WO2007094712A1

WO2007094712A1 - A method and apparatus for monitoring accuracy of measurement of volatile gas constituents

Info

Publication number: WO2007094712A1
Application number: PCT/SE2007/000071
Authority: WO
Inventors: Bertil Hök; Gert Andersson; Håkan PETTERSSON
Original assignee: Autoliv Development Ab
Priority date: 2006-02-14
Filing date: 2007-01-26
Publication date: 2007-08-23
Also published as: GB2435099A; GB0602952D0

Abstract

A sensor arrangement for detecting a reactive component of a gas mixture, the sensor arrangement comprising: an enclosure which is selectively moveable between an open configuration, in which an interior of the enclosure may communicate with surrounding gases, and a closed configuration, in which the interior of the enclosure is substantially isolated from the surrounding gases; a catalytic sensor adapted to detect the presence of the reactive component, the sensor being located within the enclosure; and a processing arrangement operable compensate for variations in the sensitivity of the catalytic sensor and to place the enclosure in the closed configuration in response to the fulfilment of a predetermined criterion. Also disclosed is a sensor arrangement for detecting a reactive component of a gas mixture, the sensor arrangement comprising a catalytic sensor adapted to detect the presence of the reactive component and being operable to: isolate the catalytic sensor substantially from surrounding gases; determine a time constant of the decay of a signal output by the catalytic sensor, or another parameter related to the rate of decay, following the isolation of the catalytic sensor; and from the time constant or other parameter, calculate the sensitivity of the catalytic sensor to the presence of the reactive element.

Description

A Method and Apparatus for Monitoring Accuracy of Measurement of Volatile Gas Constituents

THIS INVENTION relates to a method and apparatus for monitoring the accuracy of measurement of volatile gas constituents in a gas mixture, and in particular concerns a self-calibrating sensor arrangement for measuring volatile gas constituents, and an associated method of operating the sensor arrangement.

The accurate measurement of volatile gas constituents has many applications. An important one of these is the determination of alcohol concentration within the expired air flow of an individual, for instance the driver of a vehicle. Traffic accidents caused by vehicle drivers influenced by alcohol or other drugs are a major problem in modern societies, and many countries impose strict regulations on an acceptable level of blood alcohol concentration for vehicle drivers. Measurement of the blood alcohol concentration of a particular driver is clearly important but in view of the serious consequences of either a false negative or false positive determination, it is crucial that results are accurate and may be verified.

Measurement of alcohol concentration in expired air is commonly performed by devices relying on the fact that alcohol is combustible in contrast to the normal constituents of air. Combustion, or oxidation, of alcohol (C₂H₅OH) can be described by the following reaction formula:

C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O.

This formula describes the overall combustion process, including the consumption of oxygen (O₂) and the emission of carbon dioxide (CO₂) and water (H₂O). In common with all combustion processes of organic matter, the reaction is exothermic. It will be clear from this formula that if the supply of oxygen, and the removal of carbon dioxide and water from the reaction site are unlimited, the reaction rate will be determined by the availability of alcohol, and thus its concentration.

The reaction given above involves a number of steps, some of which constitute an effective energy barrier towards spontaneous onset of the reaction. The reaction rate r, i.e. the number of combusted molecules per unit time, therefore approximately obeys a temperature dependence according to:

where T is the absolute temperature [K]₁ E₃ is the activation energy [J] of the mentioned barrier, r_a is the reaction rate reached asymptotically at very high temperature, and /c=1.38*10^"23 J/K is Boltzmann's constant.

A consequence of this and the magnitude of E₃ for alcohol and comparable fuels is that the reaction rate at normal (i.e. room) temperature is extremely slow, and is noticeable only at relatively high temperatures. It is also obvious from the above equation that by decreasing the activation energy, the reaction rate at a given temperature can increase substantially. For this reason alcohol sensors normally employ catalysts to enable reasonable operating temperatures.

The presence of alcohol, and its concentration, may be recorded by measuring the heat generated by the combustion (or, indeed by the consumption of oxygen, or any other property that may directly or indirectly be related to the combustion process). Catalysts used in sensors of this type may be inorganic (such as tin oxide, platinum or other noble metals or compounds), organic, such as enzymes, catalytic devices making use of the resistivity variations occurring in a conductive polymer, or catalytic gas sensing elements based on a metal oxide semiconductor field effect transistor (MOSFET) in which the gate includes a catalytic metal, e.g. platinum or palladium, or another noble metal.

A fundamental problem relating to catalytic sensing elements arises, however, from the fact that most, if not all, catalysts may be influenced by other substances. The catalytic action may be temporarily or permanently degraded by some substances, resulting in loss of sensitivity. Temporary inhibition may be caused by adsorbed substances influencing the catalytic reaction. If the adsorbed substance can be removed, the sensitivity of the sensor will be restored, otherwise it will exhibit a permanent loss of sensitivity.

Degradation of catalytic action in an alcohol sensor may result in false negative readings which is, of course, a serious problem in view the possible legal implications. The present solution to this is to subject all alcohol sensors to periodic recalibration. Such a procedure is manual, and requires skilful and accurate handling to provide correct and reliable results. A currently recommended time interval between calibrations is typically six months. The associated costs are an important limiting factor to the widespread use of alcohol sensing devices connecting to vehicle ignition lock systems. Also, the problem of temporary inhibition of catalytic function is not solved by repeated calibrations.

It is an object of the present invention to seek to alleviate the above problems.

Accordingly, one aspect of the present invention provides a sensor arrangement for detecting a reactive component of a gas mixture, the sensor arrangement comprising: an enclosure which is selectively moveable between an open configuration, in which an interior of the enclosure may communicate with surrounding gases, and a closed configuration, in which the interior of the enclosure is substantially isolated from the surrounding gases; a catalytic sensor adapted to detect the presence of the reactive component, the sensor being located within the enclosure, characterised by a processing arrangement operable compensate for variations in the sensitivity of the catalytic sensor and to place the enclosure in the closed configuration in response to the fulfilment of a predetermined criterion.

Advantageously, the predetermined criterion comprises the detection, by the catalytic sensor, of at least a predetermined concentration of the reactive component.

Preferably, the reactive component is a combustible component.

Conveniently, the sensor arrangement is operable to measure a signal output by the catalytic sensor following the placing of the enclosure in the closed configuration.

Advantageously, the sensor arrangement is operable to determine a time constant of the decay of the signal output by the catalytic sensor, or another parameter related to the rate of decay, following the placing of the enclosure in the closed configuration.

Preferably, the sensor arrangement is operable to determine at least one further time constant of the decay.

Conveniently, the sensor arrangement is operable to calculate, from the time constant or other parameter, the sensitivity of the catalytic sensor to the presence of the reactive element. Advantageously, the sensor arrangement further comprises an inlet which may be opened to place the enclosure in the open configuration and closed to place the enclosure in the closed configuration.

Preferably, the sensor arrangement further comprises a selectively closable outlet, allowing communication between surrounding gases and the interior of the enclosure when the outlet is open.

Conveniently, when both the inlet and the outlet are open, a gas flow path between the inlet and the outlet passes near to the catalytic sensor.

Advantageously, the sensor arrangement further comprises at least one microactuator to control the opening and closing of the inlet.

Another aspect of the present invention provides a sensor arrangement for detecting a reactive component of a gas mixture, the sensor arrangement comprising a catalytic sensor adapted to detect the presence of the reactive component and being operable to isolate the catalytic sensor substantially from surrounding gases, characterised by the sensor arrangement being operable to determine a time constant of a signal output by the catalytic sensor, or another parameter related to the rate of decay, following the isolation of the catalytic sensor; and from the time constant or other parameter, calculate the sensitivity of the catalytic sensor to the presence of the reactive element.

Conveniently, the sensor arrangement is operable to compare the calculated sensitivity to a reference sensitivity. Advantageously, the sensor arrangement is operable, if the calculated sensitivity differs from the reference sensitivity by less than a first threshold amount, to apply compensation for the sensitivity.

Preferably, the sensor arrangement is operable, if the calculated sensitivity differs from the reference sensitivity by more than a first threshold amount, to provide a warning indication.

Conveniently, a heater is provided to heat the catalytic sensor.

Advantageously, the catalytic sensor measures temperature changes due to combustion of the reactive element to provide an output signal.

Preferably, the catalytic sensor comprises at least one thin film formed on a substrate.

Conveniently, the sensor arrangement is operable to perform an offset correction process to compensate for the presence of a background concentration of the reactive element in surrounding gases.

Advantageously, the isolation of the catalytic sensor is controlled in response to the detection by the catalytic sensor, of at least a predetermined concentration of the reactive component.

Another aspect of the present invention provides a vehicle incorporating a sensor arrangement according to the above.

Preferably, the vehicle is configured so that, if a determination is made by the sensor arrangement that a concentration of the reactive component is above a predetermined threshold, the vehicle may not be driven. Conveniently, if the determination is made by the sensor arrangement, the ignition of the vehicle may not be used to start the vehicle's engine.

A further aspect of the present invention provides a method of detecting a reactive component of a gas mixture, the method comprising the steps of: providing a catalytic sensor to detect the presence of the reactive component; locating the catalytic sensor within an enclosure which is selectively moveable between an open configuration, in which an interior of the enclosure may communicate with surrounding gases, and a closed configuration, in which the interior of the enclosure is substantially isolated from the surrounding gases, and allowing a gas mixture to enter the enclosure, characterised by the steps of placing the enclosure in the closed configuration in response to the fulfilment of a predetermined criterion; and compensating for variations in the sensitivity of the catalytic sensor.

Preferably, the reactive component is a combustible component.

Conveniently, the method further comprises the step of analysing an output signal from the catalytic sensor following the placing of the enclosure in the closed configuration.

Advantageously, the method further comprises the step of determining a time constant of the decay of the signal output by the catalytic sensor, or another parameter related to the rate of decay, following the placing of the enclosure in the closed configuration. Preferably, the method further comprises the step of determining at least one further time constant of the decay.

Conveniently, the method further comprises the step of calculating, from the time constant or other parameter, the sensitivity of the catalytic sensor to the presence of the reactive element.

Advantageously, the method further comprises the step of providing an inlet which may be opened to place the enclosure in the open configuration and closed to place the enclosure in the closed configuration.

Preferably, the method further comprises the step of providing a selectively closable outlet, allowing communication between surrounding gases and the interior of the enclosure when the outlet is open.

Another aspect of the present invention provides a method for detecting a reactive component of a gas mixture, the method comprising the steps of: providing a catalytic sensor adapted to detect the presence of the reactive component; and isolating the catalytic sensor, along with a gas mixture comprising the reactive component substantially from surrounding gases, characterised by the steps of determining a time constant of the decay of a signal output by the catalytic sensor, or another parameter related to the rate of decay, following the isolation of the catalytic sensor; and from the time constant or other parameter, calculating the sensitivity of the catalytic sensor to the presence of the reactive element.

Conveniently, the method further comprises the step of determining at least one further time constant of the decay.

Advantageously, the method further comprises the step of comparing the calculated sensitivity to a reference sensitivity. Preferably, the method further comprises the step, if the calculated sensitivity differs from the reference sensitivity by less than a first threshold amount, of applying compensation for the sensitivity.

Conveniently, the method further comprises the step, if the calculated sensitivity differs from the reference sensitivity by more than a first threshold amount, of providing a warning indication.

Advantageously, the method further comprises the step of heating the catalytic sensor.

Preferably, the catalytic sensor measures temperature changes due to combustion of the reactive element to provide an output signal.

Conveniently, the step of providing a catalytic sensor comprises the step of forming at least one thin film on a substrate.

Advantageously, the method further comprises the step of performing an offset correction process to compensate for the presence of a background concentration of the reactive element in surrounding gases.

Preferably, the method further comprises the step of isolating of the catalytic sensor in response to the detection by the catalytic sensor, of at least a predetermined quantity of the reactive component.

In order that the present invention may be more readily understood, embodiments thereof will now be described, by way of example, with reference to the accompanying figures, in which: Figures 1a and 1b show a sensor arrangement embodying the present invention with the inlet and outlet thereof open and closed respectively;

Figure 2 shows a schematic layout of the components of a sensor arrangement embodying the present invention;

Figure 3 shows a graph of a measurement made by a sensor arrangement embodying the present invention with respect to time; and

Figures 4a and 4b show the logarithm of portions of the graph shown in figure 3 under different conditions.

Referring firstly to figure 1a, a schematic view of a sensor arrangement 1 embodying the present invention is shown. The sensor arrangement 1 includes a sensor 2, which includes a reaction portion 3 incorporating a catalyst material. As discussed above, the catalyst material may include tin oxide, platinum, palladium or another metal or compound having catalytic properties. A communication portion 4 of the sensor 2 is placed in thermal contact with the reaction portion 3. The communication portion 4 preferably comprises a block of a material having a high thermal conductivity, and in embodiments of the invention the reaction and communication portions 3,4 are respective parts of the same block of material.

Embedded in, or bonded to, the communication portion 4 is a heating element 5. The heating element 5 may be a two-terminal resistor element, generating heat by displacing power from an electric current running therethrough. Other possible heat sources may of course be used, including mechanical, acoustic, electromagnetic, optical or chemical means. In order to obtain adequate reaction rates, it is generally necessary to heat the reaction portion 3 of the sensor 2 to around 200° to 500⁰C. Also embedded in, or bonded to, the communication portion 4 is a measurement element 6. The measurement element 6 is configured to allow measurement of the temperature of the immediate surrounds of the measurement element 6. The measurement element 6 may, for instance, be a two-terminal resistor (like the heating element 5), which has an electrical resistivity which is temperature-dependent. Several resistor materials, such as metals, metal oxides and semiconductors exhibit a predictable and stable temperature dependence of electrical resistivity. Typically, the temperature coefficient of resistivity of such materials is 0.1 to 5% per ⁰C. The measurement element 6 must be able to withstand the temperature to which the sensor 2 is heated, and to provide adequate temperature sensitivity, and these requirements may be fulfilled by many of the materials mentioned above.

Figures 1a and 1b show schematic views of the sensor arrangement 1. The sensor 2 is provided within an enclosure 7, which is preferably relatively small, having a volume of less than around 10 cm³ or preferably less than 1 cm³ and may be formed from any suitable gas-impermeable material. The enclosure is preferably designed to fit together with other functional units within a vehicle cabin.

The enclosure 7 is provided with an inlet 8 and an outlet 9, allowing communication between surrounding gases and an interior of the enclosure 7, although in some embodiments only one inlet may be provided, without a separate outlet. A flow path 10 through the enclosure 7 is defined between the inlet 8 and the outlet 9, with the flow path 10 passing over or close to the reaction portion 3 of the sensor 2, so that constituents of gas passing along the flow path may react on the reaction portion 3. In preferred embodiments of the invention, the flow path 10 also passes a heat exchanging element 11 provided within the enclosure 7. A multi-sensor 12 is also provided within the enclosure 7, for the measurement of other physical variables, such as ambient temperature, humidity and barometric pressure within the enclosure 7. In particular, the multi-sensor 12 is provided to measure parameters that may influence the rate of reaction of alcohol (or be combustible gaseous component of interest) within the enclosure 7.

A processing arrangement, in this embodiment comprising a microprocessor 13, is also provided. The microprocessor 13 is operable to control the heating element 5, and also to receive signals from the measurement element 6 and the multi-sensor 12. Further, the microprocessor 13 is operable to control movable closures 14, 15, which are provided in the vicinity of the inlet 8 and outlet 9 respectively, and are movable between an open position (shown in Figure 1a), in which gas may flow from the surroundings of the enclosure 7 through the inlet or outlet, and a closed position (shown in Figure 1b), in which gas may not flow from the surroundings of the enclosure 7 into the enclosure 7. When both of the closures 14, 15 are in the closed position, the interior of the enclosure 7 is substantially isolated from ambient gases.

Figure 2 is a schematic layout of one physical implementation of elements of the present invention. It is possible to miniaturise the sensor 1 by providing two bonded silicon dice 16, 17, which are each approximately disc-shaped, having diameters of 2 to 3mm, and thicknesses of around 0.5mm, with a lower one of the dice 17 having a recess 18 in a central portion thereof. It will therefore be understood that, when the dice 16, 17 are bonded together, the presence of the recess leads to a space between the dice 16, 17. In a central portion of the lower die 17, a catalytic sensing element 19 is provided, including a reaction portion 3 and a communication region 4 including heating and communication elements. These elements are formed by local thin film deposition, performed prior to bonding the dice 16, 17 together. First and second apertures 20, 21 are formed in an upper one of the dice 16, allowing communication between ambient gases and the space between the two dice 16, 17. Entrances to these apertures 20, 21 are controlled by two cantilever elements 14, 15, which can either be inclined to allow communication of gas between the ambient surroundings and the space between the die 16, 17 (as shown in figure 2), or in a horizontal position to close off the apertures 20, 21 and hence isolate the space within the dice from the ambient surroundings. The movement of these cantilevers 14, 15 is controlled by microactuator elements 22, 23, which may employ electrostatic, electromagnetic or electrothermal forces. The latter could use, for example, a bimorph structure of the cantilevers 14, 15 in the structure may require the addition of selected etching, for instance, using the selectivity of p- and n-doped silicon in an electrochemical process.

Operation of the sensor arrangement 1 will now be described, with reference to figures 3, 4a and 4b and 5.

Figure 3 shows a signal output by the measurement element 6 with respect to time (U(t)). At a first point in time 24, the heating element 5 and measurement element 6 are switched on. After stabilising, the measurement element 6 will output a substantially constant initial value 25. At a second point in time 26, automatic offset correction is performed by subtracting the initial value 25 from the actual signal. The resulting value 27 after offset correction will therefore be approximately zero.

At a third point in time 28, a measurement is performed by collecting a breath sample from a test subject. In the presence of alcohol in the expired breath, the sensor signal will increase and reach a stable value 29. At a fourth point in time 30, arbitrarily set as zero in figure 3, the closures 14, 15 are closed, isolating the gases inside the enclosure 7. The signal output by the measurement element 6 then declines 31 as the alcohol within the enclosure 7 is consumed. In general, the closing of the closures 14, 15 to isolate the interior of the enclosure 7 is controlled to occur in response to the fulfilment of a predetermined criterion. In preferred embodiments, the microprocessor 13 is configured to close the closures 14, 15 when it is determined that the sensor signal (predetermined by the offset correction) exceeds a certain threshold value, indicating that a certain concentration of the measurand (e.g. alcohol) is present in the gas within the enclosure 7. An alternative criterion that could be used is that the time derivative of the sensor signal exceeds a predetermined threshold value. A third possibility is to use a combination of signals from the measurement element 6 and the multisensor 12, and a skilled person will appreciate how this may be implemented to close the closures 14, 15 at an appropriate time.

It will be appreciated that the invention is not limited to a pair of closures as discussed above, and any suitable method of selectively isolating the interior of the enclosure 7 may be used.

To measure the concentration of the component including compensation for offset and sensitivity, at least three measured values are needed (assuming only one catalytic reaction, i.e. only one time constant). The sensitivity could be derived from the time constant of the exponential decay (as will be seen later). In figure 3 the following three measurements could be used: the initial value 25 gives the offset, the stable value 29 gives the sensitivity- uncompensated concentration of the component and measuring a known time interval after t=0 gives the time constant from which the sensitivity can be derived. Alternatively, the offset could be calculated from making a second measurement of the decay and from that extrapolate the zero-level. It will be appreciated that an advantage that arises from this method is that a sample of known concentration is not required. Figures 4a and 4b show logarithmic graphs of the signal output by the measurement element 6 over time. If the catalytic reaction is linear and dependent on a single reaction mechanism, the logarithmic graph will be (or will be a close approximation to) a straight line, as shown in figure 4a, the slope of which is determined by the time constant τ, as described above. If, on the other hand, the reaction rate is limited by two different parameters, the logarithmic graph will have a more complex form, for instance, as shown in shown in figure 4b. In this case, one mechanism, for instance a single step within the chain of sub-reactions, dominates the initial reaction behaviour, whereas another mechanism, for instance diffusion of reaction products, dominates the later stages. In this case, the decay may be characterised by two time constants τi and X₂. In certain reactions and situations, even more parameters than two are necessary to describe the decay.

The relationship between the signal output by the measurement element 6 and the concentration of alcohol within the enclosure 7 is given by (signal) - (offset compensation) = (sensitivity) x (concentration), where the sensitivity is R x G, R being the reaction rate coefficient (which depends on the properties of the reaction portion 3), and G is a gain factor, which is independent of the catalyst and can be calculated by conventional methods.

As discussed above, the present invention is concerned with ensuring that the sensitivity of the reaction portion 3 of the sensor 2 (and hence of the sensor as a whole) falls within acceptable parameters.

After the closures 14, 15 have been closed, it is assumed that the gas flow past the surface of the reaction region 3 remains unaffected, but that the flow is now being recirculated within the enclosure 7. Solving the first order differential equation describing the signal U output by the measurement element 6 as a function of time T gives the following expression: U(t) = G - r(t) = G - r₀ expj -

Where r (t) = R-x(t), x(t) being the concentration of the combustible component. The signal thus exhibits an exponential decay, the rate of which is determined by the time constant τ. Further analysis shows that τ is related to V₀, R and V_ref as indicated below:

Where V₀ is the volume of the enclosure 7 and V_ref is a reference volume, which is the volume of one mole of gas at 0⁰C and 10⁵N/m², i.e. 2.24*10^'2m³.

It will therefore be understood that isolating the enclosure 7 will allow for determination of the time constant τ, and therefore the reaction rate coefficient R. As shown above, the reaction rate coefficient R may then be used to calculate the sensitivity of the sensor 2. The actual value of R, the reaction rate coefficient, may then be compared to an expected value of R, based on a preset value or upon historical data, and it will be understood that this will compare the sensitivity of the sensor 2 with the expected sensitivity. If the deviation between expected and actual values is within certain limits, correctional compensation is performed. Compensation may be performed by adjusting the gain factor G in response to the actual value of R, and the sensor transfer function may be controlled to provide a calibrated output signal. If the difference between the actual and expected values of R exceeds a certain limit, an error signal is activated to indicate that the sensor 2 is not performing within the required limits. The invention is not limited to linear sensor systems described by a first order differential equation, in which the decay is exponential. In nonlinear systems, other parameters may be used to describe the decay rate.

The sensor arrangement 1 may therefore make a determination as to whether the sensitivity of the reaction portion 3 of the sensor 2 is within expected limits, or whether the sensitivity has been affected by external factors. It will be understood that, if a volatile element other than the expected one is present in gases introduced into the enclosure 7, the reaction rate will differ from that expected (due to different combustion characteristics of the volatile component) and a warning may therefore also be raised if this occurs.

The sensor arrangement 1 may therefore be maintained in calibration without the need for the sensing arrangement 1 to be calibrated by being placed completely within a controlled atmosphere which, as discussed above, is a lengthy and expensive process.

It is envisaged that a sensor arrangement embodying the present invention may be installed within a vehicle, and may be directly or indirectly connected to the ignition mechanism of the vehicle, so that individuals under the influence of alcohol may be prevented from driving the vehicle. The vehicle may be configured so that, if the sensor arrangement makes a determination that the occupant of the driver's seat is under the influence of alcohol, the ignition may be deactivated or disabled so that the car will not start. Alternatively, a warning may be displayed or emitted if such a determination is made. Other arrangements may be put in place to prevent drivers under the influence of alcohol, for instance the disabling of other parts of the vehicle such as the engine, and it will be appreciated that the ignition is not the only part of the vehicle with which the sensor arrangement may be used. Of course, sensor arrangement may simply be used as part of a hand-held or other sensor, which may be of a type suitable for use by police or other enforcement bodies.

When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

Claims

CLAIMS:

1. A sensor arrangement (1) for detecting a reactive component of a gas mixture, the sensor arrangement (1 ) comprising: an enclosure (7) which is selectively moveable between an open configuration, in which an interior of the enclosure (7) may communicate with surrounding gases, and a closed configuration, in which the interior of the enclosure (7) is substantially isolated from the surrounding gases; and a catalytic sensor (2) adapted to detect the presence of the reactive component, the sensor (2) being located within the enclosure (7), characterised by a processing arrangement (13) operable compensate for variations in the sensitivity of the catalytic sensor (2) and to place the enclosure (7) in the closed configuration in response to the fulfilment of a predetermined criterion.

2. A sensor arrangement (1) according to Claim 1 , wherein the predetermined criterion comprises the detection, by the catalytic sensor (2), of at least a predetermined concentration of the reactive component.

3. A sensor arrangement (1) according to Claim 1 or 2, wherein the reactive component is a combustible component.

4. A sensor arrangement (1 ) according to any one of the preceding claims operable to measure a signal output by the catalytic sensor (2) following the placing of the enclosure (7) in the closed configuration.

5. A sensor arrangement (1) according to Claim 4, operable to determine a time constant of the decay of the signal output by the catalytic sensor (2), or another parameter related to the rate of decay, following the placing of the enclosure (7) in the closed configuration.

6. A sensor arrangement (1) according to Claim 5, operable to determine at least one further time constant of the decay.

7. A sensor arrangement (1) according to Claim 5 or 6, operable to calculate, from the time constant or other parameter, the sensitivity of the catalytic sensor (2) to the presence of the reactive element.

8. A sensor arrangement (1) according to any preceding claim, further comprising an inlet which may be opened to place the enclosure (7) in the open configuration and closed to place the enclosure (7) in the closed configuration.

9. A sensor arrangement (1) according to Claim 8, further comprising a selectively closable outlet (9), allowing communication between surrounding gases and the interior of the enclosure (7) when the outlet (9) is open.

10. A sensor arrangement (1) according to Claim 9 wherein, when both the inlet (8) and the outlet (9) are open, a gas flow path (10) between the inlet (8) and the outlet (9) passes near to the catalytic sensor (2).

11. A sensor arrangement (1) according to any one of Claims 8 to 10, further comprising at least one microactuator (22) to control the opening and closing of the inlet (8).

12. A sensor arrangement (1) for detecting a reactive component of a gas mixture, the sensor arrangement (1) comprising a catalytic sensor (2) adapted to detect the presence of the reactive component and being operable to isolate the catalytic sensor (2) substantially from surrounding gases, characterised by the sensor arrangement (1) being operable to determine a time constant of the decay of a signal output by the catalytic sensor (2), or another parameter related to the rate of decay, following the isolation of the catalytic sensor (2); and from the time constant or other parameter, calculate the sensitivity of the catalytic sensor (2) to the presence of the reactive element.

13. A sensor arrangement (1) according to Claim 12, operable to determine at least one further time constant of the decay.

14. A sensor arrangement (1 )according to Claim 7, 12 or 13, operable to compare the calculated sensitivity to a reference sensitivity.

15. A sensor arrangement (1) according to Claim 14 operable, if the calculated sensitivity differs from the reference sensitivity by less than a first threshold amount, to apply compensation for the sensitivity.

16. A sensor arrangement (1) according to Claim 14 or 15 operable, if the calculated sensitivity differs from the reference sensitivity by more than a first threshold amount, to provide a warning indication.

17. A sensor arrangement (1) according to any preceding claim, wherein a heater (5) is provided to heat the catalytic sensor (2).

18. A sensor arrangement (1) according to any preceding claim, wherein the catalytic sensor (2) measures temperature changes due to combustion of the reactive element to provide an output signal.

19. A sensor arrangement (1) according to any preceding claim, wherein the catalytic sensor (2) comprises at least one thin film formed on a substrate.

20. A sensor arrangement (1) according to any preceding claim, operable to perform an offset correction process to compensate for the presence of a background concentration of the reactive element in surrounding gases.

21. A sensor arrangement (1 ) according to any one of Claims 12 to 20, wherein the isolation of the catalytic sensor (2) is controlled in response to the detection by the catalytic sensor (2), of at least a predetermined concentration of the reactive component.

22. A vehicle incorporating a sensor (1) arrangement according to any preceding claim.

23. A vehicle according to Claim 22, configured so that, if a determination is made by the sensor arrangement (1 ) that a concentration of the reactive component is above a predetermined threshold, the vehicle may not be driven.

24. A vehicle according to Claim 23 wherein, if the determination is made by the sensor arrangement (1 ), the ignition of the vehicle may not be used to start the vehicle's engine.

25. A method of detecting a reactive component of a gas mixture, the method comprising the steps of: providing a catalytic sensor (2) to detect the presence of the reactive component; locating the catalytic sensor (2) within an enclosure (7) which is selectively moveable between an open configuration, in which an interior of the enclosure (7) may communicate with surrounding gases, and a closed configuration, in which the interior of the enclosure (7) is substantially isolated from the surrounding gases; and allowing a gas mixture to enter the enclosure (7), characterised by the steps of: placing the enclosure (7) in the closed configuration in response to the fulfilment of a predetermined criterion; and a compensating for variations in the sensitivity of the catalytic sensor (2).

26. A method according to Claim 25, wherein the predetermined criterion comprises the detection, by the catalytic sensor (2), of at least a predetermined concentration of the reactive component.

27. A method according to Claim 25 or 26, wherein the reactive component is a combustible component.

28. A method according to any one of Claims 25 to 27, further comprising the step of analysing an output signal from the catalytic sensor (2) following the placing of the enclosure (7) in the closed configuration.

29. A method according to Claim 28, further comprising the step of determining a time constant of the decay of the signal output by the catalytic sensor (2), or another parameter related to the rate of decay, following the placing of the enclosure (7) in the closed configuration.

30. A method according to Claim 29, further comprising the step of determining at least one further time constant of the decay.

31. A method according to Claim 29 or 30, further comprising the step of calculating, from the time constant or other parameter, the sensitivity of the catalytic sensor (2) to the presence of the reactive element.

32. A method according to any one of Claims 25 to 31 , further comprising the step of providing an inlet (8) which may be opened to place the enclosure (7) in the open configuration and closed to place the enclosure (7) in the closed configuration.

33. A method according to Claim 32, further comprising the step of providing a selectively closable outlet (9), allowing communication between surrounding gases and the interior of the enclosure (7) when the outlet (9) is open.

34. A method for detecting a reactive component of a gas mixture, the method comprising the steps of: providing a catalytic sensor (2) adapted to detect the presence of the reactive component; and isolating the catalytic sensor (2), along with a gas mixture comprising the reactive component substantially from surrounding gases, characterised by the steps of: determining a time constant of the decay of a signal output by the catalytic sensor (2), or another parameter related to the rate of decay, following the isolation of the catalytic sensor (2); and from the time constant or other parameter, calculating the sensitivity of the catalytic sensor (2) to the presence of the reactive element.

35. A method according to Claim 34, further comprising the step of determining at least one further time constant of the decay.

36. A method according to Claim 25, 34 or 35, further comprising the step of comparing the calculated sensitivity to a reference sensitivity.

37. A method according to Claim 36 further comprising the step, if the calculated sensitivity differs from the reference sensitivity by less than a first threshold amount, of applying compensation for the sensitivity.

38. A method according to Claim 36 or 37 further comprising the step, if the calculated sensitivity differs from the reference sensitivity by more than a first threshold amount, of providing a warning indication.

39. A method according to any one of Claims 25 to 38, further comprising the step of heating the catalytic sensor (2).

40. A method according to any one of Claims 25 to 39, wherein the catalytic sensor (2) measures temperature changes due to combustion of the reactive element to provide an output signal.

41. A method according to Claim 25 to 40, wherein the step of providing a catalytic sensor (2) comprises the step of forming at least one thin film on a substrate.

42. A method according to any one of Claims 25 to 41 , further comprising the step of performing an offset correction process to compensate for the presence of a background concentration of the reactive element in surrounding gases.

43. A method according to any one of Claims 34 to 42, further comprising the step of isolating of the catalytic sensor (2) in response to the detection by the catalytic sensor (2), of at least a predetermined quantity of the reactive component.