GB2223582A

GB2223582A - A pressure responsive device having a resonantly vibrating element in an elastic tube

Info

Publication number: GB2223582A
Application number: GB8823234A
Authority: GB
Inventors: John Christopher Greenwood
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1988-10-04
Filing date: 1988-10-04
Publication date: 1990-04-11
Anticipated expiration: 2008-10-04
Also published as: GB2223582B; GB8823234D0

Abstract

In a pressure responsive device for the measurement of high pressures, typically 500 to 1,000 atmospheres, the pressure responsive element comprises an elastic tubular member (11) e.g. of silicon which is deformed radially inwardly by the pressure. Deformation of the tubular member (11) is measured by a resonator sensor element (17) which is both driven and read optically via an optical fibre 16 in a ferrule 13. The resonator element is shaped as a square frame with a diagonal filament of silicon and it is soldered to the tube wall by a glass frit. The device may be used in oil or gas wells. <IMAGE>

Description

TRANSDUCER DEVICE This invention relates to tranducers, and in particular to devices for the measurement of high pressures.

Pressure sensing transducers are employed in a wide variety of monitoring and/or control applications.

Their use for the measurement of high pressures, e.g.

500 to 1000 atmospheres, has however been restricted by the lack of devices that can withstand such pressures whilst providing an accurate and reproducable pressure measurement. Particularly hostile conditions are to be found in oil or gas wells where high pressures are accompanied by high temperatures thus exacerbating the difficulty.

The object of the invention is to minimise or to overcome this disadvantage.

According to the invention there is provided a pressure responsive device, including an elastic tubular member, and a strain responsive element disposed within the tubular member and so arranged that in use, application of pressure to the outer surface of the tubular member inwardly deforms that member so as to apply a corresponding strain to the strain responsive element whereby to provide a measure of the applied pressure.

According to the invention there is further provided a pressure responsive device, including an elastic tubular member, a strain responsive element disposed within the tubular member and retained in abutment with the inner surface thereof, and fibre optic means associated with the strain responsive element whereby a signal from that element indicative of that strain applied thereto maybe transmitted to a remote station, the arrangement being such that application of a isostatic pressure to the outer surface of a tubular member causes that member to deform radially inwardly so as to apply a corresponding strain to the strain responsive element whereby to provide a measure of the applied pressure.

An embodiment of the invention will now be described with reference to the accompanying drawings in which: Fig. 1 is a longitudinal sectional view of the pressure responsive device; Fig. 2 is a cross sectional view of the plane x-x of the device of Fig. 1; and Fig. 3 is a general view of a strain responsive element for use in the device of Figs. 1 and 2.

Referring to figures 1 and 2, the pressure responsive device includes a tubular member (11) of annular cross section and preferably of cylindrical symmetry. The tubular member (11) is formed from an elastic material whose physical properties are such as to respond to an applied pressure but undergoing elastic rather than plastic deformation. Typically we employ single crystal silicon for the tubular member, but other elastic material such as silica, borosilicate glass, ceramic materials and steel may also be employed. One end of the tubular member (11) is sealed with a plate (12) e.g. of silicon or glass which may be electrostatically bonded to the tube end. Preferably the plate (12) has a thermal expansion co-efficient matched to that of the tube (11).

The other end of the tubular member (11) is mounted on an optical fibre ferrule (13). The tube end is located in a correspondingly shaped recess (14) in the ferrule and is secured thereto e.g. by brazing.

Advantageously, the ferrule (13) is constructed from KOVAR (Registered Trade Mark) which has a thermal expansion coefficient similar to that of silicon. The thermal properties of two materials are such that, when assembly is cooled to ambient temperature after brazing, the ferrule exacts a compressive force or prestress on the silicon tubular member. This enhances the strength of the bond between the ferrule and the tubular member.

Advantageously the tubular member is evacuated to provide an absolute pressure reference.

The ferrule (13) has an axial bore (15) in which an optical fibre (16) is located. In use this fibre provides a coupling between the pressure sensor and a remote station (not shown).

When subjected to an external pressure, typically an isostatic pressure, the wall of the tubular member (11) deforms radially inwardly. The extent of this deformation is determined by the pressure differential across the tube wall, the tube dimensions and the bulk elastic modulus of the tube material. For example, a single crystal silicon tube of 5 mm internal diameter and 1 mm wall thickness deforms radially by 1-2 microns at pressures from 500 to 1000 atmospheres (50-100 M pascals).

Deformation of the tubular member (11) in response to an applied pressure is detected and measured by a strain responsive element retained within the tubular member and in abutment with the tube inner wall.

Typically the element (17) is a resonant sensor formed from single crystal silicon, the resonator frequency being a function of the strain being applied to the sensor. Advantageously, the element (17) is bonded to the tube wall by soldering with a low melting point glass frit. The element (17) is maintained in its resonant state from energy supplied by the optical fibre (16). This fibre also transmits light modulated at the resonant frequency to the remote station.

The strain responsive element is shown in detail in Fig. 3 of the accompanying drawings. The element comprises a generally square frame (31) of single crystal silicon between two diagonally opposite corners of which a taut flexible silicon filament (32) is disposed. The filament (32) provides the resonator whose frequency is determined by the tension therein.

The other two corners of the frame (31) have mounting members (33) extending therefrom whereby, in use, the strain responsive element is mounted in the tubular member (11) with the mounting members (33) in abutment with the tube wall. When the tube (11) is subjected to an external pressure, the consequent radially inward deformation of the tube wall applies a compressive force to the members (33). This in turn distorts the frame (31) and applies a tensile force to the filament (32) thus increasing its resonant frequency, the increase corresponding to the applied pressure. Typically, the strain responsive element is formed by selective etching from a body of single crystal silicon.

The filament (32) is maintained in its oscillatory state by synchronised light pulses transmitted from the remote station via the optical fibre (16). Each pulse causes momentary heating and expansion of one side of the filament thus converting each light pulse into a mechanical pulse whereby the filament is driven at its resonant frequency. A further steady light signal transmitted via the fibre is reflected from the filament and is thereby modulated at the filament frequency. This reflected signal is used by the remote station to synchronise the drive pulse and to provide a measure of the external pressure to which the tubular member is subjected.

The pressure responsive device described above is particularly suitable as a 'down well' sensor in oil drilling and exploration. It may however also be used in other high pressure measurement applications.

Claims

CLAIMS:

1. A pressure responsive device, including an elastic tubular member, and a strain responsive element disposed within the tubular member and so arranged that, in use, application of pressure to the outer surface of the tubular member inwardly deforms that member so as to apply a corresponding strain to the strain responsive element whereby to provide a measure of the applied pressure.

2. A pressure responsive device, including an elastic tubular member, a strain responsive element disposed within the tubular member and retained in abutment with the inner surface thereof, and fibre optic means associated with the strain responsive element whereby a signal from that element indicative of that strain applied thereto may be transmitted to a remote station, the arrangement being such that application of a isostatic pressure to the outer surface of a tubular member causes that member to deform radically inwardly so as to apply a corresponding strain responsive element whereby to provide a measure of the applied pressure.

3. A pressure responsive device as claimed in claim 1 or 2, wherein said tubular member is formed from single crystal silicon.

4. A pressure responsive device as claimed in claim 1,2 or 3, wherein said strain responsive element comprises a mechanical resonator structure formed from single crystal silicon.

5. A pressure responsive device as claimed in claim 1, 2, 3 or 4, wherein said tubular member is mounted on an optical fibre ferrule whereby the device may be coupled to a remote measurement station.

6. A pressure responsive device substantially as described herein with reference to and as shown in the accompanying drawings.

7. Well logging apparatus provided with a pressure responsive device is claimed in any one of the preceding claims.