GB2182433A

GB2182433A - Remote sensor

Info

Publication number: GB2182433A
Application number: GB8527046A
Authority: GB
Inventors: Gillies David Pitt
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1985-11-02
Filing date: 1985-11-02
Publication date: 1987-05-13
Also published as: GB8527046D0; GB2182433B

Abstract

A remote pressure sensor includes means for temperature compensation. The sensor (11) is coupled to a remote station (12) via an optical fibre link (13) and modulates incident light with a first signal corresponding to pressure and a second signal corresponding to temperature. The remote station recovers the two signals to provide a temperature compensated pressure measurement. In the embodiment the pressure sensor (11) is of the kind including a flexible element which vibrates at a frequency representing the pressure, and thereby amplitude- or frequency-modulates the incident radiation. The sensor may be provided with a temperature-dependent layer (16) of e.g. gallium arsenide, which returns frequency-shifted light, the amount of the frequency shift representing the temperature. <IMAGE>

Description

SPECIFICATION Remote sensor This invention relates to remote sensors e.g.

for use in well logging applications.

For down-well pressure sensing a high degree of resolution is required. Typically a pressure measurement should have a relative accuracy between 1 part in 104 and 1 part in 105, e.g. +1 psi in 20,000 psi. The allowable tolerance on the absolute accuracy is a little less stringent and is about + 10 psi. At such a high degree of resolution the temperature variation of the sensor response must be taken into account. At present this requires the use of two sensors, one for temperature and one for pressure, each with its associated monitoring equipment.

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

According to the invention there is provided a remote pressure sensor for providing a temperature compensated measure of pressure, the sensor including a pressure responsive element, means for illuminating the element with substantially monochromatic light, means associated with the element for modulating the incident light with a first signal corresponding to a pressure condition adjacent the sensor, and temperature responsive means associated with the element for modulating the incident light with a second signal corresponding to the temperature of the element whereby, in use, temperature compensation of the pressure signal may be effected.

In use the sensor is illuminated by a continuous light signal which signal may comprise a free space wave or a signal guided via an optical fibre. The resultant modulated signal is then demodulated to recover the temperature and pressure information. The temperature measurement is then used to provide a compensated pressure reading.

An embodiment of the invention will now be described with reference to the accompanying drawing in which the single figure is a schematic diagram of a remote sensor system.

Referring to the drawing, the sensor system comprises a pressure sensor 11 optically coupled to a remote station 12 via an optical fibre 13. The sensor 11, which is typically of the type in which a flexible element vibrates on a frequency corresponding to an applied pressure, modulates the incident light with a first signal corresponding to that pressure.

The modulated light is returned to the remote station via the fibre 13. This modulation may comprise an amplitude or frequency modulation of the optical carrier.

The remote station includes a light source 14, e.g. a solid state laser or a light emitting diode, which source is coupled to the fibre 13 via a beam splitter 15. The function of the beam splitter is to separate the 'go' and 'return' signals. In some applications separate fibres may be employed for these two signals and the beam splitter can then be dispersed with.

The sensor also modulates the incident light with a second signal corresponding to the sensor temperature. This may be achieved by a number of methods. In a preferred embodiment the sensor surface is coated with a wavelength conversion phosphor 16 the wavelength shift of which is a function of temperature. Typically we employ a gallium arsenide film for this purpose. We have found for example that a gallium arsenide surface film, when illuminated with light of a wavelength of 800 nm returns a reflected wavelength in the range 850 to 900 nm, the magnitude of the wavelength shift corresponding to the film temperature.

At the remote station 12 the wavelength of the return signal is measured by a first detector 17 and the modulation corresponding to the pressure signal is recovered via a second detector 18. The outputs of the detectors are then appropriately combined in an output circuit 19 to give a compensated pressure measurement.

Other methods of providing the temperature compensation signal may of course be employed. Thus the sensor may be provided with a coating having an optical absorption band the cut-off edge of which is temperature sensitive. If such a sensor is illuminated with light of a wavelength adjacent this cut-off the intensity of the returned signal will provide a measure of the sensor temperature.

In a further embodiment the sensor includes a temperature refractory film, e.g. of silicon, which is ion implanted with transition and/or rare earth metal ions. The presence of these ions gives rise to optical absorption peaks the positions of which are a function of temperature.

The sensor system described herein is intended for use in oil well logging applications.

It is not however limited to such applications.

1. A remote pressure sensor for providing a temperature compensated measure of pressure, the sensor including a pressure responsive element, means for illuminating the element with substantially monochromatic light, means associated with the element for modulating the incident light with a first signal corresponding to a pressure condition adjacent the sensor, and temperature responsive means associated with the element for modulating the incident light with a second signal corresponding to the temperature of the element whereby, in use, temperature compensation of the pressure signal may be effected.

2. A sensor as claimed in claim 1, wherein the temperature responsive means comprises

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Remote sensor This invention relates to remote sensors e.g. for use in well logging applications. For down-well pressure sensing a high degree of resolution is required. Typically a pressure measurement should have a relative accuracy between 1 part in 104 and 1 part in 105, e.g. +1 psi in 20,000 psi. The allowable tolerance on the absolute accuracy is a little less stringent and is about + 10 psi. At such a high degree of resolution the temperature variation of the sensor response must be taken into account. At present this requires the use of two sensors, one for temperature and one for pressure, each with its associated monitoring equipment. The object of the invention is to minimise or to overcome this disadvantage. According to the invention there is provided a remote pressure sensor for providing a temperature compensated measure of pressure, the sensor including a pressure responsive element, means for illuminating the element with substantially monochromatic light, means associated with the element for modulating the incident light with a first signal corresponding to a pressure condition adjacent the sensor, and temperature responsive means associated with the element for modulating the incident light with a second signal corresponding to the temperature of the element whereby, in use, temperature compensation of the pressure signal may be effected. In use the sensor is illuminated by a continuous light signal which signal may comprise a free space wave or a signal guided via an optical fibre. The resultant modulated signal is then demodulated to recover the temperature and pressure information. The temperature measurement is then used to provide a compensated pressure reading. An embodiment of the invention will now be described with reference to the accompanying drawing in which the single figure is a schematic diagram of a remote sensor system. Referring to the drawing, the sensor system comprises a pressure sensor 11 optically coupled to a remote station 12 via an optical fibre 13. The sensor 11, which is typically of the type in which a flexible element vibrates on a frequency corresponding to an applied pressure, modulates the incident light with a first signal corresponding to that pressure. The modulated light is returned to the remote station via the fibre 13. This modulation may comprise an amplitude or frequency modulation of the optical carrier. The remote station includes a light source 14, e.g. a solid state laser or a light emitting diode, which source is coupled to the fibre 13 via a beam splitter 15. The function of the beam splitter is to separate the 'go' and 'return' signals. In some applications separate fibres may be employed for these two signals and the beam splitter can then be dispersed with. The sensor also modulates the incident light with a second signal corresponding to the sensor temperature. This may be achieved by a number of methods. In a preferred embodiment the sensor surface is coated with a wavelength conversion phosphor 16 the wavelength shift of which is a function of temperature. Typically we employ a gallium arsenide film for this purpose. We have found for example that a gallium arsenide surface film, when illuminated with light of a wavelength of 800 nm returns a reflected wavelength in the range 850 to 900 nm, the magnitude of the wavelength shift corresponding to the film temperature. At the remote station 12 the wavelength of the return signal is measured by a first detector 17 and the modulation corresponding to the pressure signal is recovered via a second detector 18. The outputs of the detectors are then appropriately combined in an output circuit 19 to give a compensated pressure measurement. Other methods of providing the temperature compensation signal may of course be employed. Thus the sensor may be provided with a coating having an optical absorption band the cut-off edge of which is temperature sensitive. If such a sensor is illuminated with light of a wavelength adjacent this cut-off the intensity of the returned signal will provide a measure of the sensor temperature. In a further embodiment the sensor includes a temperature refractory film, e.g. of silicon, which is ion implanted with transition and/or rare earth metal ions. The presence of these ions gives rise to optical absorption peaks the positions of which are a function of temperature. The sensor system described herein is intended for use in oil well logging applications. It is not however limited to such applications. CLAIMS

2. A sensor as claimed in claim 1, wherein the temperature responsive means comprises a wavelength conversion phosphor the wavelength shift of which is temperature sensitive.

3. A sensor as claimed in claim 2, wherein the phosphor is gallium arsenide.

4. A pressure and temperature sensor substantially as described herein with reference to and as shown in the accompanying drawings.

5. A sensor system comprising a sensor as claimed in any one of claims 1 to 4 coupled to a remote sensing station via an optical fibre link.

6. A sensor system as claimed in claim 5, wherein said fibre link comprises a single optical fibre.

7. A pressure and temperature sensor system substantially as described herein with reference to and as shown in the accompanying drawings.