of second plate  12 ′ is exposed to the fluid in second chamber  20 ′ via second fluid connection  22 ′. A mechanical stop  24  is axial di to pressure enclosure  16  to limit the axial displacement of force sensing assembly  11 . 
     Displacement of the force sensing assembly  11  is sensed by capacitor  50  comprised of first capacitor plate  52  and second capacitor plate  54 . Position sensing unit  60  is connected to capacitor plate  54  by a connection  62  which is insulated from the frame  32  while capacitor plate  52  is connected to position sensing unit  60  via electrically conductive permanent magnet  40 , the electrically conductive rigid link  30 , the electrically conductive plate  12 , the electrically conductive flexible first member  14 , the electrically conductive pressure enclosure  16  and thence to the position sensing unit  60  via the grounds  64  and  66 . Second capacitor plate  54  is annular and is rigidly attached to annular insulator  56 . Insulator  56  is rigidly attached to frame  32 . First capacitor plate  52  is rigidly attached to permanent magnet  40  and therefore undergoes the same displacement as does the force sensing assembly  11 . 
     Force balancing unit  70  supplies current to coil  76  via electrical connections  74  which are insulated from frame  32 . Coil  76  is wound on core  78 . Core  78  is rigidly attached to core support  80 . Core support  80  is rigidly attached to frame  32 . 
     During assembly of force balancing manometer  10  and prior to rigidly linking the plates  12  and  12 ′ coaxially, the common axis of the plates  12  and  12 ′ is set horizontal and P 1  and P 2  (the respective pressures in chambers  20  and  20 ′) are set equal to the prevailing atmospheric pressure so that the pressure differential across each of plates  12  and  12 ′ is zero. The axial spacing S n , between plates  12  and  12 ′ and the position of first plate  12  relative to said frame  32  is then measured and recorded with said pressure differential equal to zero. The position of first plate  12  relative to frame  32  with zero axial force on first plate is defined as the null position. Mechanical stop  24  is adjusted to be a distance ΔY from first plate  12  with first plate  12  in the null position. ΔY is preferably about 0.005 to 0.002 inches and may be 0.0005 to 0.005 inches. Rigid links  30  and  30 ′ are then installed to maintain the axial spacing S n . 
     Also, during assembly of the force balancing manometer  10 , the position of the second capacitor plate  52  is adjusted to provide a spacing D n  between capacitor plates  52  and  54  when first plate  12  is in the null position. Spacing D n , is preferably set at about 0.0005 to 0.0015 inches and may range from about 0.0005 to 0.008 inches. The capacitance of capacitor  50  when first plate  12  is in the null position is defined as the null capacitance. 
     After assembly, force balancing manometer  10  is operated preferably with the axis of force balancing assembly  11  in a vertical orientation. In this orientation with P 1 =P 2 , the weight of assembly  11  will cause the assembly to rest on mechanical stop  24  resulting in a capacitance in capacitor  50  smaller than the null capacitance. 
     The capacitance of capacitor  50  is measured by position sensing unit  60  by means well known in the art. In operation, the capacitance is continuously compared with the null capacitance and a difference signal is produced. The difference signal is sent to the force balancing unit  70  via connections  72 . Force balancing unit  70  provides a corresponding current in coil  76 . The magnetic field of permanent magnet  40  interacts with the current in coil  76  to produce a balancing force on permanent magnet  40 . The balancing force tends to move said plate  12  toward the null position. As the capacitance of capacitor  50  approaches the null capacitance, the difference signal tends to zero. 
     The resulting current in coil  76  may be accurately related to the prevailing pressure differential P 1 −P 2  by calibrating force balancing manometer  10 . First, the current required in coil  76  to restore the capacitance in capacitor  50  to the null capacitance with P 1 =P 2  is measured and preferably stored electronically in the position sensing unit  60 . 
     Then one or more known pressure differentials P 1 −P 2  are created and the current required in coil  76  to restore the capacitance in capacitor  50  to the null capacitance is measured and preferably stored electronically in the position sensing unit  60  for each known pressure differential P 1 −P 2 . The required currents and corresponding known pressure differentials P 1 −P 2  are converted to a differential pressure vs. required current calibration curve as is well known in the art. 
     An unknown pressure differential may then be determined by measuring the current required in coil  76  to restore the capacitance in capacitor  50  to the null capacitance and using the calibration curve to identify the corresponding pressure differential where the current measuring means (not shown) may be disposed in force balancing unit  70 . 
     In operation, the differential pressure is applied to displaceable force sensing assembly  11  and deviations of the force sensing assembly from the null position are sensed by sensing unit  60 . A servosystem controls force balancing unit  70  acting on the force sensing assembly  11  so as to return the force sensing assembly to its null position. The force required to restore the force sensing assembly to the null position is calibrated to correspond to the differential pressure acting on the force sensing assembly. 
     The present invention provides flexibly suspended circular first plate  12  to isolate first chamber  20  at pressure P 1  from the atmosphere and a flexibly suspended circular second plate  12 ′ to isolate a second chamber  20 ′ at pressure P 2  from the atmosphere. A rigid link joins the two plates and locates the plates coaxially. The flexibly suspended plates are provided with mechanical stop  24  and capacitor plates  52  and  54  to limit axial displacement of the plates to a very small range Δx. Δx is preferably about 0.003-0.006 inches but may range from 0.002 to 0.030 inches. 
     The first and second circular plates  12  and  12 ′ are preferably flat and relatively thick and stiff. First and second flexible suspension members  14  and  14 ′ are preferably very thin shells of revolution of a segment of a circle. A thin shell of revolution has a relatively high spring rate when it is clamped at both its inner and outer edges. However, it has been found in accordance with the present invention that by clamping the outer edge of a thin shell of revolution such as to pressure enclosure  16  or  16 ′ and joining the inner edge of the shell to the outer edge of a stiff circular plate such as plate  12  or  12 ′ that the resulting assembly surprisingly has a very low spring rate but can still withstand relatively high pressure differentials without damage. 
     A flexible plate with very low spring rate would suffer from the same overpressure problems as a thin diaphragm in a capacitance manometer if mechanical deformation were relied upon to provide the opposing force. The present invention uses force balancing means such as unit  70  well known in the art to provide the opposing force. In operation, the flexibly suspended plates are constrained to move within a range of axial displacement Δx where the manometer is extremely sensitive. When the manometer is not in operation the force balancing assembly rests on mechanical stop  24 . 
     When the force balancing assembly rests on stop  24 , the spring rate of the thin shells  14 ,  14 ′ increases dramatically compared to the spring rate when the assembly is not against a stop. This large change in spring rate is achieved without the need for elaborate accurately shaped stops for the thin shells as are required in capacitance manometers. Only simple mechanical stops are required. When the force balancing assembly rests against a mechanical stop, the thin flexible shells can resist relatively high pressure differentials without damage if the higher pressure is applied to the concave side. When the force balancing means is in operation the thin flexible shells are in effect clamped at their peripheries and can resist relatively high pressure differentials. However, in operation the force balancing assembly in the present invention has a very low spring rate and can thus measure very low pressure differentials as well as relatively high differentials all in the same manometer. The combination of fixed clamping when not in operation plus pseudo clamping by the force balancing means when in operation provides overpressure protection together with high sensitivity. These advantages have not been achieved simultaneously in the prior art. 
     The dual circular plates  12 ,  12 ′ are arranged with their flat faces substantially parallel. The effective areas of the flexibly suspended plates are preferably made substantially equal. The plates are joined together by a rigid link comprising rigid links  30 ,  30 ′, and permanent magnet  40  connected to that face of each plate which is exposed to atmospheric pressure P atm  although it should be understood that a pressure other than atmospheric may be employed within the manometer. In this regard, atmospheric pressure is the preferred pressure in that it can be simply introduced in the manometer through the openings through which connections  62  and  74  pass. The chambers  20 ,  20 ′ are typically arranged in a fixed location relative to rigid frame  32 . 
     As discussed above, during assembly prior to rigidly linking the two plates coaxially, the common axis of the two plates  12 ,  12 ′ is made horizontal and P 1  and P 2  in chambers  20  and  20 ′ are set equal to the prevailing atmospheric pressure so that the axial force on each plate is zero. The axial spacing S n , between the plates and the position of the first plate  12  relative to frame  32  are then measured and recorded with zero axial force present. The position of the first plate relative to the frame with zero axial force on the first plate is defined as the null position. The mechanical stop in the first chamber is spaced a distance Δy from the first plate with the first plate in the null position. The rigid link is then installed to maintain the axial spacing S n . 
     The axial displacement of the plates is preferably measured with a variable capacitor as is well known in the art. As stated above, the capacitor  50  consists of two annular capacitor plates  52  and  54 . The first capacitor plate  52  is rigidly attached to the flexibly suspended assembly. The second capacitor plate  54  is rigidly attached to the frame. During assembly the position of the second capacitor plate  54  is adjusted to provide a distance D n , between the capacitor plates when the first plate  52  is in the null position. The capacitance of the position sensing capacitor  50  when the first plate  52  is in the null position is defined as the null capacitance. 
     In operation, the manometer is operated preferably with the axis of the assembly vertical. In this orientation, with P 1 =P 2 , the weight of the assembly will cause the assembly to rest on the lower stop  24  resulting in a capacitance much smaller than that corresponding to null position. When not in operation the capacitor plates  52  and  54  may function as a mechanical stop for displacement in one direction. 
     The resulting difference in capacitance corresponding to the null position and the capacitance at any other position of the force balancing assembly is used to electronically control force balancing unit  70  which exerts a force on the plates  12 ,  12 ′ to return the plates to their null position. Force balancing means well known in the art may be used wherein an electrical current in a coil immersed in a magnetic field generates a balancing force. 
     In operation, there is a pressure difference (P atm −P 1 ) across the first plate producing a force F 1  directed toward the first chamber where 
     
       
           F   1 =( P   atm   −P   1 ) a   e1   
       
     
     and a e1  is the effective area of the flexibly suspended first plate. In operation there is a pressure difference (P atm −P 2 ) across the second plate producing a force F 2  directed toward the second chamber where 
     
       
           F   2 =( P   atm   −P   2 ) a   e2 . 
       
     
     If 
     
       
           a   e1   =a   e2 , 
       
     
     the net pressure force on the dual plate assembly is 
     
       
           F   1 − F   2 =−( P   1 − P   2 ) a   e2 . 
       
     
     Directed toward that chamber with the lower pressure. 
     When P 1 &gt;P 2  there will be a net pressure related force normal to the surface of the plates  12 ,  12 ′ directed towards the lower pressure region which net force will cause the plates to be displaced from the null position. The additional current required to counterbalance the net pressure related force can be related to the pressure difference P 1 −P 2  by calibration as has been described above and is well known in the art. 
     As shown in FIG. 2, one embodiment of the present invention provides a thin shell of revolution to suspend each plate where the diameter of the semi-circle which is used to generate the shell is perpendicular to the axis of revolution  13 . It has been found in accordance with the present invention that flexible suspensions with this geometry can readily be made from extremely thin sheet metal and still withstand relatively high pressure differences when all surfaces exposed to high pressure are concave. For example, a stainless steel flexible member with such preferred geometry and outside diameter of 1.5 inches and a thickness of only 0.001 inches will be stressed to only approximately 3600 psi when subjected to atmospheric pressure differential. Under the same conditions a flat diaphragm of the same thickness and outside diameter in a conventional capacitance manometer will be stressed to over 1,000,000 psi. Such a high stress will, of course, cause the diaphragm to burst. The flat diaphragm is subject to large bending moments whereas a thin shell of revolution of a segment of a circle is subject to negligible bending moments. 
     As shown in FIG. 3, when the lowest pressure differentials are to be measured (for example, less than 10 −4  Torr), multiple shells of revolution  100  may be employed to provide a very small spring rate where each shell may be nearly circular in cross section, two shells being shown for purposes of illustration where the number of shells may vary. The multiple shells may be joined on their inner peripheries to short inflexible spacers  102   a ,  102   b , and  102   c  where the spacers may be cylindrical in configuration and where spacer  102   a  is sealingly connected at its upper surface to pressure enclosure  16 ′ and at its lower surface is sealingly connected to the upper shell  100  shown in FIG.  3 . Intermediate spacer  102   b  is connected at its upper surface to the lower surface of the upper shell  100  and at its lower surface to the upper surface of the lower shell  100 . Cylindrical spacer  102   c  is connected at its upper surface to the lower surface of the lower shell  100  and to plate  12 ′ at its lower surface. 
     The arrangement shown in FIG. 3 would be duplicated at the lower end of the manometer with respect to plate  12  and pressure enclosure  16 . Where the shells  100  utilized at both the upper and lower portions of the manometer would have the same characteristics as that of the shells  14  and  14 ′ of FIG. 1, the only difference being that the shells  14  and  14 ′ constitute semicircular segments of a complete circle while the shells  100  of FIG. 3 are larger segments of a circle such that they are nearly circular, as described above. 
     The spacers  102   a ,  102   b  and  102   c  are not flexible and thus the flexibility of the arrangement of FIG. 3 results from the flexibility of the shells  100 . 
     Because the suspended plate assembly has a very small spring rate in the null position, extremely small pressure differentials can be readily measured. Higher pressure differentials can also be readily measured by simply increasing the balancing force. Because of the special geometry of the flexible suspensions, reasonable over pressure does not affect the accuracy of the device as commonly occurs with diaphragm manometers which rely on metal deformation to provide the opposing force. Simple mechanical stops restrict the displacement of the plates to a very small range when the restoring force means is not active and when overpressure is present. Thus, objectives 1 and 2 of the present invention mentioned hereinbefore are achieved by using thin shells of revolution with a circular segment cross-section concave on the high pressure side. 
     Because the restoring force is generated by means independent of the material properties of said plates and flexible members, the new device of the present invention provides pressure measurement which is not influenced by changes in material properties. Thus, the new device is significantly more stable than prior art devices which depend on maintaining material properties unchanged to preserve stability. Thus, objectives 3 and 4 of the present invention mentioned hereinbefore are met by the combination of the above features. 
     A preferred embodiment of the present invention provides for locating the position sensing means (capacitor  50 ) and the force balancing means (permanent magnet  40  and coil  76 ) between plates  12  and  12 ′. Thus, all required electrical circuitry may be located outside the vacuum at atmospheric pressure. Therefore, there is no need for costly and troublesome vacuum feedthroughs from the atmosphere side of the device to the vacuum side. Locating all of the position sensing and force balancing means outside the vacuum also avoids the creation of additional vacuum surfaces which if present would hinder pumpdown. Thus, the present invention achieves objectives 4 and 5 mentioned hereinbefore. 
     Locating the force balancing means (permanent magnet  40  and coil  76 ) midway between the plates minimizes several other problems as will now be explained. In order to minimize power dissipation in the force balancing means and the resulting temperature rise in the manometer, it is desirable to maintain very close spacing between the magnet  40  and the coil  76 . It is also desirable for the flexibly suspended assembly  11  to move freely and not rub against the fixed coil. However, in order to measure very low pressures, very thin flexible suspensions are required which tend to yield readily to small torque on the assembly, produced, for example, by vibration. 
     The present invention uses dual flexibly suspended plates  12  and  12 ′ spaced relatively far apart axially so as to provide a large rotational spring rate about the midpoint of the axis between the circular plates where the spacing should preferably be at least about 70% of the outer diameter of the first and second pressure deformable members. Thus, the present invention is relatively immune to small torque on the displaceable portion of the manometer quite unlike the behavior of the device described in Soviet Union Patent RU2010201. Therefore, the spacing between the magnet  40  and coil  76  may be made very small for optimum force balancing with minimum power. By locating the force balancing means (magnet  40  and coil  76 ) midway between the plates much smaller clearances between the magnet and coil may be used. With this location for the force balancing means, a significant advantage is obtained without increasing the overall axial length of the manometer. 
     In U.S. Pat. No. 3,832,618 mentioned hereinbefore, a mechanical link of unspecified length is disclosed which connects two differential pressure capsules. In U.S. Pat. No. 5,457,999 a mechanical link is disclosed which is longer than the length of an elastic vibrating member in tension employed to determine the pressure differential. 
     The present invention provides for preferably locating the position sensing capacitor plates  52  and  54  midway between the circular plates for the same reason as described above for locating the force balancing means (magnet  40  and coil  76 ) midway between the circular plates  12  and  12 ′. The spacing between the capacitor plates  52  and  54  may be made much smaller than if the capacitor plates were located elsewhere along the axis of the circular plates for the same rotational displacement of the axis. Closer spacing results in higher capacitance and thus improves the capability of resolving smaller axial displacements and thus lower pressures. By using two electromagnets the capacitor plates may be located optimally on axis and midway between the plates  12  and  12 ′ as shown in FIG.  4 . 
     Referring to FIG. 4, two electromagnets  40  and  40 ′ are illustrated where the magnets are separated by a plurality of posts  200  where typically three or four posts are disposed around the periphery of the magnets  40  and  40 ′. Movable capacitor plate  52  is mounted on permanent magnet  40  and electrically insulated therefrom by insulating spacer  202 . Stationary capacitor plate  54  is fixedly mounted with respect to frame  32  by electrically conductive plate  204  where plate  204  is provided with a plurality of openings  210  through which the posts  200  pass to thereby permit movement of the magnets  40  and  40 ′ with respect to frame  32  while capacitor plate  52  remains stationary with respect thereto. 
     The electrical connection  74  from and to force balancing unit  70  extends through the upper coils  76 ′ and thence via line  206  to the coils  76  and then back to the force balancing unit where line  206  is appropriately insulated. 
     Moreover, the electrical connection from position sensing unit  60  to capacitor plates  52  and  54  is such that a direct connection is made via insulated line  208  to movable plate  52  while the connection to stationary plate  54  is via ground  66 , ground  64 , pressure enclosure  16 , frame  32 , electrically conductive support plate  204  and thence to the stationary capacitor plate  54 . From the foregoing, it can be seen that the capacitor plates  52  and  54  are located optimally on the manometer axis and midway between the plates  12  and  12 ′. 
     Each shell used for the suspension responds predictably in a direction parallel to the axis of the plates but because each shell is very thin, each shell has a tendency to deform non-predictably when exposed to lateral or radial forces. It has been in accordance with the invention found that spacing the plates relatively far apart and tying them rigidly together significantly decreases the tendency to buckle. Thus, placing the force restoring means (magnet  40  and coil  76 ) between the plates  12  and  12 ′ rather than beyond one plate decreases the lever arm acting on the plates and helps to decrease torque on the suspensions and thus helps prevent buckling. This major benefit is also achieved without increasing the overall length of the device. 
     As stated hereinbefore, the effective areas of the flexibly suspended plates  12 ,  12 ′ are preferably made substantially equal. However, by making one of the plates substantially larger than the other, the absolute pressure may be measured where the pressures in chambers  20  and  20 ′ to be measured would be substantially equal and where the pressure in the space  85  between plates  12  and  12 ′ would be different from that of the pressure in chambers  20  and  20 ′. For instance, the space between plates  12  and  12 ′ could be evacuated rather than having atmospheric pressure therein. 
     As also stated hereinbefore, it should be understood that the various dimensional values given hereinbefore such as those for Δx, ΔY, D n , the thickness of the shells of revolution, the manometer dimensions, etc. are intended to be illustrative and not limiting on the present invention. 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.