Liquid capacitance pressure transducer technique

• US 4,425,799 A
• Filed: 06/03/1982
• Issued: 01/17/1984
• Est. Priority Date: 06/03/1982
• Status: Expired due to Term

• Current Assignee: Kavlico Corporation (Sensata Technologies Holding Plc)
• Original Assignee: Kavlico Corporation (Sensata Technologies Holding Plc)
• Inventors: Park, Kyong M.
• Primary Examiner(s): Woodiel, Donald O.
• Application Number:
  US06/384,510
• Time in Patent Office: 593 Days
• Field of Search: 73/718, 73/724, 361/283, 29/595, 29/25.41
• US Class Current: 73/718
• CPC Class Codes: G01L 9/0075 using a ceramic diaphragm, e.g. alumina, fused quartz, glass
• CPC Class Codes: Y10T 29/43 Electric condenser making

Abstract

A pressure transducer for measuring the difference in pressures between two liquids, includes two very closely spaced plates, at least one of which is a diaphragm, and opposed conductive coatings on these two closely spaced plates. The space between the two plates is evacuated, and a low viscosity silicon oil is drawn in through a conduit to completely fill the space between the two plates. The conduit leading to the space between the plates is filled up to a certain point with the low viscosity fluid, and a thin membrane or bellows tightly seals this conduit. One of the two liquids, the pressure of which is to be measured, is channelled to the outer side of the membrane or bellows; and the other of the two liquids is conducted to the outer surface of the diaphragm of the pressure transducer. Two concentric capacitors are formed by the opposed conductive coatings, with an inner variable capacitor having substantial changes as the diaphragm flexes, and the outer reference capacitor having very little change; but both of the capacitors being separated by the same dielectric liquid.

Claims

1. A capacitive sensor for measuring differential pressure between a first and a second liquid comprising:

first and second flat insulating plates, at least one of said plates being a diaphragm;

means for mounting said plates spaced apart from one another and sealed to one-another along a narrow zone spaced out from the center of said plates for facilitating free movement of said plates toward and away from one-another as the pressure across the diaphragm varies, the spacing between said plates being in the order of 0.5 to 20 thousandths of an inch,

means including conductive coatings on the inner surfaces of the facing opposed portions of said plates for forming a variable capacitor located toward the center of said plates which changes substantially with changes in differential pressure, and a reference capacitor located outward from the center of said plates near said narrow zone, said reference capacitor only charging slightly with changes in capacitance;

means for coupling and directing said first liquid to the outside of said diaphragm;

a conduit containing a thin membrane or bellows; and

a low viscosity dielectric fluid completely filling the space between said plates and said conductive coatings and filling said conduit up to and engaging said membrane or bellows;

means for conducting and directing said second liquid to the other side of said bellows,

whereby the changes in capacitance of said sensor directly indicate the pressure differential between said first and second liquid.

2. A capacitive pressure sensor as defined in claim 1 wherein said low viscosity fluid is a silicon oil.

3. A capacitive pressure sensor as defined in claim 1 wherein said variable and said reference capacitor have substantially the same area of conductive coating, and therefore have substantially the same capacitance at zero pressure differential across the diaphragm.

4. A capacitive pressure sensor as defined in claim 1 wherein the spacing between said plates is less than 5 thousandths of an inch.

5. A capacitive pressure sensor as defined in claim 1 wherein said plates are made of aluminum oxide.

6. A capacitive pressure sensor as defined in claim 1 wherein said plates are made of glass.

7. A capacitive pressure sensor as defined in claim 1 wherein one of said plates is at least several times thicker than the other, with the thinner plate being the diaphragm, and wherein said conduit includes a hole extending through the thicker plate adjacent to but inside said zone where said plates are mounted together and sealed to one-another.

8. A capacitive pressure sensor as defined in claim 7 further including an output electronic circuit chip mounted on said thicker plate.

9. A capacitive pressure sensor as defined in claim 1 wherein said membrane or bellows is a thin rubber bellows

10. A capacitive pressure sensor as defined in claim 1 wherein said space between said plates is a vacuum gas purged zone.

11. A method for manufacturing a capacitive pressure sensor for measuring differential liquid pressure between a first liquid and a second liquid, comprising:

applying conductive coatings to first and second insulating plates, at least one of which plates is a diaphragm, to form a reference and a variable capacitor when said plates are assembled;

assembling said first and second insulating plates, at least one of which is a diaphragm by firing a peripheral path of glass frit to space said conductive coatings on said plates apart by about from 0.5 to 20 thousandths of an inch;

evacuating the space between said plates and said conductive coatings;

providing a conduit extending to the space between said plates;

drawing a low viscosity dielectric liquid into the previously evacuated space between said plates, whereby the very limited space is completely filled with the dielectric liquid with no included air bubbles and with said dielectric liquid extending into said conduit;

mounting a thin very flexible membrane or bellows in said conduit in contact with the low viscosity liquid;

providing means for coupling said first liquid to the other side of said membrane or bellows; and

providing means for coupling said second liquid to the outer side of said diaphragm;

whereby the deflection of said diaghragm and the resultant capacitance change reflects the difference in pressure between the first and second liquids.