Differential pressure sensor capsule with low acceleration sensitivity

• US 4,120,206 A
• Filed: 01/17/1977
• Issued: 10/17/1978
• Est. Priority Date: 01/17/1977
• Status: Expired due to Term

• Current Assignee: Rosemount Incorporated (Emerson Electric Company)
• Original Assignee: Rosemount Incorporated (Emerson Electric Company)
• Inventors: Rud, Stanley E. Jr.
• Primary Examiner(s): Woodiel, Donald O.
• Application Number:
  US05/759,837
• Time in Patent Office: 638 Days
• Field of Search: 73/398 C, 73/393, 73/407 R, 73/398 R, 73/398 AR, 73/718, 73/724, 361/283
• US Class Current: 73/718
• CPC Class Codes: G01L 19/02 Arrangements for preventing, or for compensating for, effects of inclination or acceleration of the measuring device; Zero-setting means for aneroid barometers G01L7/14
• CPC Class Codes: G01L 9/0072 using variations in capacitance

Abstract

A capacitance type differential pressure sensor which includes isolation diaphragms and incompressible fluid filling so that the sensing element does not have to be exposed to hostile fluids for pressure sensing and which isolation diaphragms are arranged in a manner to minimize sensitivity to acceleration forces encountered during use.

Claims

1. A pressure sensor for operation in environments requiring a desired design minimum temperature, comprising a housing, means defining a cavity in said housing at one end thereof, a first diaphragm fixed to said housing and enclosing said cavity, said first diaphragm having portions which deflect relative to the housing under pressure, a second flexible diaphragm sealingly fixed to said housing within said cavity and separating the first diaphragm from first portions of said cavity and defining an isolation chamber with respect to said first portions of said cavity, means to supply a pressure to be measured to said isolation chamber, said second diaphragm having a surface facing the first diaphragm, an incompressible fluid layer between substantially the entire surface of said second diaphragm facing said first diaphragm, said fluid layer being of minimal dimension in direction substantially normal to said first diaphragm and sufficient only to form a fluid layer between said diaphragms which transfers pressures in said isolation chamber acting on said second diaphragm through said fluid to change deflection of said deflecting portion of the first diaphragm and prevent the first and second diaphragms from contacting each other in the deflecting portions of the first diaphragm at said desired minimum temperature during operation of said sensor to minimize the fluid head in direction substantially normal to said first diaphragm, and means supported on second portions of said housing cavity different from the first portions to sense deflection of said first diaphragm from a reference position.

2. The combination as specified in claim 1 wherein said means to sense deflection comprises means to sense differences in electrical impedance including a generally flat plate member defining a surface facing said first diaphragm, said plate member being fixed to said housing at said second portion, and said first diaphragm including means to affect electrical impedance between the first diaphragm and said plate member, and said first diaphragm being sufficiently close to said plate member at its reference position to permit sensing changes in impedance as said first diaphragm deflects, said second flexible diaphragm having all deflecting portions thereof spaced from said first diaphragm not substantially greater than the spacing of said flat plate member from said first diaphragm.

3. The combination as specified in claim 1 wherein said second flexible diaphragm is at least part annular, and wherein said second portions are adjacent to the central axis of said first diaphragm.

4. A pressure sensor comprising a housing having an outer peripheral edge, and an end surface defined by said outer peripheral edge, a diaphragm sealingly mounted to said housing around said outer peripheral edge, said diaphragm being deflectable under pressure with respect to said end surface, a second flexible diaphragm joined to said housing on first portions only of said end surface to define an isolation chamber fluidly separated from said first mentioned diaphragm, second portions of said surface being open to said first mentioned diaphragm, means mounted on said second portions of said surface to detect deflections of said first mentioned diaphragm under pressure, a pressure connection from a pressure source to be measured open to said isolation chamber, an incompressible fluid filling all spaces between said first mentioned diaphragm said flexible diaphragm and the second portions of said surface, said fluid spacing said flexible diaphragm sufficiently far from said first mentioned diaphragm to permit a minimal layer of said incompressible fluid to remain between said first mentioned and second diaphragms during operation of said pressure sensor.

5. A pressure sensor for operation at a minimum design temperature comprising a housing having a generally central axis along which axis acceleration effects are to be minimized, said housing having an end surface generally normal to said axis at one end thereof, a first isolation diaphragm sealed with respect to first portions of said surface, and spaced from said first portions to form an isolation chamber between said first portions and said first isolation diaphragm, a pressure fitting open to said isolation chamber, a second sensing diaphragm mounted to said housing enclosing said first isolation diaphragm and second portions of said housing end surface to define a sealed chamber open to said first isolation diaphragm across substantially the entire surface of said first isolation diaphragm on an opposite side of said first isolation diaphragm from said isolation chamber and open to said second portions of said housing end surface, said first isolation diaphragm being positioned at a minimal spacing from said second sensing diaphragm across generally the entire deflecting surface of said first isolation diaphragm, and an incompressible fluid filling the chamber formed by said second sensing diaphragm with respect to said first isolation diaphragm and said second portions of said housing end surface, said incompressible fluid being minimized in volume so that when the fluid is at said minimum design temperature during use the incompressible fluid forms a thin fluid film across all portions of said first isolation diaphragm, means positioned on said second portions of said housing end surface to sense deflections of said second sensing diaphragm from a reference position comprising AC electrical impedance sensing means including a plate element mounted on said second portions of said housing end surface adjacent to said second sensing diaphragm, said second sensing diaphragm comprising means to affect electrical impedance relative to said plate element, and said plate element being sufficiently close to sense the impedance changes caused by deflection of said second sensing diaphragm, all portions of said first isolation diapharm being spaced not substantially farther from said second sensing diaphragm then said plate element in direction along said central axis.

6. The combination as specified in claim 5 wherein said planar plate element comprises a first capacitor plate, and said second diaphragm comprises a second capacitor plate.

7. A differential pressure sensor comprising a housing having a central axis, an active sensing element comprising a deflecting diaphragm extending transverse to said central axis of said sensor and deflecting in response to differentials in pressure on opposite sides of said diaphragm, separate isolation diaphragms mounted to said housing on opposite sides of said deflecting diaphragm to define two separated chambers each open to one surface of the deflecting diaphragm, said deflecting diaphragm being mounted substantially midway between said isolation diaphragms, said chambers being filled with a substantially incompressible fluid, said isolation diaphragms each being capable of being subjected to pressure on sides thereof opposite from said chambers and each comprising an annular diaphragm spaced from the deflecting diaphragm in direction along said central axis a minimal distance, said distance being selected to insure that deflection of the deflecting diaphragm caused by acceleration effects of the incompressible fluid when the pressure sensor is accelerated in direction along its central axis is not substantially greater than deflection thereof caused by the mass of the isolation diaphragms and the deflecting diaphragm during the same acceleration, said deflecting diaphragm comprising capacitor plate means, and separate fixed capacitor plate means on said housing facing opposite surfaces of said deflecting diaphragm.

8. The combination of claim 7 wherein a separate one of said isolation diaphragms surrounds each of said fixed capacitor plate means, said housing having recessed surfaces defined therein on sides of said isolation diaphragms opposite from the chambers formed by the respective isolation diaphragms, said recessed surfaces being open to pressure being sensed and forming supports for supporting the respective isolation diaphragm when each isolation diaphragm is deflected toward the respective recessed surface by high pressure on the other of said isolation diaphragms.

9. A pressure sensing module comprising a housing having a central axis and a peripheral rim, a central mounting portion surrounding the central axis and having a mounting portion peripheral edge spaced inwardly toward the central axis from the peripheral rim, a first flexible diaphragm surrounding said mounting portion and sealingly fixed to said housing adjacent the mounting portion peripheral edge and also sealingly fixed to the housing adjacent the peripheral rim, a deflecting sensing diaphragm sealingly fixed to the peripheral rim of the housing and extending across said housing to form an enclosed chamber in cooperation with the first diaphragm and the mounting portion, sensing means mounted on said mounting portion sensing displacement of the deflecting diaphragm relative to said mounting portion, said chamber being filled with a substantially incompressible fluid, and pressure sensing passage means open to a side of said first diaphragm opposite from said chamber, said sensing means comprises a first electrical sensing element carried by said mounting portion, said deflecting diaphragm comprising a second electrical sensing element, said electrical sensing elements being sensitive to relative displacement between said mounting portion and said deflecting diaphragm.

10. The sensor of claim 9 wherein said sensor module comprises two housings each constructed substantially identically and joined to said sensing diaphragm, said sensing diaphragm comprising a bisecting member.

11. The sensor of claim 10 and a peripheral weld means joining the peripheral rim of said housings and sealing the edges of the respective isolation diaphragms and the sensing diaphragm.

12. The sensor of claim 9 wherein said mounting portion comprises an electrical insulation on material, and said first and second sensing elements comprise capacitor plate means.

13. A pressure sensor comprising a housing portion having a generally central axis, said housing portion having a surface extending laterally to said axis, a first isolation diaphragm forming an isolation chamber with respect to first portions of said surface, to form a pressure receiving isolation chamber, a sensing diaphragm mounted to said housing enclosing said isolation diaphragm and second portions of said surface to define a sealed chamber open to said isolation diaphragm on one side of said isolation diaphragm opposite from said isolation chamber and open to said second portions of said surface, an incompressible fluid filling the sealed chamber, and means on said second portions of said surface to sense deflections of said sensing diaphragm from a reference position, said isolation diaphragm and said sensing diaphragm being positioned at a minimal distance from each other by said substantially incompressible fluid to reduce acceleration effects of said incompressible fluid on said sensing diaphragm during acceleration of the sensor along said central axis.

14. The sensor as specified in claim 13 wherein said means to sense deflection of said sensing diaphragm comprises AC electrical impedance sensing means.

15. The sensor as specified in claim 13 wherein said first surface portions are at least part annular around and spaced from said central axis, and said second portions of said surface are adjacent said central axis.