GB2091426A

GB2091426A - Pressure gauge

Info

Publication number: GB2091426A
Application number: GB8201290A
Authority: GB
Original assignee: Danfoss AS
Current assignee: Danfoss AS
Priority date: 1981-01-19
Filing date: 1982-01-18
Publication date: 1982-07-28
Also published as: DK6582A; JPS646688B2; FR2498324A1; DE3101449A1; JPS57141527A

Abstract

A pressure gauge device comprises a displaceable wall (14) impinged on one side by the pressure (Pm) to be measured and in the opposite direction by a compensating pressure (Pk) produced by means of a vapour pressure. A sensor (13) detects the position of the wall and controls heating means (10) for changing the vapour pressure. The vapour temperature determined with the aid of temperature measuring means (12) serves as a control and/or measuring signal for the pressure. <IMAGE>

Description

SPECIFICATION Improvements in and relating to a pressure transducer and/or a pressure gauge This invention relates to a pressure transducer for converting a pressure into an electric signal and more particularly to a transducer (in this case, a gauge too) which uses the signal to provide an indication of pressure, the gauge comprising a displaceable wall, such as a corrugated tube bellows or a diaphragm impinged on one side by the pressure to be measured and in the opposite direction by a compensating force, a sensor for detecting the position of the wall and setting means which are controlled thereby and which cause the compensating force to follow the pressure being measured, and a signal producer for producing an electric signal corresponding to the measured pressure.

A pressure gauge device of this kind has been proposed (SAPS 3 908 460) in which the compensating force is produced by an electromagnet with a controllable exciter current.

The displaceable wall is connected to the coil or to the core of the magnet. The position of the displaceable wall is monitored with the aid of a light sensor and, depending on departures from the normal position, the exciter current is changed by way of a pulse generator and an integrator. To display the measured pressure, a counter fed by the pulse generator acts on a display device.

Such electromagnets are expensive and require considerable space to be accommodated if elevated pressures have to be measured. Also, the control circuit is quite expensive.

The present invention is based on the problem of providing a pressure gauge device of the aforementioned kind which does not require the use of an electromagnet.

This problem is solved according to the present invention in that, for producing the compensating force, there is a vapour filled chamber on the side of the wall remote from the pressure being measured, that the setting means are heating means for changing the vapour pressure, and that the signal producer has on the input side temperature measuring means for determining a temperature corresponding to the vapour pressure.

In such a device, the vapour pressure can be very readily changed with the aid of the heating means. The subsisting temperature is a clear measure of the compensating pressure and thus of the pressure being measured. Further, the temperature can be measured electrically in a simple manner. The compensating pressure device can be kept small. The electric control circuit can be of simple construction. The manner of operation is very stable because adequately large time constants are available by the mass that is to be heated or that is cooling off.

Preferably, the vapour-filled chamber contains a liquid-vapour filling which makes it possible to associate a comparatively large pressure range with a limited temperature range.

With particular advantage, the sensor controls bistable switching means which switch the heating means on when the pressure being measured exceeds the vapour pressure being measured. This results in a very simple electric circuit because it is not necessary to control the size of the heating current, which only needs to be switched on and off.

From a constructional point of view, it is advisable to have a cup-shaped capsule of thermally conductive material which carries the heating means on the outside, bounds the vapour filled chamber together with the displaceable wall and carries on the opposite side of the wall a cover with a connection for the medium of which the pressure is to be measured. This results in a compact construction in which the measured pressure producer and the compensating pressure producer are unified in a space-saving manner.

The sensor can for example be a magnetically working proximity sensor having a permanent magnet on the wall within the capsule and a magnetic flux-responsive sensor beyond the end wall of the capsule. The end wall of the capsule should be of a material having poor magnetic conductivity in the region of the magnetically working proximity sensor. In this way it is possible to derive switching signals from the position of the displaceable wall disposed in a closed space.

Particularly simple temperature measuring means comprise a temperature responsive resistor in the vapour-filled chamber or in thermally conductive contact therewith. Such a resistor is a cheap component adapted to provide an electric temperature signal with sufficient accuracy.

In particular, the heating means may comprise a PTC resistor. This has the advantage that the heating means cannot overheat if they are defectively switched on permanently.

Further, in such a case the heating means may also form the temperature measuring means because the temperature of the heating resistor influences its resistance.

It is particularly favourable if the heating means comprise a heating wire coiled about the circumference of the capsule. The capsule therefore serves as a winding core so that the heating wire is securely positioned.

Further, the capsule should be surrounded by thermally insulating material. This reduces the energy consumption so that the arrangement can operate with comparatively small currents, the heating means may be of correspondingly weak design and the electric circuit can be constructed of small components. However, the thermal insulation should permit a certain amount of cooling such as is necessary for intermittent operation.

It is particularly recommended to have an electronic circuit which switches the heating energy on and off in response to the sensor signal and, after switching off, measures the resistance of the heating means. Consequently, only two conductors need be provided between the electric circuit and the vapour-filled chamber.

The electronic circuit may be built together with the other parts of the pressure gauge means to form an integrated circuit. This will result in a compact overall construction.

The present invention also provides a pressure transducer and/or gauge comprising a displaceable member to be acted on, in use, in one direction by a fluid pressure to be converted (and/or measured) and in the opposite direction by the pressure of a reference fluid, a sensor responsive to the position of the member, temperature-varying means controlled by the sensor to vary the temperature and hence the pressure of the reference fluid in correspondence with changes in the pressure to be converted (and/or measured), means for sensing the temperature of the reference fluid and means for producing an electric signal in response to that temperature.

A pressure gauge constructed in accordance with the present invention will now be described, by way of example, with reference to the accompanying drawing, the single figure of which diagrammatically illustrates the gauge in crosssection.

Referring to the accompanying drawing, applied to an annular plate 1 there is a cover 2 of poor thermally conductive material having a connection 3 for the pressure Pm being measured, a capsule 4 of good thermally conductive material on the other side and corrugated tube bellows 5 inserted in the middle. This produces a first chamber 6 at the pressure being measured and a second chamber 7 filled at the base with a liquid 8 and in the remaining space with vapour 9 (in equilibrium with the liquid) at a compensating pressure Pk.

A wire 11 of PTC resistance wire coiled about the capsule 4 serves as a heating means 10, and, at the same time, as a temperature measuring means 12. Switching on and off of these heating means 10 takes place depending on a sensor 13 which detects the position of displaceable wall 14 of the corrugated tube bellows 5. The sensor comprises a permanent magnet 1 5 on the movable wall 14 and a magnetic flux-dependent electronic switching device 1 6. Between the two elements, the capsule 4 has a magnetic window in the form of a diaphragm 1 7 of rust-proof steel.

The switching device 1 6 may be temperature compensated.

The heating means 10 as well as the switching device 1 6 are connected to an electronic circuit 18 which comprises a signal producer, has various electronic components 20 on a plate 19, and can for example be in the form of a thick-film circuit.

The circuit 1 8 also comprises leads 21 which can be connected to a voltage source, as well as signal leads 22 which go to a display device 23.

The parts 1 to 20 are surrounded by thermally insulating material 24 selected so that cooling is retarded but not prevented.

The device as described operates as follows.

When the compensating pressure Pk is less than the pressure Pm being measured, the displaceable wall 14 and thus the permanent magnet 15 is moved towards the steel diaphragm 1 7 so that the switching device 1 6 switches the heating means 10 on. The current flowing therethrough produces heat.The temperature within the capsule 4 rises, as does the compensating pressure P until its value is slightly above the pressure P being measured. The wall 14 with its magnet 1 5 then moves away from the steel diaphragm 17 so that the heating means 10 are switched off again.

As a result of cooling, the compensating Pk drops until the heating means are switched on again.

The displaceable wall 14 thus alternately moves up and down. The amount of movement depends on the sensitivity of the magnet fluxresponsive switching device 16 and is generally small. Since the very smallest of pressure differences are sufficient for the movement, a temperature will result for the capsule 4 and the liquid-vapour filling 8, 9 in the chamber 7 which is a very accurate replica of the compensating pressure Pk and thus the pressure Pm being measured. This is particularly so if the temperature is always measured in a certain operating condition, preferably shortly after switching the heating means 10 off.

With the aid of the electronic circuit it is possible to measure the resistance of the PTC resistance wire 11 after switching the heating means 10 off. A preferred time interval is between 0.5 and 5 seconds, particularly 1 second. After such a delay, the material of the wire has assumed the temperature of the capsule 4 and the vapour filling 9. Since the resistance depends on the temperature, the resistance corresponds to the vapour pressure.

The result of the measurement can thereafter be converted to a different electric signal in known manner, e.g. into a voltage signal between O.V and 1 O.V or into a current signal from O.mA to 20.mA, in order to produce a display in the device 23.

The temperature measuring means 1 2 can also be provided separate from the heating means; they can for example also be formed by an NTC resistor or by a thermoelement. Instead of the display device 23 it is also possible to influence a control device in response to the pressure Pm which is converted -- in this case it need not actually be measured -- to an electric signal for controlling the control device. This is, for example, of interest if an installation is to be controlled in dependence on the pressure, e.g. a refrigeration installation. The heating current can also flow continuously and have its value controlled. It is also possible to use a diaphragm instead of the corrugated tube bellows.

Claims

1. A pressure transducer (and/or gauge) comprising a displaceable member to be acted on, in use, in one direction by a fluid pressure to be converted (and/or measured) and in the opposite direction by the pressure of a reference fluid, a sensor responsive to the position of the member, temperature-varying means controlled by the sensor to vary the temperature and hence the pressure of the reference fluid in correspondence with changes in the pressure to be converted (and/or measured), means for sensing the temperature of the reference fluid and means for producing an electric signal in response to that temperature.

2. A device as claimed in claim 1, in which the reference fluid is a liquid/vapour mixture.

3. A device as claimed in claim 1 or claim 2, in which the sensor controls a bistable switching means which switches the temperature-varying means on when the pressure being converted (and/or measured) exceeds the reference pressure and off when the reference pressure exceeds the pressure being converted (and/or measured).

4. A device as claimed in any one of claims 1 to 3, in which a thermally conductive cup-shaped capsule is provided which defines, with the displaceable member, a chamber for the reference fluid and which defines, with a cover for the capsule, another chamber for receiving the fluid the pressure of which is to be converted (and/or measured), the temperature-varying means being supported by the capsule.

5. A device as claimed in claim 4, in which the sensor is magnetic and comprises two parts, one part being a permanent magnet and the other part a magnetic flux-responsive member, one part being fixed to the displaceable member and the other part being fixed outside the base of the capsule.

6. A device as claimed in claim 5, in which, at least in the region of the sensor, the capsule is made of a material having a poor magnetic conductivity.

7. A device as claimed in any one of claims 1 to 6, in which the temperature sensing means comprises a temperature-responsive resistor arranged in the reference fluid or in thermally conductive relation with that fluid.

8. A device as claimed in any one of claims 1 to 7, in which the temperature-varying means is a heating means.

9. A device as claimed in claim 8, in which the heating means is a P.T.C. resistor.

10. A device as claimed in any one of claims 1 to 9, in which a single means constitutes the temperature-varying means heating and the temperature sensing means.

11. A device as claimed in claim 4 or any one of claims 5 to 10 when appendant to claim 4, in which the temperature-varying means comprises a heating wire coiled round the circumference of the capsule.

12. A device as claimed in claim 4 or any one of claims 5 to 11 when appendent to claim 4, in which the capsule is surrounded by thermally insulating material.

1 3. A device as claimed in any one of claims 1 to 12, in which the temperature-varying means is an electrical component, and an electric circuit is provided which switches the temperature-varying means on and off in response to a signal from the sensor and which after switching off, measures the resistance of the temperature-varying means.

14. A device as claimed in claim 13, in which the circuit is built together with other parts of the device to form an integrated circuit.

1 5. A pressure gauge device comprising a displaceable wall, such as corrugated tube bellows or a diaphragm, impinged on one side by the pressure to be measured and in the opposite direction by a compensating force, a sensor for detecting the position of the wall and setting means which are controlled thereby and which cause the compensating force to follow the pressure being measured, and a signal producer for producing an electric signal corresponding to the measured pressure, characterised in that for producing the compensating force there is a vapour-filled chamber on the side of the wall remote from the pressure being measured, that the setting means are heating means for changing the vapour pressure, and that the signal producer has on the input side temperature measuring means for determining a temperature corresponding to the vapour pressure.

1 6. A pressure gauge device substantially as hereinbefore described with reference to, and as illustrated by, the accompanying drawing.

1 7. Apparatus including a device as claimed in any one of claims 1 to 1 5, wherein the electric signal is used to control operation of the apparatus.

18. Apparatus as claimed in claim 16, in which the apparatus is a refrigeration system.