GB2227566A - Pressure transducer - Google Patents

Abstract

A remote pressure sensor for depth measurement comprises a pressure measuring device (50) operatively connected to a conduit (52) and extending to a lowermost measuring position, and indicator means responsive to a signal generated by the pressure measuring device to give an audible and/or visual indication of the pressure measured. The lowermost surface of device (50) is provided with a pressure aperture (55) closed by a diaphragm (54) so that external fluid apples pressure to a piezo crystal pressure sensing element (57) via a resilient elastomer (56). In a preferred embodiment the crystal (57) is vented to the atmosphere via a space (60) with a collapsible seal (58) interposed therebetween. <IMAGE>

Description

PRESSURE TRANSDUCER The present invention relates to pressure transducers, and particularly, although not exclusively, to silicon diaphragm gauge pressure transducers. These transducers are of a well known type and are known to include a transducer body operatively connected via a pressure tube either to a monitoring device, or where the transducer includes the monitoring device integrally, via a vent pipe to the exterior of the measured fluid.

In the first case the accuracy of the reading obtained from the device tends to be impaired by rapid temperature changes because of the expansion and/or contraction of air in the pressure tube.

This effect can, to some extent, be alleviated by use of a thermistor, but the results are still subject to error.

In the second case the transducer includes a diaphragm in direct contact with the fluid to be measured. This arrangement is adapted to give an ouput signal proportional to the pressure exerted directly on the diaphragm. In this instance, for example, transducers may incorporate a laser trimmed bridge resistor operatively connected to a pressure sensing element including a silicon chip with integral sensing diaphragm and attendant piezo resistors. Transducers of this type work satisfactorily but are subject to corrosion in the long term by fluid such as diesel and some agrochemicals.

The present invention is concerned with the use of this latter type of transducer in a corrosive atmosphere such as is to be found in diesel tanks, or agrochemical spray tanks.

In my co-pending GB-A-8818320.7 I have described a pressure sensor especially for depth measurement comprising; a pressure measuring device operatively connected to a conduit extending to a lowermost measuring position, and indicator means responsive to to a signal generated by the pressure measuring device to give an audible or visual indication of the pressure meansured, characterised in that the conduit is closed by a flexible and/or resilient film whereby the pressure sensed is transmitted to the pressure measuring device by captive fluid.

I have now found that when it is necessary to obtain direct readings from corrosive liquids such as diesel, the captive fluid is preferably replaced by a resilient adhesive elastomer.

According, therefore, to the present invention there is provided a pressure transducer for use in a corrosive atmosphere, said transducer comprising a pressure responsive element (for example, a silicon chip with integral sensing disphragm), characterised in that the pressure responsive element is overlaid by a diaphragm inert to the corrosive atmosphere, and in that a resilient adhesive elastomer is disposed between the diaphragm and the element.

The diaphragm is preferably a silicon diaphragm, but may be any suitable material, such as nitrile, depending upon the intended corrosive agent with which it is to come into contact. It will also be appreciated that if, during the period of use, the diaphragm becomes minutely perforated, as may happen as a result of physical damage or because of the reaction of corrosive liquids, this will not necessarily incapacitate the device.

The resilient adhesive elastomer is preferably a roomtemperature vulcanising silicon rubber, although other types of elastomer may be readily used so long as they do not tend to harden with time. The elastomer must also be adhesive in its non-cured form so that there is adhesive contact between the diaphragm and the pressure-measure responsive element.

It is preferred that the resilient adhesive elastomer has a Durometer hardness shore A of 23 to 30 and preferably of about 27.

It may also have an elongation percent of 350-450% and preferably about 400. The brittle point inoC should be significantly below the 0 intended operating temperature of the device, say at least 20 C below; a good value for the brittle point is therefore -500C for devices which are intended to operate in ambient air.

A suitable resilient adhesive elastomer for use in the present invention is Silastic (Registered Trade Mark) Q37062 RTV Non-corrosive adhesive sealant.

I have found that by use of the resilient elastomers in accordance with the present invention, no allowance need be made for air temperature changes relative to expansion and contraction of the ambient temperature. Thus, although corrective devices such as thermistors may be- applied if desired, these are often not necessary for most applications.

I have also found that results are significantly more accurate over a period utilising the arrangement in accordance with the invention as against the arrangements in accordance with the prior art. I have also found that corrosive agents do not attack the nitrile diaphragm which is the diaphragm of preference for use with diesel tanks.

The invention will now be described, by way of illustration only, with reference to the accompanying drawing which -- shows diagrammatically in vertical cross-section a transducer in accordance with the present invention.

In accordance with the present invention, therefore, a transducer (50) is formed with a housing (51) of a generally rectangular cross-section. The housing is formed on its upper surface with an upstanding connector portion (61) adapted in use to intimately contact the lower end of a nitrile pressure vent (52) through which passes an electric lead (53) for connection to a suitable monitor. The pressure vent (52) is sealed to the connector piece (61) by suitable adhesive means.

On the lowermost surface of the transducer (50), remote from the connector piece (51), is a pressure aperture (55). The pressure aperture (55) is obturated by a diaphragm of nitrile or silicon which extends across the floor of the lower end of the housing (51).

A pressure element (62) is provided in a down-turned condition such that it is in contact with the uppermost surface of the diaphragm (54) and has its axis substantially coaxial with that of the transducer (50). The pressure element (62) contains a pressure-responsive element which may be a piezo crystal disposed in a central aperture. The piezo crystal is in direct adhesive contact with a resilient elastomer (56) such as Silastic. The uppermost aperture in the pressure element (62) is closed by a collapsible seal (58) to which it is sealingly attached. Lead wires (63) connect the lead (53) to the pressure responsive element (57). The pressure responsive element (57) is sealed into the housing (51) by means of a sealant (59) which, nevertheless, leaves a vented space (60) extending substantially coaxially through the device.

In use, the transducer (50) in accordance with the present invention is located at the bottom of a tank containing a corrosive fluid, such as diesel or agrochemical spray. The diaphragm aperture (55) allows the liquid to come into contact with the diaphragm (54) and hence apply a pressure to the piezo crystal (57). The nitrile tube (52) is lead to the monitor which is positioned outside the body of the corrosive fluid and is vented to the exterior.

Accordingly the uppermost, non-pressure side of the piezo crystal (57) is vented to the atmosphere via the space (60). It will be appreciated that the collapsible seal (58) acts to ensure that the pressure responsive element is not subjected to corrosive atmospheres, while simultaneously ensuring that the pressure readings are not compromised.

Pressure exerted by the corrosive fluid or liquid to be measured acts upon the diaphragm (54) via the aperture (55); the pressure being transmitted via the elastomer (56) to the piezo crystal (57).

I have found that in long-term trials the pressure transmission to the piezo crystal is not impaired, but rather is enhanced in that accuracy is no longer dependent to some extent on air pressure.

Claims (15)

1. A remote pressure sensor for depth measurement comprising a pressure measuring device operatively connected to a conduit extending to a lowermost measuring position, and indicator means responsive to a signal generated by the pressure measuring device to give an audible and/or visual indication of the pressure measured; characterised in that the conduit is closed by a flexible and/or resilient film whereby the pressure sensed is transmitted to the pressure measuring device by a captive fluid.

2. A sensor according to claim 1 characterised in that the pressure measuring device is a pressure transducer including a pressure responsive element overlaid by a diaphragm inert to the atmosphere at the intended site of operation; and in that a resilient adhesive elastomer is positioned between the film and the transducer.

3. A sensor according to claim 1 or 2 characterised in that the sensor is a piezo-electric pressure sensor and the indicator means is an LED array or a digital or analogue readout.

4. A sensor according to claim 1 characterised in that the conduit is in the form of a short tube or a diaphragm which is responsive to pressure, which tube or diaphragm is operatively fixed across the transducer.

5. A sensor according to claim 4 characterised in that the captive fluid is a gas selected from air or a higher molecular weight gas.

6. A sensor according to claims 4 or 5 characterised in that the conduit is protected by an open-ended guard.

7. A sensor according to claim 1 characterised in that the conduit is elongate and the sensor is constructed and arranged to be disposed remote from the substance to be measured.

8. A sensor according to any of claims 1 to 6 characterised in that the sensor is constructed and arranged for immersion in a liquid.

9. A sensor according to claim 8 characterised by a generally cylindrical form provided with a hollow body portion provided at a first end with a chamber, said chamber having a bore for communication with the conduit, the said bore being obturated by the transducer.

10. A sensor according to claim 9 characterised in that the conduit is a short closed tube formed of a gaseous diffusion resistant plastics material, and in that the tube is protected by a hollow annular guard.

11. A method of measuring depth of a liquid which comprises measuring the pressure exerted on a closed column of a captive material contained by a flexible and/or resilient material, and converting said measurement into an audible or visual readout.

12. A method according to claim 11 characterised in that said material is a resilient adhesive elastomer.

13. A method according to claim 11 characterised iri that the material is a captive fluid.

14. A method according to claim 13 characterised in that the captive fluid is a gas having a molecular weight sufficient to ensure that the gas does not permeate the flexible and/or resilient material when under pressure.

15. A method according to either of claims 13 or 14 characterised in that the measurement is readable remotely at a maximum distance dictated by the length of multicore cable connecting the readout to the captive fluid.