GB2125561A - Improved transducer for measuring the pressure of a fluid, in particular an aggressive fluid and at high temperature - Google Patents

Abstract

The invention relates to a transducer for measuring the pressure of fluids, in particular aggressive fluids and at high temperature. The transducer 10 comprises two members 11, 12 one inserted into the other so as to define an annular chamber 17 into which said fluid is admitted. On the surface of one of said members 11, which is relatively yieldable elastically, there is applied a plurality of strain gauges 18, 19 disposed in a substantially symmetrical manner. <IMAGE>

Description

SPECIFICATION Improved transducer for measuring the pressure of a fluid, in particular an aggressivefluidand at high temperature Pressuretransducerswhich use strain gauges in the form of metal wire, metal film, cermet or semiconductors are known, butthese have constructional and operational characteristics which are too closely dependent on the component materials of the resistors, with the result that such transducers are of relatively poor versatility in terms of their application.

Considerable difficulties also arise from the different deformation characteristics of the strain gauge and of the substrate on which it is positioned.

To obviate this, pressure tranducers are used comprising a circular alumina membranewhich carries strain gauges in the form of resistors applied by a silk-screen process on the region ofthe mem brane centre. Atransducer of this type is described and illustratedforexamplein Italian patentapplication 28682 An8 filed on 12October1978 in the name of Fabbrica Italiana Magneti Marelli S.p.A.

However, even in this case great difficulties remain deriving from the need to find a position forthe resistors on the substrate such as to ensure that the deformations due to the fluid pressure are as independent as possible from the fictitious deformations which deriveforexamplefromtheexpansion ofthe supports on which it rests, which takes place in the presence of thermal stresses.

The main object of the present invention is to provide a transducer able to preventthe measured data from being influenced by fictitious deformations (ie those not caused directly by the fluid) which derive for example from high temperature and other extraneous stresses, so as to obtain precise readings.

This object is attained according to the present invention by a fluid pressure measurement transducer characterised by comprising a first and second memberwhich areinserted one intothe other in such a mannerasto definean interspace into which the fluid under pressure is admitted,thefirst of said members being constructed of elastically yieldable material and carrying a plurality of strain gauges in the form of resistorswhich are isolatedfrom thefluid and are interconnected electrically in orderto emit an output signal which is a function of the fluid pressure.

The structural and operational characteristics of the present invention will be apparent from the accompanying diagrammatic drawings in which: Figure 1 is a diagrammatic longitudinal section through a pressuretransducer according to the invention, Figure 2 is a perspective view of the substrate mlative to the transducer of Figure 1 carrying strain gauges applied internally by a silk-screen process, Figures 3 showthe electrical connection circuit for the strain gauges, Figure 4 is a diagrammatic longitudinal section through a second embodiment of a pressure transducer according to the invention, Figure 5 is a perspective view of the substrate relative to the transducer of Figure 4 carrying strain gauges applied externally by a silk-screen process.

With referenceto Figurel,atransduceraccordingto the invention, indicated overall by 10, is composed of atubular member 1 1,which is relativelyyieldable elastically and is contained in a main cylindrical container 12 which receives fluid under pressure from an axial conduit 13.

The member 1, preferably of alumina, is closed at one end by a base 14 and is connected circumferentially at its other end 15 to a support ring nut 16 screwed and welded to the container 12. The member 11 thus defines, in cooperation with the inner walls of said container 12, an annular reception chamber 17 forthe fluid under pressure.

The member 11 can be joined to the base 14 and to the ring nut 16 by the known vacuum braze welding method.

On the inner walls of the member 11 there are disposed pairs of resistor strain gauges 18,19 (Figure 2) which are connected electrically into a Wheatstone bridge as in Figure 3, in such a mannerthatthe resistors subjected to the same type of deformation are connected into opposing arms ofthe bridge.

Said strain gauges 18, 19 are applied in the form of filmtothewalls by a silk-screen process using resistive pastes suitable for use up to 4000C.

One application of a transducer constructed in accordance with the invention is for example as follows.

The transducer 10 is inserted into a system containing a fluid ofwhich the pressure is to be measured, and which is at a certain temperature, for example 350"C.

The fluid penetrates through the conduit 13 into the chamber 17,then compresses and deforms the lateral walls ofthe member 11, which maintain their elastic yieldability characteristics even at high temperature.

The strain gauges 18, 19 disposed on said walls become subjected to tension, so varying their electrical resistance and in known manner emitting a signal proportional to the resistance variation.

This signal produces the pressure measurement, which is read on an instrument operationally connected to the transducer by a connector of known type, indicated overall by 20.

The advantage of this pressure transducer over the known art is firstly thatthe pairs of strain gauges 18, 19 positioned on the cylindrical member 11 as illustrated heretofore are in no way influenced by fictitious deformation due to expansion ofthe constraining and supporting parts (14,15,16). The formation of the chamber 17 is also very important, it being of minimum volume to allow a ready and precise measurementto be obtained from a negligible amount of entering fluid.

Moreover, in this first embodiment, the pressure of the fluid being measured can be very high because the alumina member 11 has a high ultimate compressive stress.

Figure4shows a further possible embodiment of the present invention in which equal or equivalent parts carry the same reference numerals increased by 100. The main difference betweenthetwoembodiments is represented by a nozzle 121 with radial bores 122 which enablesthefluidto be fed into the tubular member 111, or more specifically into the interspace 117 defined by the nozzle 121 and the inner wall of said member 111.

In this second embodiment, the pressures con cerned must necessarily be less because the walls of the tubular member 111 are subjected to tensile stress, forwhich the material used has a lower ultimate strength.

It is importantto note that in thefirst embodiment the walls ofthe main container 12 actto oppose and contain the fluid under pressure, whereas in the second embodiment said walls ofthe container 112 are not in contact with the fluid under pressure, but merely isolate and protectthestrain gauges 118,119 from the outer environment.

The positioning ofthe strain gauges is also a characteristic of the invention because in the first embodiment said strain gauges 18,19 are disposed on the inside ofthe tubular member 11, whereas in the second embodiment said pairs of strain gauges 118, 119 are disposed on the outer wall ofthe member 111 for isolation from the fluid under pressure.

Claims (5)

1. Atransducerfor measuring the pressure of a fluid, characterised by comprising a first and second member which are inserted one into the other in such a mannerasto define an interspace into which the fluid under pressure is admitted,thefirst of said members being constructed of elastically yieldable material and carrying a plurality of strain gauges in the form of resistors which are isolated from the fluid and are interconnected electrically in order to emit an output signal which is a function of the fluid pressure.

2. Atransducer as claimed in claim 1, characterised in that said first member is hollow and is internal to the second, and that said strain gauges are applied to the innerwall of the cavity of said first member.

3. Atransducer as claimed in claim 1, characterised in that said first member is hollow in order to contain the second, and carries said strain gauges on its outerwall, there also being provided a third outer memberfor containing the firsttwo.

4. Atransducer as claimed in claim 1, characterised in that the first member, oftubularform, carries strain gauges on its innerwalls, and is closed atone end by a base wheras at its other end it is rigidly constrained circumferentiallyto a ring nut connected to the second member into which the first member is inserted, to define said interspace which is fed by a conduit formed axially in said second member.

5. Atransducer as claimed in claim 1, characterised in thatthefirst member, oftubularform, carries strain gauges on its outerwalls, and is closed at one by a base whereas at its other end it is rigidly constrained circumferentiallyto a ring nut connectedto the second member which, inserted into said first member, defines said interspace which is fed by a conduit formed axially in the second member, there being presentathirdtubularmemberwhich emclosesthe two aforesaid members.