Differential pressure sensing apparatus

• US 4,398,194 A
• Filed: 02/25/1982
• Issued: 08/09/1983
• Est. Priority Date: 04/26/1979
• Status: Expired due to Term

• Current Assignee: Emerson Process Management New Zealand Ltd. (Emerson Electric Company)
• Original Assignee: Rosemount Engineering Company, Limited
• Inventors: Johnston, James S.
• Primary Examiner(s): Crosland, Donnie L.
• Application Number:
  US06/352,301
• Time in Patent Office: 530 Days
• Field of Search: 340/870.37, 340/870.05, 340/870.16, 340/626, 324/60 R, 324/60 C, 324/60 CD, 73/768, 73/774, 73/780, 73/745, 73/716, 73/718, 73/724, 323/356, 318/662, 361/283
• US Class Current: 73/718
• CPC Class Codes: G01L 13/025 using diaphragms
• CPC Class Codes: G01L 9/0072 using variations in capacitance
• CPC Class Codes: G01L 9/125 with temperature compensating means non electric temperature compensating means G01L19/04

Abstract

A differential pressure transducer is a body with a pair of opposed sensing chambers bounded by diaphragms and associated surfaces of the body. The surfaces have conducting layers and form capacitors with the metal diaphragms. The chambers are connected together and fitted with a fixed mass of oil. A signal generating circuit produces electrical signals representing the values of the two sensing capacitors which are fed to a computer to calculate the differential pressure independent of temperature.

Claims

1. A differential pressure sensing apparatus including a transducer having a body and comprising a pair of sensing chambers of substantially identical geometry and pressure response, each said chamber being defined by a diaphragm and an associated surface of the body, the diaphragm having a respective outer pressure sensing surface, and each chamber having capacitor plate means including the associated diaphragm and means on the associated body surface, whereby a corresponding deflection of each of the two diaphragms causes a corresponding change in the electrical capacity of the respective capacitor plate means,

the transducer further comprising fluid passage means between the two sensing chambers, and a fixed mass of a substantially non-compressible dielectric fluid filling said two chambers and the fluid passage means whereby a change in the differential pressure between the outer pressure sensing surfaces of the two diaphragms causes one diaphragm to deflect inwards relative to its associated sensing chamber and the other to deflect by a corresponding amount outwards relative to its associated sensing chamber,

the apparatus further including electrical signal generating means responsive to said capacitor plate means to provide at least respective signals representative of the electrical capacities of the two capacitor plate means, and means for receiving at least respective signals and for providing from such signals a compensation term which is representative of variations of the volume and hence the density of the fixed mass of said fluid and for further providing from such signals and said compensation term a value for the differential pressure which is substantially independent of variations with temperature in the dielectric constant of the dielectric fluid.

2. Apparatus as claimed in claim 1 wherein said means for receiving at least said respective signals provides an output signal substantially corresponding with the expression:

##EQU10## where C₁ and C₂ are the capacity values represented by the respective signals and a₃ is a constant.

3. Apparatus as claimed in claim 1 wherein the transducer body is moulded of electrically insulating material and has outer surfaces defining said chamber defining surfaces.

4. Apparatus as claimed in claim 3, wherein said outer surfaces of the body are each concave and shaped to conform with and support the associated diaphragm at its maximum inwards deflection.

5. Apparatus as claimed in claim 4, wherein the transducer comprises a hollow circular cylinder of metal with the body moulded into the cylinder so that said concave surfaces are at opposite ends of the cylinder.

6. Apparatus as claimed in claim 5, wherein the diaphragms are secured about their peripheries to respective ends of the metal cylinder.

7. Apparatus as claimed in claim 6, wherein the diaphragms are of metal and are connected together via the metal cylinder.

8. Apparatus as claimed in claim 3 wherein the body is moulded of a polyphosphate material.

9. Apparatus as claimed in claim 3 wherein said means on the associated body surfaces, forming said capacitor plate means with the associated diaphragms, comprise respective conducting films.

10. Apparatus as claimed in claim 1 wherein the electrical signal generating means is arranged to provide a third signal representative of the difference between the electrical capacities of the two capacitor plate means, and the means for receiving is arranged to receive the third signal also and use the value represented by the third signal in computing the value of said differential pressure.