GB2162319A - Pressure sensitive capacitor - Google Patents

Abstract

A pressure transducer which comprises a circular diaphragm 10 mounted between two annular rings 11, 12, the annular rings and diaphragm being electron beam welded together. Two plate members 13, 14, each carry a respective electrode 33, 34 and are mounted to the annular rings 11, 12 by set screws 16. One of the plate members is used, during an assembly, to tension the diaphragm through a confronting ring surface 19, the confronting ring surface 20 of the other plate member abutting the surface 19. Pressure is applied to the two sides of the diaphragm and electrodes 33, 34 mounted to the plate members 13, 14 detect movements of the diaphgram. <IMAGE>

Description

SPECIFICATION Pressure transducer The present invention relates to a pressure transducer, preferably a differential pressure transducer.

In conventional pressure transducers there is normally provided a diaphragm of conductive material, usually metal, which separates two chambers, the two chambers being connected respectively to the differential pressure source. Electrodes are provided adjacent each side of the diaphragm and changes of differential pressure are reflected in movement of the diaphragm which is in turn reflected by a change of capacitance between each of the electrodes and the diaphragm. Measurement of the changes of capacitance can be used to measure the differential pressure.

Conventional differential pressure transducers have incorporated two rings mounted on respective sides of the diaphragm around the outer circumference of the circular shaped diaphragm. These two rings have been bolted together. The two chambers have been defined by two opposite plate members, one plate member and the diaphragm defining one chamber and the other plate member and diaphragm defining the opposite chamber.

The electrodes have been separately mounted to each plate member by means of screw connections. It is necessary for the diaphragm to be tensioned and this has been done by moving the two plate members with respect to the rings in one axial direction.

This has been a particularly skilled job.

Manufacture and assembly of such a pressure transducer has been an expensive, time consuming and skilled job.

There will now be described a differential pressure transducer according to the invention which incorporates a number of substantial improvements over the prior art transducers.

The pressure transducer to be described allows for relatively simple assembly, a reduction in the number of components, and has been found in practice to provide a pressure transducer which is more accurate and provides results which are considerably more reproducible.

A differential pressure transducer according to the invention will now be described by way of example only and with reference to the accompanying drawing which shows an axial section therethrough.

The differential pressure transducer shown in the drawing comprises a circular diaphragm 10 (typically of stainless steel and of a thickness 0.025mm of and of 50mm diameter), the circular diaphragm 10 being connected to two rings 11, 1 2 at its circumferential edge by means of electron beam welding. The inner edge of each ring 11, 1 2 adjacent the diaphragm 10 is chamfered at 15. In the prior arrangement described above the connection was by means of a screw connection but the use of electron beam welding maintains a consistent connection between the two rings 11, 12 and the diaphragm 10 and simplifies assembly.

The two rings 11, 1 2 (and indeed all of the other components of the trapsducer unless otherwise stated) are of stainless steel.

Plate members 13, 14 are mounted to respective rings 11, 1 2 on opposite sides of the diaphragm 10, the plate members 13, 1 4 being connected to the rings 11, 1 2 by means of set screws 16, four of which are shown, which engage in threaded bores 1 7 in the rings 11, 12, the threaded bores extending axially of the transducer, Normally three set screws 1 6 are provided for mounting each plate member 13, 14 to its respective ring 11, 1 2 although more may be provided if desired.

Each plate member 13, 14 is circular so that its outer circumference aligns with the outer circumference of the rings 11, 1 2 and the circular diaphragm 10 and includes a circular shoulder 18, 25 which extends towards the opposite plate member, the opposite circular shoulders 18, 25 having confronting surfaces 1 9, 20 respectively. In fact, in use, the circular shoulders 18, 25 of the two plate members 13, 1 4 clamp the circular diaphragm 10 between their confronting surfaces 19, 20. The circular shoulders 18, 25 of each plate member thereby define respective chambers 21, 22 between the plate members 13, 14 respectively and the diaphragm 10.Respective bores 23, 24 pass through each plate member 13, 14 and mounted within the bores 23, 24 are hose connectors 26, 27 so that pressure may be applied to the chambers 21, 22. The hose connectors 26, 27 may be mounted in their respective bores 23,24 by means of a screw threaded engagement.

Each plate member 13, 14 includes a central bore 28, 29 respectively therethrough, through which is mounted a respective electrode 31, 32. Each electrode comprises a flat circular plate electrode 33, 34 having a spacer shoulder 36, 37 therebehind, each shoulder 36, 37 being spaced from the respective plate 13, 14 by means of a respective mica ring 38, 39.

Behind the shoulder 36, 37 each electrode 31, 32 includes a respective axially extending post 41, 42 which is spaced from the central bore 28, 29 of the respective plate member 13, 14 by means of a shaped insulating plastics sleeve 43, 44 respectively. The outermost end of each post 41, 42 is threaded and carries a respective nut 47, 48 underneath the respective nuts 47, 48 are mounted insulating washers 49, 50 so that each electrode 31, 32 is clamped to the respective plate member 13, 14 by means of the nut 47, 48, washer 49, 50 and mica ring 38, 39. Between the back nut 47, 58 and the washer 49, 50 is mounted a respective conventional electrical connector 51, 52 which may be of copper or other electrically conductive metal.

An electrical connector wire 53 is welded to the rings 11, 1 2 when they are electron beam welded to the diaphragm 10.

The mode of assembly of the apparatus will now be described. The electrodes 31, 32 are assembled to their respective plate members 13, 14 and it will be noted that because the thickness of the mica rings 38, 39 are accurate that the front surface of the plate electrodes 33, 34 are accurately spaced from the plane of the confronting surfaces 19, 20 of the respective plate member 13, 14. As is clear from the Figure the confronting surfaces 19, 20 are disposed slightly forward of the front surface of the plate electrodes 33, 34.

The assembled plate member 1 3 is mounted to the assembly of rings 11, 1 2 which have already been welded to the circular diaphragm 10 and by tightening the screws 1 6 the plate member 1 3 will be moved into engagement with the ring 11. The confronting surface 1 9 of circular shoulder 1 8 of plate member 1 3 engages the circular diaphragm 10 and by viewing the circular diaphragm 10 from below in the Figure (remembering that plate member 14 has not yet been assembled to the rings 11, 1 2) the tension in the diaphragm 10, may be visibly adjusted so that all wrinkles and deformities in the diaphragm in its untensioned state are removed.This tensioning process requires that the confronting surface 1 9 of the shoulder 1 8 should be below the plane of the joint between rings 11, 1 2 as is illustrated in the Figure although this displacement is somewhat exaggerated in the Figure for clarity.

When the diaphragm 10 is suitably tensioned the second plate member 14 is attached to the rings 11, 1 2 by means of the set screws and in this instance the set screws 1 6 are tightened because movement of the plate member 14 is limited by engagement of the confronting surface 20 with the confronting surface 1 9 through the diaphragm 10.

Because the two plate members 13, 1 4 are identical in dimensions as are the two electrodes 31, 32 the arrangement is such that the diaphragm 10 extends in a plane which is exactly equidistant from the front surface of the plate electrodes 33, 34.

In use of the apparatus electric potential is applied between the terminals 51, 53 and 52, 53 as is well known and a first pressure is applied to hose connector 26 and a second pressure to hose connector 27 If these two pressures are different then the diaphragm 10 will move from its position equidistant between the two plate electrodes 33, 34 and there will be a change of capacitance between the terminals 51, 53 and 52, 53. Measurement of these capacitances and calibration of the apparatus will be similar to that with prior known differential pressure transducers.

The differential pressure transducer of the invention described is very accurate and reproducible. Further there are no or substantially no hysteresis problems, that is the electrical outputs follow the changes of pressure accurately without any lag which depends upon the history of pressure change.

The invention is not restricted to the details of the foregoing example.

Claims (5)

1. A differential pressure transducer comprising a circular diaphragm mounted between two annular rings, the annular rings and diaphragm being electron beam welded together to form a unitary member; two substantially identical plate members, a respective plate member being mounted to each ring, each plate member being circular and having a circumference greater than the inner circumference of its associated ring and including a circular shoulder which extends towards the opposite plate member, the opposite circular shoulders of the two plate members having confronted surfaces to clamp the diaphragm therebetween, each plate member, its respective circular shoulders and the diaphragm defining respective chambers which in use are connected to receive the two pressures to be measured, each plate member including a central bore through which is mounted a respective electrode, the electrode being spaced from the plate member by an insulated ring of predetermined thickness to define the exact position of the front face of the electrode in respect to the plate member.

2. A transducer as claimed in claim 1 in which means is provided to move one of the plate members, during assembly, with respect to its associated ring so that its confronting surface tensions the diaphragm.

3. Apparatus as claimed in claim 1 or 2 in which the insulation ring of predetermined thickness comprises a washer of mica.

4. Apparatus as claimed in any of claims 1 to 3 in which the diaphragm, the two annular rings and the two plate members are of stainless steel.

5. A differential pressure transducer as claimed in claim 1 substantially as hereinbefore defined with reference to the accompanying drawings.