Sub-atmospheric pressure sensor and method

Abstract

A surface acoustic wave (SAW) pressure sensor 40 for detecting sub-atmospheric gas pressures in molecular and viscous flow pressure ranges. A substrate 15 has a sensing surface 17 preferably of a piezoelectric material. Surface acoustic waves are generated in the surface of the substrate (e.g. via interdigital transducers IDT) and detection means 20 detect a change in at least one property of the SAW (e.g. phase, velocity, amplitude, resonant frequency) caused by a damping effect. The substrate maybe mounted on a diaphragm (60, fig 3). There maybe a reference pressure sensing unit having a reference sensing surface 18 in contact with a chamber 70 at reference pressure wherein noise correcting circuitry compensates for detected changes in the pressure sensor.

Description

FIELD OF THE INVENTION The field of the invention relates to pressure sensing and in particular to the 5 sensing of pressures in the vacuum pressure region. BACKGROUND The accurate measurement of pressure across a range of vacuum pressures can be challenging. One way of performing high accuracy measurement in the sub-10 atmospheric pressure range down to 1 CHmbar is to use capacitive manometers. These measure pressure via the displacement of a diaphragm due to a change in the differential pressure across the diaphragm against a sealed evacuated chamber. One common method of measurement of the displacement is capacitive sensing. Alternative SAW sensors have been proposed which 15 measure the displacement of the diaphragm through the measurement of the induced strain on the diaphragm, by detecting the effect of the displacement on a surface acoustic wave SAW. Any device based on displacement of a diaphragm is limited in its measurement 20 range with several devices being required to measure across the full range from 10OOmbar to 10'4mbar. The other disadvantage is that they require a constant reference pressure to be maintained which may change over temperature and time. The diaphragm itself is also susceptible to change in its mechanical properties over time and through use / misuse. 25 An alternative pressure measurement technique is the spinning rotor gauge. These devices are based on the principle of measuring the gas damping, viscous or molecular damping, of a spinning ball. In contrast to a diaphragm gauge this does not require a reference pressure and can provide lower pressure 30 measurement and potentially a wider measurement range, 1 mbar to <10~6mbar, may be possible from a single device. These instruments however cannot measure up to 1000mbar. The instruments are also more costly and bulky than diaphragm devices and the spinning ball is also susceptible to damage. As such these are only suitable for use in a controlled laboratory environment. It would be desirable to provide an alternative compact, robust pressure sensor 5 able to measure across a wide sub-atmospheric pressure range in an accurate and repeatable manner. SUMMARY One aspect provides a pressure sensor for sensing pressures in the sub-10 atmospheric pressure region comprising: a substrate comprising a surface in contact with a gas whose pressure is to be measured; a driving means for generating a surface acoustic wave in said surface of said substrate; a detecting means for detecting a change in at least one property of said surface acoustic wave caused by a damping effect due to momentum transfer or drag force from 15 gas molecules interacting with said sensing surface, said detecting means being configured to determine a pressure of said gas from said detected at least one property change. It was recognised that the properties of a surface acoustic wave SAW in a 20 substrate surface are affected by the damping effect of gas molecules interacting with the surface and that this damping effect is pressure dependent. Thus, detection of changes in one or more properties of the SAW may be used to determine a pressure of a gas. This provides a compact, low cost pressure sensor that is able to measure pressures across a wide sub-atmospheric 25 pressure range. In some embodiments, said pressure sensor is for sensing pressures in the molecular flow region and said change in said at least one property of said surface acoustic wave is caused by a damping effect due to momentum transfer 30 from gas molecules impacting said sensing surface. In other embodiments, it may be for sensing pressures in the viscous flow region and the changes in the property of the SAW may be due to drag forces from gas interacting with said sensing surface. The damping effect on the oscillating structure on the surface acoustic wave due 5 to momentum transfer provides the change in property that is detected and relates to the pressure of the gas. In molecular flow it is dependent on the impacts of the gas molecules on the surface and measuring the effect of these on one or more properties of the SAW provides a repeatable and accurate way of determining the pressure in this pressure range. The pressure range may be 10 between 10-2 to 10-9 mbar, the lower limit being restricted by the sensitivity of the equipment, noise levels and intrinsic material damping of the substrate. Factors affecting the noise levels include external influences such as thermal vibrations and electronic noise. 15 In some embodiments, said detected change in said at least one property comprises at least one of the following: a change in phase, a change in velocity, a change in amplitude or a change in resonant frequency of the surface acoustic wave. 20 The damping effect of the gas will affect different properties of the SAW and the property that is measured to determine the pressure may be any one (or more) of the affected properties. The change may be measured between the input wave generated by said driving means and the output wave detected at said detecting means. 25 In some embodiments, said substrate comprises a piezoelectric material. In some embodiments, said driving means comprise one or more electrodes, in some embodiments said one or more electrodes comprise one or more interdigital transducer. The wave may be a surface acoustic wave formed in the 30 surface of a piezoelectric material, such as quartz, Lithium Niobate, Lithium Tantalate, GaAs. ln some embodiments said driving means is configured to generate said surface acoustic wave with a set frequency and amplitude, at least one of said set frequency or amplitude being selected from a number of possible values in dependence upon a pressure range to be measured. 5 The effect of the gas molecules interacting with the surface may be dependent upon the pressure range being measured and the effect may be greater if the wave has certain properties such as frequency or amplitude. In some cases, a pressure sensor that is configured to determine pressures across a relatively 10 wide pressure range may be configured to select different frequencies and / or amplitudes in dependence upon the sub-range of pressure that it is determined is to be detected. In some embodiments, the pressure sensor comprises a surface facing, and at a 15 distance of less than 10mm, preferably less than 1 mm, and more preferably less than 0.1 mm from at least a portion of said sensing surface. In order to increase the effect of the molecules on the surface, an opposing surface close to the sensing surface may be used to increase gas interactions. 20 This may cover the entire surface or just a portion of the surface. This may be particularly effective when the pressures measured are in the molecular flow range as the number of gas molecules impacting the surface will be increased as they are reflected off the facing surface and back towards the sensing surface. A potential drawback is that variations in the pressure being measured may take 25 longer to manifest at the sensing surface due to restriction of the flow of molecules into the space between the surfaces. Having the sensing surface cover only a portion may alleviate this drawback. In some embodiments, the pressure sensor is configured to monitor pressure in 30 both molecular and viscous flow pressure ranges, said detecting means comprising a plurality of methods of processing detected signals to determine a pressure, said detecting means being configured to select one of said plurality of methods in dependence upon a pressure range to be measured. Measuring pressures across a substantial pressure range may mean that the 5 process causing the damping of the oscillating structure changes and thus, the algorithm linking changes in property(s) to changes in pressure may also change. For example, there may be both molecular and viscous flow at these different pressures and the interactions and effect on the SAW will be different. Thus, the detecting means will select a particular method or algorithm for determining the 10 pressure in dependence upon the pressure range to be measured. In some embodiments, said substrate is mounted on a diaphragm, deflection of said diaphragm affecting properties of said surface acoustic wave at pressures above said molecular flow region, said detecting means being configured to 15 detect changes in at least one property of said surface acoustic wave caused by deflection of said diaphragm and to determine said pressure of said gas from said detected at least one property change. Higher pressures may be measured if the substrate is mounted on a diaphragm 20 and if in addition to detecting changes in properties due to the interaction of gas molecules with the surface, changes in the SAW properties due to the deflection of the diaphragm are also measured. Here again a different method of processing the signals may be used to convert the changes in the properties of the SAW to pressure changes that caused these property changes. In some 25 cases a different amplitude or frequency may be used for the input wave in these different scenarios. The property of the wave that is measured may also change depending on the pressure range and the physical process causing the change in wave properties (diaphragm deflection or damping). 30 In some embodiments, the pressure sensor comprises a plurality of driving means, said plurality of driving means being configured to generate surface acoustic waves in said substrate at different frequencies. Although, the pressure sensor may only have a single driving means, where it is desirable, for example, to measure in different pressure ranges and where this requires the use of different frequencies then a plurality of driving means may be 5 provided. For example, if the driving means are interdigital transducers then the frequency of the generated wave depends on the finger spacing, so an additional drive means with a different finger spacing may be added. It should be noted that different frequencies can also be achieved by activating a subset of the fingers however, a greater range of different frequencies may be achieved with a plurality 10 of driving means. In some embodiments, said plurality of driving means are arranged at different points along a length of said pressure sensor. 15 The plurality of driving means may be arranged in effect side by side, in some cases, they may be mounted on different substrates. The different substrates may have different properties and some may be more easily damped by certain pressure ranges and / or may be suitable to be driven at different frequencies and / or amplitudes. This may allow the accuracy of the pressure sensor to be 20 increased across a wide pressure range. In some embodiments, said driving means are arranged substantially orthogonally to each other, such that they generate surface acoustic waves that propagate orthogonally to each other. 25 The driving means may alternatively be arranged orthogonally to each other which may make for a more compact pressure sensor and one or other of the driving means may be activated at different times depending on the pressure range being measured. In some embodiments, the pressure sensor comprises a plurality of detecting means corresponding to said plurality of driving means. Where there are a plurality of driving means there may well be a plurality of detecting means that detect the properties of the generated and propagated wave. In some cases, one of the driving means may be configured to be driven 5 at a frequency that is high enough to be used for cleaning the sensing surface by ejecting particulates or condensates that have deposited thereon. If the driving means is to be used for cleaning then there may not be an associated detecting means. 10 A further aspect provides a pressure sensing system comprising a pressure sensor according to one aspect, and a reference unit, said reference unit comprising a reference pressure sensor comprising a substrate, driving and detecting means that correspond to said substrate, driving and detecting means of said pressure sensor, said reference pressure sensor comprising a reference 15 sensing surface in contact with a chamber at a reference pressure, said pressure sensing system comprising noise correcting circuitry configured to detect changes in signals received from said detecting means of said reference pressure sensor and to adjust signals from said detecting means of said pressure sensor to compensate for said detected changes. 20 In order to be able to more accurately measure a pressure and to be able to mitigate for changes in the environment such as temperature affecting the measurements, a reference pressure sensor may be used that preferably is substantially equivalent to the pressure sensor such that the effect of noise and 25 temperatures is substantially the same. This reference pressure sensor is measuring the pressure of a known reference pressure. In this way, changes in the measurement of the reference pressure sensor are indicative of noise in the system and can be used to adjust signals of the pressure sensor to mitigate for the effects of this noise. ln some embodiments, said reference pressure sensor substrate is a same substrate as said pressure sensor substrate, said sensing surface and said reference sensing surface being on opposing sides of said substrate. 5 An effective and compact way of providing the reference sensor may be to mount it on the opposite side of the same substrate as the pressure sensor. In this way, the sensor is both compact and also has many of the same properties and is subject to the same noise and temperature variations as the sensor. 10 In some embodiments said reference pressure sensor and said pressure sensor are mounted on either side of a flexible diaphragm. Mounting the reference pressure sensor and pressure sensor on either side of a flexible diaphragm allows the distortion of the diaphragm due to higher pressures 15 on the sensing side to affect both the sensing and reference SAW in a corresponding manner. When lower pressures are sensed and the diaphragm is no longer distorted to any appreciable amount, then the two sensors will sense either the pressure in the reference chamber or that on the sensing side. Processing circuitry may determine when the two sensors are no longer 20 indicating a similar pressure and may determine at this point that an algorithm for determining pressure from the effect of the damping of the SAW due to interaction with gas molecules should be used, as opposed to an algorithm for determining the effect of the diaphragm deflection on the SAW and relating this to pressure. 25 A yet further aspect provides a method of measuring pressures in the sub-atmospheric pressure region, said method comprising: placing a sensing surface mounted on a substrate in contact with a gas whose pressure is to be measured; generating a surface acoustic wave in said sensing surface of said substrate 30 using driving means; detecting a change in at least one property of said surface acoustic wave caused by a damping effect due to momentum transfer or a drag force from gas molecules interacting with said sensing surface; and determining a pressure of said gas from said detected property change. In some embodiments, said method is for measuring pressures in the molecular 5 flow region and said step of detecting a change in said at least one property of said surface acoustic wave comprises detecting said change caused by a damping effect due to momentum transfer from gas molecules impacting with said sensing surface; 10 Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims. 15 Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function. BRIEF DESCRIPTION OF THE DRAWINGS 20 Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which: Figure 1 schematically illustrates a pressure sensor according to an embodiment; Figure 2 schematically illustrates a pressure sensor configured to increase gas interactions according to an embodiment; 25 Figure 3 schematically illustrates a pressure sensor for measuring pressures across an increased pressure range according to an embodiment; Figure 4 schematically illustrates a pressure sensing system with associated reference pressure sensor according to an embodiment; Figure 5 schematically illustrates a pressure sensing system for measuring 30 pressures across an increased pressure range according to a further embodiment; and Figure 6 schematically illustrates a method of sensing a pressure according to an embodiment. DESCRIPTION OF THE EMBODIMENTS 5 Before discussing the embodiments in any more detail, first an overview will be provided. Embodiments utilise the sensitivity of Surface Acoustic Wave transducers to detect pressures below atmosphere and in some embodiments pressures in the io molecular flow region. Embodiments provide a device that measures the damping effect on the oscillating structure of a SAW in the surface of a substrate. This damping effect in molecular flow, is due to the momentum transfer of molecules hitting the 15 surface of the substrate and this is dependent on the number of molecules and thus, the pressure of the gas. In viscous flow it is due to the viscous drag force of the gas interacting with the surface, which again is pressure dependent. Sensors of embodiments are able to measure pressures across a wide range and 20 with good resolution at low pressures using low cost components. Sensors of embodiments may be configured as small, compact device with a simple compact readout technique making them suitable as a pressure measurement device for general use outside of a controlled laboratory environment. 25 Embodiments expose a SAW wave to the gas whose pressure is to be measured, and the difference in a property between the input and output waves are detected and used to determine pressure. The output wave will be influenced by gas damping and the damping of the wave can be measured and correlated to a given pressure. The SAW will penetrate the surface of the material by 1 30 wavelength which can be of the order of <1 urn. This means that the intrinsic material damping is very low allowing the pressure sensor to accurately measure to very low pressures. One method to measure the damping is by taking a measure of the phase difference between the input and output signals. Further enhancements of the signal sensitivity and hence the ability to measure 5 to lower pressures can be achieved by placing a second surface in close proximity, of the order of a fraction of a mm, to the SAW sensing surface to increase the number of molecule impacts. The gas molecules are effectively trapped in the gap between the two surfaces until they pass out of the structure. This increases the number of surface interactions of a given gas molecule with 10 the SAW. A disadvantage of this arrangement is that the reduction in gas conductance to the sensing surface will result in a slower response time to a change in pressure. As is the case with a spinning rotor gauge, for the proposed device at pressures 15 below approximately 10’2 mbar the damping is predicted to be linearly proportional to the pressure and has a dependence on the gas molecular mass and also on temperature. This limit of 10-2 mbar may represent the upper pressure measurement range for the device of some embodiments, however at higher pressures damping will occur due to viscous damping of the surface wave, 20 which with appropriate calibration of the device, will allow the pressure measurement range to be extended to higher pressures (=1mbar). A combination of equipment sensitivity and wavelength and frequency could be selected to increase the accuracy of this method, these factors will also be 25 related to the pressure range that the device may have acceptable accuracy for. A datum or reference wave generated by a corresponding driving means that is in contact with a controlled atmosphere, in a reference sensor, may be used to cancel out or at least reduce noise from other external elements and correct for 30 temperature induced variations in response. ln some embodiments the SAW sensor of embodiments may be combined with a diaphragm, by mounting the substrate on such a diaphragm. At lower pressure the gas damping phenomenon could be used to detect the pressure and at higher pressure the same or a secondary SAW sensor might detect the bending of the 5 diaphragm. This could provide a single compact device capable of accurately measuring pressures from WOOmbar to <10-5mbar. With such a device pressures above atmospheric pressures might also be measured. 10 The substate material might be a quartz or Lithium Niobate. The wave might be generated by electrodes in some cases by a piezoelectric actuator or an interdigital transducer (IDT). 15 Figure 1 shows a pressure sensor 40 comprising a piezoelectric substrate 15 such as quartz with a driving means 10 which in this embodiment comprises an input interdigital transducers IDT, and control means 12. The input IDT is configured to generate a surface acoustic wave SAW in a sensing surface 17 of the substrate. There is a detecting means 20 that in this embodiment comprises 20 an output interdigital transducer IDT and processing circuitry 22 configured to process the signals received from the IDT 20. Gas molecules 30 within the chamber of the pressure sensor 40 will impact the sensing surface 17 of the substrate 15 and interact with the SAW providing a damping effect of the oscillation due to momentum transfer which is detected in this embodiment as a 25 phase shift between the input and output signals. In this embodiment, the detecting means 20 will detect the phase difference that this damping effect has on the SAW and processing circuitry 22 will determine a pressure within the sensor 40 from this detected difference. 30 In some embodiments, processing circuitry 22 may be configured to execute one of a plurality of algorithms when processing signals received from the IDT 20 of the detecting means. The algorithm selected will depend on the pressure range being measured. In this regard, during molecular flow at high vacuums then the damping of the SAW due to the number of impacts and the pressure measurements can be determined based on an algorithm for these interactions. As pressures increase then the damping mechanism changes and the 5 relationship between the change in phase and the pressure may differ and require a different algorithm. In some embodiments, control means 12 may be operable to control the IDT 10 to generate SAWs of different amplitudes and in some cases different io frequencies depending on the pressure range that is to be measured. In this regard, although only one drive means 10 is shown there may be a plurality of drive means each operable to be driven at different frequencies. In some cases, these may be arranged orthogonally such that the waves are transmitted in different directions, alternatively they may be arranged in a side by side manner, 15 increasing the length of the sensor. In some embodiments, a wave may be generated at an amplitude and frequency high enough to have a cleaning effect on the surface and remove condensates or particulates that have become deposited thereon. 20 Figure 2 shows an alternative embodiment where there is a further opposing surface 50 facing the sensing surface 17. The surface is close to but spaced from the sensing surface 17, by a distance A. This additional surface 50 provides a reflecting surface for gas molecules that impact the sensing surface 17 and 25 thereby increases the number of impacts and therefore the sensitivity of the pressure sensor. This figure also shows an input 42 to the pressure sensor 40, this is where the pressure sensor may be attached to a vacuum chamber whose pressure is to be measured. A potential drawback to having the additional surface 50, is the reduced conductance for gas flowing through the inlet 42 to the 30 gap between the surfaces. This leads to a delay in pressure changes within the vacuum chamber being monitored manifesting at the sensing surface. In some embodiments, in order to address this potential drawback, the additional surface 50 may only cover a portion of the sensing surface 17. Figure 3 shows an alternative embodiment where pressure sensor 40 comprises 5 a substrate 15 similar to substrate 15 of Figures 1 and 2, with driving and detecting means (not shown) mounted on a diaphragm 60, the diaphragm covers an evacuated chamber 62. The pressure sensor may measure high vacuums in the molecular flow region by determining the damping of the SAW on the surface of the substrate 15 and it may measure higher pressures by measuring the effect io on the SAW of deflection of the diaphragm 60 at these higher pressures. The processing circuitry (not shown) associated with the detecting means will apply a different algorithm to determine the pressure from the change in the SAW at low pressures due to the damping effect of the gas compared to the change in the properties of the SAW at the higher pressures due to the deflection of the 15 diaphragm 60. Figure 4 shows a further pressure sensing system 40. Pressure sensing system 40 comprises driving means 10 and detecting means 20 on either side of a substrate 15. In this embodiment the lower side of the substrate 15 is the 20 reference portion and has a reference sensing surface 18 which is distorted with a SAW by driving means 10A of the reference sensor. Changes in the SAW in the reference surface 18 that are due to interactions with gas within reference chamber 70 are detected by detecting means 20A. Reference chamber 70 is filled with a known gas at a predetermined pressure. 25 The upper side of the substrate comprises the sensing portion of the sensing system 40 and comprises a sensing surface 17 which is distorted by a SAW using driving means 10. Changes in the SAW due to the damping effect of gas molecules impacting the sensing surface 17 is detected by detecting means 20. 30 As the reference sensing surface 18 and the pressure sensing surface 17 are formed on the same substrate and are at a substantially same temperature as each other and as the electronics that control the driving and the detecting means are also substantially the same and at a substantially same temperature then any noise that affects the reference sensor will be substantially the same as the noise that affects the sensor itself. As the reference sensor is sensing the pressure in a chamber 70 that is constructed to be held at a constant pressure, any changes 5 detected in the measurement of the reference pressure can be considered to be due to noise. Thus, the effect of noise on the pressure measurements can be detected using the reference portion and the signal from the sensing portion can be adjusted to mitigate for this noise. These measurements and adjustments may be done using processing circuitry associated with each detecting means. io In this way noise is both detected and reduced and the accuracy of the sensor is increased. Figure 5 shows a further embodiment where pressure sensing system 40 comprises two substrates mounted on either side of a flexible diaphragm 60 with 15 driving and detecting means 10, 10A, 20 20A mounted on the substrates on either side of diaphragm 60. In this embodiment the diaphragm covers and seals an evacuated reference chamber 70 at a reference pressure. The lower side of the sensing system 40 has a reference sensing surface 18 20 which is distorted with a SAW by driving means 10A of the reference sensor. Changes in the SAW in the reference surface 18 that are due to interactions with gas within reference chamber 70 and / or deflection of the diaphragm are detected by detecting means 20A. Reference chamber 70 is filled with a known gas at a predetermined pressure. 25 The upper side of the substrate comprises the sensing portion of the sensor 40 and comprises a sensing surface 17 which is distorted by a SAW using driving means 10. Changes in the SAW due to the damping effect of gas molecules impacting the sensing surface 17 and / or deflection of the diaphragm is detected 30 by detecting means 20. The pressure sensing system 40 may measure high vacuums in the molecular flow region by determining the damping of the SAW on the surface of the substrate and it may measure higher pressures by measuring the effect on the SAW of deflection of the diaphragm 60. The processing circuitry (not shown) 5 associated with the detecting means will apply a different algorithm to determine the pressure from the change in the SAW at low pressures due to the damping effect of the gas compared to the change in the properties of the SAW at the higher pressures due to the deflection of the diaphragm 60. The processing circuitry may determine the transition between these states by comparing the io measurements from each sensing surface. At higher pressures, the deflection of the diaphragm will be the predominant effect and will be felt equally by both sensing surfaces. At lower pressures where the molecular impact of gases predominates, then the effect will depend on the reference pressure in chamber 70 for surface 18 and the pressure being sensed for surface 17. Thus, 15 processing circuitry may determine which effect predominates and which algorithm to use by comparing the measurements from each sensing surface. Figure 6 shows a flow diagram schematically illustrating steps in a method according to an embodiment. In a first step S10 a sensing surface on a substrate 20 is placed in contact with the gas whose pressure is to be measured. At step S20 a surface acoustic wave is generated in the sensing surface of the substrate using driving means. At step S30 a change in at least one property of the surface acoustic wave is detected. At step S40 a pressure of the gas is determined from the detected property change. 25 In some embodiments, the method may determine the pressure range that the pressure sensor is sensing within prior to performing step S40 and in some cases prior to performing step S20. In some embodiments, in dependence upon the determined pressure range, a surface acoustic wave with a particular frequency 30 and / or amplitude appropriate for that range may be generated at step S20. In some embodiments, in dependence upon the determined pressure range an appropriate algorithm for determining pressure from the detected change in dependence upon this pressure range may be selected prior to performing step S40. 5 Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing 10 from the scope of the invention as defined by the appended claims and their equivalents. REFERENCE SIGNS 10, 10A driving means 12 control means 15 substrate 5 17 18 sensing surface reference surface 20, 20A detecting means 22 processing circuitry 30 gas molecules 10 40 pressure sensor or sensing system 50 opposing surface 60 diaphragm 62 low pressure chamber 70 reference pressure chamber 15

Claims

1. A pressure sensor for sensing pressures in the sub-atmospheric pressure region comprising:5 a substrate comprising a surface in contact with a gas whose pressure isto be measured;a driving means for generating a surface acoustic wave in said surface of said substrate;a detecting means for detecting a change in at least one property of said io surface acoustic wave caused by a damping effect due to momentum transfer or drag force from gas molecules interacting with said sensing surface, said detecting means being configured to determine a pressure of said gas from said detected at least one property change.15 2. A pressure sensor according to claim 1, wherein said detected change insaid at least one property comprises at least one of the following: a change in phase, a change in velocity, a change in amplitude or a change in resonant frequency of the received wave.20 3. A pressure sensor according to any preceding claim, wherein said drivingmeans is configured to generate said surface acoustic wave with a set frequency and amplitude, at least one of said set frequency or amplitude being selected from a number of possible values in dependence upon a pressure range to be measured.

254. A pressure sensor according to any preceding claim, comprising a surface facing, and at a distance of less than 10mm , preferably less than 1mm, more preferably less than 0.1 mm from, at least a portion of said sensing surface.30 5. A pressure sensor according to any preceding claim, said pressure sensorbeing configured to monitor pressures in both molecular and viscous flow pressure ranges, said detecting means comprising a plurality of methods ofcoprocessing detected signals to determine a pressure, said detecting means being configured to select one of said plurality of methods in dependence upon a pressure range to be measured.

56. A pressure sensor according to any preceding claim, said substrate being mounted on a diaphragm, deflection of said diaphragm affecting properties of said surface acoustic wave at pressures above said molecular flow region, said detecting means being configured to detect changes in at least one property of said surface acoustic wave caused by deflection of said diaphragm and to10 determine said pressure of said gas from said detected at least one property change.

7. A pressure sensor according to any preceding claim, said pressure sensor comprising a plurality of driving means, said plurality of driving means being15 configured to generate surface acoustic waves of different frequencies.

8. A pressure sensor according to claim 7, wherein said plurality of driving means are arranged at different points along a length of said pressure sensor.20 9. A pressure sensor according to claim 8, wherein said pressure sensorcomprises at least two different substrates, at least two of said plurality of driving means being mounted on said at least two different substrates.

10. A pressure sensor according to claim 7, wherein said driving means are25 arranged substantially orthogonally to each other, such that they generate surface acoustic waves that propagate orthogonally to each other.

11. A pressure sensor according to any one of claims 7 to 10, comprising a plurality of detecting means corresponding to said plurality of driving means.3012. A pressure sensing system comprising a pressure sensor according to any preceding claim, and a reference unit, said reference unit comprising a referencepressure sensor comprising a substrate, driving and detecting means that correspond to said substrate, driving and detecting means of said pressure sensor, said reference pressure sensor comprising a reference sensing surface in contact with a chamber at a reference pressure, said pressure sensing system5 comprising noise correcting circuitry configured to detect changes in signals received from said detecting means of said reference pressure sensor and to adjust signals from said detecting means of said pressure sensor to compensate for said detected changes.10 13. A pressure sensing system according to claim 12, wherein said referencepressure sensor substrate is a same substrate as said pressure sensor substrate, said sensing surface and said reference sensing surface being on opposing sides of said substrate.15 14. A method of measuring pressures in the sub-atmospheric pressure region,said method comprising:placing a sensing surface mounted on a substrate in contact with a gas whose pressure is to be measured;generating a surface acoustic wave in said sensing surface of said20 substrate using driving means;detecting a change in at least one property of said surface acoustic wave caused by a damping effect due to momentum transfer or a drag force from gas molecules interacting with said sensing surface; anddetermining a pressure of said gas from said detected property change.2515. A method according to claim 14, wherein said method is for measuring pressures in the molecular flow region and said step of detecting a change in said at least one property of said surface acoustic wave comprises detecting said change caused by a damping effect due to momentum transfer from gas30 molecules impacting with said sensing surface.

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