GB2466929A

GB2466929A - Pressure sensor device comprising flexible diaphragm with integral optical sensor

Info

Publication number: GB2466929A
Application number: GB0900358A
Authority: GB
Inventors: Crispin Doyle
Original assignee: SMART FIBRES Ltd
Current assignee: SMART FIBRES Ltd
Priority date: 2009-01-09
Filing date: 2009-01-09
Publication date: 2010-07-14
Also published as: GB0900358D0

Abstract

A pressure sensing device comprises a housing having walls. A flexible diaphragm (shown in fig. 2) is mounted to the housing, the diaphragm having first and second sides. The housing walls and diaphragm define at least a first chamber positioned within the housing upon the first side of the diaphragm for containing a fluid. The diaphragm has an integral optical sensor 15 having an optical response in accordance with the deformation of the diaphragm caused by a differential fluid pressure between the first and second sides. The deformation of the diaphragm may be monitored by providing optical signals to the sensor and detecting the optical response of the sensor. The response may then be used to calculate a pressure differential between the first and second sides of the diaphragm. Burying / embedding the optical sensor such that it is integral with the diaphragm reduces susceptibility to damage, increases operational life and increases the temperature at which the sensor may be used.

Description

Pressure Sensing Device

Field of the Invention

The present invention relates to an optical pressure sensing device which is particularly suited for use in oil or gas wells.

Background to the Invention

The ability to monitor pressure accurately is crucial in numerous industries and applications. One particularly demanding application is in well-hole pressure monitoring in the oil and gas industry where sensing devices are used to measure absolute or differential (relative) pressure.

One known device for well-hole applications subjects a flexible diaphragm to local pressure down a well. An optical fibre containing a Fibre Bragg Grating (FBG) is attached to the diaphragm and is subjected to a mechanical strain by the deformation of the diaphragm. The magnitude of the strain can be measured optically by monitoring the corresponding change in the optical response of the FBG. The pressure differential experienced by the diaphragm can then be derived by use of a predetermined relationship between the strain in the fibre and the pressure, for example due to previous calibration experiments.

Optical sensors are advantageous in that they allow pressure monitoring without the need for the supply of electrical energy to the sensing element. Such sensors do however have associated problems and disadvantages. A particular problem is poor performance at high temperatures caused by mechanical creep in the bonding medium used to attach the fibre to the diaphragm. The effect is that the sensors lose accuracy when used in high temperature environments. This causes a shortened operational life and reduces the temperatures at which they are deemed useable by

engineers in the field.

Another problem is the fibres are relatively delicate. This means that they are not suitable for positioning on the side of the diaphragm that is in contact with the external environment. Furthermore, the fibres are susceptible to damage during fabrication of the device and also during the installation and use of the sensors in the well-hole environment.

There is need for improved devices which address these and other limitations encountered by current devices.

Summary of the Invention

In accordance with the invention there is provided a pressure sensing device comprising a housing having walls; a flexible diaphragm mounted to the housing, the diaphragm having first and second sides, wherein the housing walls and diaphragm define a first chamber positioned within the housing upon the first side of the diaphragm; wherein the diaphragm further comprises a first integral optical sensor having an optical response in accordance with the deformation of the diaphragm.

The device according to the invention provides a significant advantage over fibre-based sensors. The provision of the optical sensor as an integral part of the diaphragm means that the former problems encountered at high temperatures due to the use of bonding material are obviated since the sensor is an integral part of the diaphragm. In order to benefit to the greatest extent from the use of the integral sensor, preferably the diaphragm has a first integral waveguide for the propagation of optical signals and the first optical sensor is "written" into the waveguide. This can be achieved using the interaction of a laser with a suitable waveguide material. An integral waveguide and sensor results in the device as a whole having greatly improved performance in that it may be operated in more aggressive environments and in particular at higher temperatures. In comparison with a bonded fibre, the integral sensor and waveguide provide a more robust structure.

One advantageous structure for the waveguide and diaphragm is the use of a layered diaphragm structure having at least first and second layers, where the waveguide is formed in the first layer. As an example, the first layer may comprise a material such as silica, preferably doped with an element such as germanium for beneficial optical properties. The second layer may be formed from silicon which in such a case may act as a substrate layer.

The optical sensor itself functions by causing light signals at certain frequencies to interfere with one another. This may be achieved by the provision of a region of the first waveguide in which the refractive index of the waveguide material exhibits a periodic spatial modulation. This modulation may have a wavelength that is of similar order to the wavelength of light being propagated along the optical waveguide, which in turn may be infra-red, visible or ultra-violet light. A Waveguide Bragg Grating (WBG) may be used as the sensor. This is preferred because a WBG may be written into materials such as silica which have physical and optical properties which are highly advantageous in the demanding oil and gas well applications.

Another example is the provision of the sensor in the form of a Surface Relief Grating.

In either case, the gratings preferably operate by reflecting a specific wavelength of light in accordance with their grating periodicity. By launching a band of frequencies into the waveguide and monitoring the frequency of the reflecting light, the spatial periodicity of the grating can be measured. This can then be equated to strain within the sensor and therefore the diaphragm, this strain being a function of temperature and mechanical force upon the diaphragm. Techniques used in the measurement of pressure and temperature for fibre-based gratings can be adapted for use with the presently discussed sensors.

It is also noted here that, as an alternative to sensors having a spatial periodicity of the order of the wavelength to be monitored, the sensor may be instead formed as a Long Period Grating (LPG), which has a periodicity many times larger than that of the wavelength in question. Long Period Gratings are however transmissive devices and therefore in order to measure the periodicity of the LPG, the light which has been transmitted through such a grating, rather than reflected light, must be monitored. Long Period Gratings are typically tens of millimetres in length and therefore, even when placed at the centre of the diaphragm, the amount of strain in the LPG may vary by a significant amount at different points along its length.

The light signals are preferably provided to the sensor using a first optical fibre which is optically coupled to a first end of the first optical sensor. This may typically be coupled to an entry location of the waveguide containing the sensor. A single fibre may be used to launch signals into and receive signals from the sensor by reflection. This is advantageous since it simplifies the apparatus needed to perform the monitoring of the wavelengths. However, a second optical fibre may be optically coupled to a second end of the first optical sensor. In the case of a reflective sensor such as a WBG the reflected wavelength could be determined by monitoring for the absence of that wavelength within the transmitted signal. Similarly the second fibre would be used in the case where an LPG sensor is used since this is transmissive in nature.

Whilst the diaphragm could be provided as effectively one wall of the housing such that the housing as a whole contains a single chamber, it is preferred that the diaphragm is within the interior of the housing. In such cases the housing walls and diaphragm further define a second chamber within the housing, on the second side of the diaphragm. This is advantageous in that the diaphragm is therefore protected from the external environment by the housing.

A number of different configurations are contemplated for implementing the devices having two or more chambers. For example one of the first and second chambers may be sealed and contain a fluid at a reference pressure (which may be a vacuum).

The device in such a case may be used to measure a pressure differential with respect to the reference pressure. The other of the first and second chambers may be placed in pressure communication with the environment external to the device so as to measure the external pressure relative to that of the fluid. Alternatively each of the first and second chambers may be placed in pressure communication with the environment external to the device and in this case the relative pressure is measured between the chambers. Such a differential pressure is often useful in process monitoring, for instance in the calculation of flow.

Any chamber placed in fluid communication with the external environment may be simply "open" to the external environment through one or more apertures or conduits. Alternatively, where the external pressure is required to be transmitted to the diaphragm and yet it is desired to isolate the diaphragm physically from the environment then a pressure transfer member may be used. The pressure transfer member provides the pressure communication with the external environment whilst ensuring that the chamber in question remains sealed. A flexible diaphragm or a set of bellows may be used as such a member.

Further advantages may be provided by the use of a second optical sensor. Such a sensor may take the form of a Fibre Bragg Grating and may be positioned within an optical fibre at a location within the housing which is isolated from the pressure differential. The second sensor may be positioned within a fibre used to supply and/or receive optical signals from the first integral sensor, or indeed may be positioned within a separate fibre. Preferably however, the second optical sensor is also an optical sensor that is integral with the diaphragm. It is noted here that whilst the first optical sensor is positioned within the diaphragm at a location where it it experiences a strain in response to differential pressure across the diaphragm, this may not be the case for the second optical sensor. For example, the diaphragm has a part which undergoes a displacement or strain due to a relative pressure change between its first and second opposing surfaces. The diaphragm may also typically have a part which is not directly subjected to such relative pressures and such a part is therefore substantially unaftected by pressure changes in the external environment. It is in such an isolated part, for example at a location clamped by the walls of the housing, that the second integral sensor may be located. Nevertheless the second optical sensor is preferably located in the same waveguide as the first optical sensor. It may alternatively be located in a separate second waveguide to the first optical element.

The second optical sensor may be used to measure temperature within the device, this information being useful in order to monitor the local well-hole conditions and also for calculating the strain within the first optical sensor which is due solely to the pressure rather than the temperature. Insofar as is possible, the materials used in the construction of the sensors and diaphragm are preferably arranged to be matched in thermal properties so as to reduce strain caused by differential thermal expansion coefticients.

It is also envisaged that the housing comprises two further chambers (such as third and fourth chambers) separated by a further flexible diaphragm similar to the first.

The second optical sensor may be integral with the further diaphragm. As for the first and second chambers, one of the further chambers may be sealed and contain a fluid at a reference pressure (which may be similar or different from a reference fluid in another chamber). The other of the further chambers may be placed in pressure communication with the environment external to the device. Furthermore, the other of the further chambers may also be sealed and contain a fluid at a reference pressure which is typically the same as that of the other further chamber. In this case the further chambers may replicate the arrangement of the first and second chambers and yet be isolated from the external environment. The second sensor in this case provides a highly accurate means of measuring the effect of the temperature upon the first sensor.

Thus the devices discussed herein provide enhanced performance in terms of operational temperatures, operational life and robustness over known sensor devices.

Brief Description of the Drawings

Some examples of devices according to the invention are now described, with reference to the accompanying drawings, in which:-Figure 1 shows a schematic section of a first example device which measures relative external pressures; Figure 2 illustrates the diaphragm of the first example; Figure 3 shows a second example which uses a reference pressure and temperature compensation; Figure 4 shows a third example which measures relative external pressures and uses temperature compensation; Figure 5 shows a fourth example which measures relative external pressures and uses two reference chambers for temperature compensation; and, Figure 6 shows a fifth example which is a dual sensor device using reference pressures.

Description of Examples

A number of devices for sensing differential pressure in an oil or gas well-hole application are now discussed. A first example is shown in Figure 1 which is a schematic representation of a pressure sensing device 100. This has a housing 1 which has walls enclosing an internal space. The internal space is divided into two separate parts by a flexible diaphragm 2. The diaphragm 2 entirely separates one part of the space from the other thereby defining a first chamber 3 and a second chamber 4. In the example of Figure 1, each of the chambers 3, 4 is of approximately equal size with the housing taking the form of a hollow cylinder, the flexible diaphragm 2 defining a plane normal to the cylinder major axis. Of course the housing walls may be formed in other shapes, such as a cuboid, although a cylinder having a circular cross-section is relatively easy to fabricate. A typical wall thickness for the housing is a few hundred micrometres whereas the length and diameter of the cylinder is typically a few millimetres in the case of a sensing device having a maximum operational pressure of about 10 to 70 MPa.

The first chamber comprises a first aperture 5 which provides a channel connecting the external environment, generally indicated at 10, with the inside of the first chamber 3. Similarly a second aperture 6 is positioned in a corresponding location for the second chamber 4 such that the diaphragm 2 essentially acts as a mirror plane within the device.

The housing 1 may be formed from any suitable material such as a metal and in the present case the housing walls are formed from nickel-iron alloy 42. The mechanical requirements of the housing are that it does not undergo significant strain when subjected to operational pressures, it is corrosion resistant and is capable of withstanding repeated use. The intended field of use of the pressure sensing device is in measuring the pressure down a well-hole for example in gas or oil production optimisation. A typical maximum temperature at which the device is expected to operate in such an application is 350 degrees Celsius and a typical maximum operational pressure is 20000 psi (around 140 MPa).

Referring to the flexible diaphragm 2 in more detail, the diaphragm has a two layer structure. This is formed from a substrate layer 7, which constitutes the majority of the diaphragm material, upon which is formed an optical layer 8 which is bonded by atomic forces to the substrate layer 7. The optical layer 8 may be deposited upon the substrate layer 7 by a number of known techniques used for the fabrication of optical devices such as chemical vapour deposition. The optical layer 8 is used to contain one or more optical waveguides and sensors. The substrate layer 7 is formed from silicon and the optical layer 8 is formed from germanium doped silica in the present example. As indicated in Figure 1, the diaphragm 2 extends into the walls of the housing and it is clamped and sealed securely within the walls such that a large pressure difference may be withstood between the first and second chambers 3, 4. The part of the diaphragm 2 which forms the dividing barrier between the first and second chambers and is therefore effectively cantilevered between the walls, is a flexible part 9.

As is shown in Figure 1, positioned substantially equidistant from each of the walls of the housing, an optical sensor in the form of a Waveguide Bragg Grating (WBG) 15 is formed within the optical layer 8 by a laser writing method.

Figure 2 shows a schematic representation of the diaphragm in more detail. Here, each of the layers 7, 8 are shown as circular planar discs. A waveguide 16 is formed within optical layer 8, this spanning the surface layer 8 and passing through its centre. The waveguide is a rectangular buried structure with an increased refractive index created by for example UV laser writing (in which point defects are created in the photosensitive layer) so as to create a rectangular buried waveguide. Other alternative processes could be used to fabricate the waveguide such as mask-etched processes analogous to those used in the semiconductor fabrication industry.

At a position approximately halfway along the waveguide at the centre of the diaphragm 2, the WBG 15 is written into the waveguide 16. A first optical fibre 17 is shown coupled to one side of the waveguide 16 at the edge of the diaphragm 2. Due to the use of the WBG 15, the single optical fibre 17 may be used to direct a band of wavelengths of light along the optical waveguide 16 to the grating 15. This is illustrated by the arrow 18 with the narrow band of wavelengths being denoted NA.

The WBG 15 reflects one particular wavelength (ignoring any harmonics for simplicity of discussion) and this wavelength is indicated by the arrow 19 and shown as A1 within Figure 2. The grating 15 therefore selectively reflects a wavelength of A1 whereas the remaining wavelengths (NA-A1) are transmitted through the grating as is shown by arrow 20. These transmitted wavelengths may be simply absorbed and not used further. By measuring the frequency of the reflected light of wavelength A1 the periodicity of the WBG 15 can be measured very accurately.

As is illustrated in Figure 2, the grating 15 comprises a number of equally spaced (spatially periodic) modulations in refractive index within the waveguide 16 and it is these which cause the reflection of a specific wavelength which is determined by their physical spacing. Any distortion or strain within the WBG 15 causes the period of the grating to change slightly and this causes the wavelength of the reflective light to change accordingly.

Returning now to Figure 1, the WBG 15 and optical layer 8 are illustrated as being located upon the first chamber side of the diaphragm 2. When the pressure sensing device 100 is placed in an operational position, for example down a well-hole, a local pressure within the external environment 10 is transmitted through the aperture 5 into the first chamber 3, this pressure being represented as P2. Similarly, at a different location within the external environment, the local pressure there is transmitted through the second aperture 6 into the second chamber 4 as illustrated at P1. It will be appreciated that Figure 1 is only a schematic representation and therefore a difference in pressure P1 and P2 may be experienced due to a flow of fluid within the external environment 10. Flow measurement is used widely in process monitoring and control, including in hydrocarbon wells.

In the event that the pressure P2 is greater than that of P1 then the flexible part 9 of the diaphragm will be caused to deflect so as to produce a slightly concave surface in the first chamber 3 and a slightly convex surface in the second chamber 4. Due to the position of the WBG 15 adjacent the first chamber, then this will cause a negative strain within the WBG 15 by shortening the wavelength of the FBG 15. This in turn will result in the reflection of a shorter wavelength from the band of wavelengths NA provided to the WBG 15. Conversely, if P1 is greater than P2 then the wavelength of the grating 15 will increase, thereby causing an increase in the reflected wavelength.

Known equipment situated at or near the surface of the well can be used to determine the precise wavelength of the reflected light.

Turning now to Figure 3, a second example sensor 300 is illustrated. Reference numerals which are similar to those of Figure 1 denote similar components. In Figure 3, the first aperture 5 is dispensed with and the first chamber 3 is therefore sealed from the external environment. This first chamber 3 contains a reference fluid such as oil or brine which is pre-pressurised at a pressure PREF as shown in Figure 3. The second aperture 6 in the second chamber 4 is still present, although this time it is positioned to one side of the second chamber 4. In Figure 3, the part of the housing to one side of the chambers, illustrated at 25, is extended in thickness and the area of the diaphragm 2 is extended accordingly. Within this extended thickness of wall 25, a second WBG 26 is provided within the waveguide 16. In this case in particular, the materials for the diaphragm 2 and housing 1 are constructed from materials having a similar thermal expansion to the waveguide structure 16 such as nickel-iron alloy 42.

It should be noted here that the second WBG 26 is in an area of the waveguide 16 which is mechanically isolated from the flexible part of the diaphragm 9. The WBG 26 is preferably placed in the same waveguide 16 as that containing the first WBG 15 although it may be placed in a second separate waveguide which could be shorter.

The WBG 26 is used as a temperature sensor which enables the signal received from the WBG 15 to be compensated for temperature effects. In Figure 3, the optical fibre 17 is illustrated as bonded and aligned to the waveguide 16 such that the second WBG 26 is encountered by the input optical signals before the WBG 15 although the reverse situation is just a appropriate.

The pressure sensing device 300 monitors the pressure in the external environment in a different way to that of the device 100. The pressure within the external environment (PEn) is illustrated in Figure 3 and this is communicated to the inside of the second chamber 4 by virtue of the second aperture 6. Thus within the second chamber a pressure of PE is experienced. The diaphragm 2 therefore undergoes either a concave or convex flexure in respect to one of the chambers depending on whether or not P is greater than PREF. It should be noted here that PREF may be arranged at a pressure which provides the most accurate reading of P for the device in question at a target operational external pressure P. The wavelength reflected by the WBG 15 not only includes the effect caused by the differential pressure of PREF -P, but also an effect based upon the temperature. At higher temperatures, due to the coefficients of thermal expansion of the device materials, the reflected wavelength of the WBG 15 increases for any given pressure differential. In this device 300 the second WBG 26 can be used to determine the extent of the temperature effect. This can therefore be taken into account within the calculations for E)(1.* A further advantage of the device 300 and indeed devices with an analogous arrangement is that the WBG 15 is located on the first chamber side of the diaphragm 2. The first chamber is not exposed to the external environment 10 and therefore the operational life of the WBG 15 may be increased.

Referring now to Figure 4, a third example pressure sensing device 400 is shown, this essentially combining the features of the first device 100 and the second device 300. In this case therefore, the device 300 is modified such that the first aperture 5 is reintroduced and therefore provides fluid communication between the external environment and the interior of the first chamber 3. Thus the differential pressure between the locations of the first and second apertures 5, 6 (PE2 and P1) can be monitored by the device 400.

Figure 5 shows a further example pressure sensing device 500 having four chambers. The left hand side of the figure shows an arrangement similar to that in the device 300. In this case however, as is illustrated on the right hand side of the figure, the housing 1 is further extended to include a third chamber 30 and a fourth chamber 31. The chambers 3, 4, 30, 31 are each divided by a common diaphragm structure and therefore the diaphragm 2 is extended to span the chambers 30, 31 through a separating section of the wall of the housing. Although the diaphragm 2 is formed from a single substrate and optical layer 7, 8, the separating section of the housing between the two sets of chambers clamps the diaphragm 2 such that any movement of the diaphragm in the first and second chambers 3, 4 does not affect the part of the diaphragm dividing chambers 30, 31. Thus as is shown at 32 in Figure a second flexible part of the diaphragm is provided between the chambers 30, 31.

A single waveguide passes within the optical layer 8 between both chambers. The waveguide therefore contains the WBG 15. It also contains the second WBG 26, this second WBG 26 being positioned in a corresponding manner to that of the WBG 15, at a position centrally within the second flexible part 32 of the diaphragm between the chambers 30, 31.

The first chamber 3 contains a fluid at a reference pressure REF (which may be a vacuum). Notably there is no aperture connecting this chamber with the external environment. The second chamber 4 is connected to the external environment via the second aperture 6 and therefore has an operational pressure PE. Each of the third and fourth chambers 30, 31 does not have an aperture and is therefore enclosed and isolated from the external environment. Each of the chambers 30, 31 is filled with a fluid at a pressure PREF. As before, the WBG 26 may be in a second separate waveguide or in, as is the present case, a common waveguide. The advantage of the use of the chambers 30, 31 and the fluid with a common diaphragm 2 is that the temperature effect upon the second WBG can be very accurately measured since the arrangement is essentially identical to that of the first WBG 15 and therefore has a similar thermal response.

Turning now to Figure 6, a fifth example is shown, this being a pressure sensing device 600. The device 600 is similar in construction to the device 500, the main difference being that the chamber 31 of the device 600 is in communication with the external environment by means of a third aperture 34. Therefore the first chamber 3 and third chamber 30 are each isolated from the external environment and contain fluids at a common reference pressure PREF. The second chamber is in communication with the external environment 10 at a pressure P1 and the fourth chamber 31 is likewise in communication with its local environment at a pressure ExT2. The sensor 600 therefore provides for a dual pressure monitoring device.

Given that the temperature response of each of the sensors 15 and 26 is likely to be the same, the difference between the reflected wavelengths measured will be related to the difference between the pressures PE and P2. With a single waveguide 16 containing each of the WBGs 15, 26, a single first optical fibre 17 may be used to launch the signals into and receive the selectively reflected wavelengths from each of the WBGs.

Whilst the above examples have been described with the use of Waveguide Bragg Grating, it will be appreciated that they may also be implemented with other types of gratings such as surface relief and long-period gratings. A long-period grating would require the use of a second optical fibre, for example at the other end of the waveguide 16, due to the transmissive nature of the grating.

Claims

Claims 1. A pressure sensing device comprising: a housing having walls; a flexible diaphragm mounted to the housing, the diaphragm having first and second sides, wherein the housing walls and diaphragm define a first chamber positioned within the housing upon the first side of the diaphragm; wherein the diaphragm further comprises a first integral optical sensor having an optical response in accordance with the deformation of the diaphragm.
2. A pressure sensing device according to claim 1, wherein the diaphragm comprises a first integral waveguide for the propagation of optical signals and wherein the first optical sensor is formed within the waveguide.
3. A pressure sensing device according to claim 2, wherein the diaphragm comprises first and second layers and wherein the first waveguide is formed within the first layer.
4. A pressure sensing device according to claim 3, wherein the first layer comprises silica and the second layer comprises silicon.
5. A pressure sensing device according to claim 4, wherein the silica layer is doped with germanium.
6. A pressure sensing device according to any of the claims 2 to 5, wherein the first optical element comprises a region of the first waveguide in which the refractive index of the waveguide material exhibits a periodic spatial modulation.
7. A pressure sensing device according to claim 6, wherein the first optical sensor is a Waveguide Bragg Grating or a Surface Relief Grating.
8. A pressure sensing device according to claim 6 or claim 7, wherein the first optical sensor selectively reflects light propagating along the first waveguide according to the frequency of the light.
9. A pressure sensing device according to any of claims 1 to 6, wherein the sensor is a Long Period Grating.
10. A pressure sensing device according to any of the preceding claims, further comprising a first optical fibre which is optically coupled to a first end of the first optical sensor.
11. A pressure sensing device according to claim 10, further comprising a second optical fibre which is optically coupled to a second end of the first optical sensor.
12. A pressure sensing device according to any of the preceding claims, wherein the housing walls and diaphragm further define a second chamber within the housing, on the second side of the diaphragm.
13. A pressure sensing device according to any of the preceding claims, wherein one of the first and second chambers is sealed and contains a fluid at a reference pressure.
14. A pressure sensing device according to any of claims ito 12, wherein one of the first and second chambers is in pressure communication with the environment external to the device.
15. A pressure sensing device according claim 13 or claim 14, wherein the other of the first and second chambers is in pressure communication with the environment external to the device.
16. A pressure sensing device according to claim 14 or claim 15, wherein a pressure transfer member provides the pressure communication with the external environment.
17. A pressure sensing device according to any of the preceding claims, wherein the device contains a second optical sensor.
18. A pressure sensing device according to claim 16 when dependent upon claim 2, wherein, when a waveguide is provided in the diaphragm, the second optical sensor is located in the same waveguide as the first optical sensor.
19. A pressure sensing device according to claim 16 when dependent upon claim 2, wherein the second optical sensor is located in a separate second waveguide to the first optical sensor.
20. A pressure sensing device according to claim 18 or claim 19, wherein the second optical sensor is positioned so as to be substantially unaffected by pressure changes within the external environment.
21. A pressure sensing device according to claim 20, wherein the housing comprises two further chambers separated by a further flexible diaphragm and wherein the second optical sensor is integral with the further diaphragm.
22. A pressure sensing device according to claim 20, wherein one of the further chambers is sealed and contains a fluid at a reference pressure.
23. A pressure sensing device according to claim 21, wherein the other of the further chambers is in pressure communication with the environment external to the device.
24. A pressure sensing device according to claim 21, wherein the other of the further chambers is sealed and contains a fluid at a reference pressure.Amendments to the claims have been filed as follows: Claims 1. An oil or gas well-hole pressure sensing device comprising: a housing having walls; a flexible diaphragm mounted to the housing, the diaphragm having first and second sides, wherein the housing walls and diaphragm define a first chamber positioned within the housing upon the first side of the diaphragm and define a second chamber within the housing on the second side of the diaphragm and wherein at least one of the first and second chambers is adapted to be placed in pressure communication with the well-hole environment external to the device; wherein the diaphragm further comprises first and second layers and wherein a first integral waveguide is formed in the first layer for the propagation of optical signals, the first integral waveguide being adapted to be coupled optically to an optical fibre, wherein the first integral waveguide has a first integral optical sensor formed from a region of the first waveguide in which the refractive index of the waveguicie material exhibits a periodic spatial modulation, such that the first integral optical sensor has an optical response in accordance with the deformation of the diaphragm.2. A pressure sensing device according to claim 1, wherein the first layer comprises silica and the second layer comprises silicon. ****. : 3. A pressure sensing device according to claim 2, wherein the silica layer is * .* doped with germanium.4. A pressure sensing device according to claim 1, wherein the first optical sensor is a Waveguide Bragg Grating or a Surface Relief Grating.5. A pressure sensing device according to ciaim 1, wherein the first optical sensor selectively reflects light propagating along the first waveguide according to the frequency of the light.6. A pressure sensing device according to any of claims 1 to 3, wherein the sensor is a Long Period Grating.7. A pressure sensing device according to any of the preceding claims, further comprising a first optical fibre which is optically coupled to a first end of the first optical sensor.8. A pressure sensing device according to claim 7, further comprising a second optical fibre which is optically coupled to a second end of the first optical sensor.9. A pressure sensing device according to any of the preceding claims, wherein one of the first and second chambers is sealed and contains a fluid at a reference pressure.10. A pressure sensing device according to claim 1, further comprising a pressure transfer member which provides the pressure communication with the external environment.11. A pressure sensing device according to any of the preceding claims, wherein the device contains a second optical sensor.***. 12. A pressure sensing device according to claim 11, wherein the second optical sensor is located in the same waveguide as the first optical sensor. * * S* S. 13. A pressure sensing device according to claim 11, wherein the second optical * sensor is located in a separate second waveguide to the first optical sensor. * *S ( S SS*14. A pressure sensing device according to claim 12 or claim 13, wherein the * second optical sensor is positioned so as to be substantially unaffected by pressure changes within the external environment.15. A pressure sensing device according to claim 11, wherein the housing comprises two further chambers separated by a further flexible diaphragm and wherein the second optical sensor is integral with the further diaphragm.16. A pressure sensing device according to claim 15, wherein one of the further chambers is sealed and contains a fluid at a reference pressure.17. A pressure sensing device according to claim 16, wherein the other of the further chambers is in pressure communication with the environment external to the device.18. A pressure sensing device according to claim 16, wherein the other of the further chambers is sealed and contains a fluid at a reference pressure.S S...SS..... * S ** * * . . * S.S *SS * S. * SSSS.....