WO2001065211A2

WO2001065211A2 - Method for operating a sensor

Info

Publication number: WO2001065211A2
Application number: PCT/EP2001/002246
Authority: WO
Inventors: Renier J. L. C. Goossens; Robert Schinkel; Guido Pauels
Original assignee: Health And Safety Systems Holding S.A.
Priority date: 2000-03-01
Filing date: 2001-02-28
Publication date: 2001-09-07
Also published as: LU90536B1; EP1259803A2; AU2001246486A1; WO2001065211A3

Abstract

A method for operating a sensor sensitive to a measurable variable comprises the operation of the sensor in a measuring mode in which the sensor is activated so as to be able to generate at the end of a required initiation period a sensor signal that can be related to a calibrated value of the measurable variable. The sensor is deactivated during the initiation period if a signal parameter based on the sensor signal exceeds a protection threshold.

Description

Method for operating a sensor

Introduction

The present invention generally relates to a method for operating a sensor, a method for calibrating a detector, in particular a gas detector, and a method for cleaning a gas detector

Most detectors f ex chemical, biological and physical sensors deteriorate when exposed to high quantities of the variable they measure In the case of gas detectors, for instance, after a long exposure to a relatively high concentration of gas, even within -4heιr measurement range, those detectors show a loss of sensitivity This phenomenon is known as "drift" Detectors with drift characteristic produce measured values which are unstable or deviate from their initial calibrated values This is due to the fact that gas sensors, when under power, are damaged when the concentration of the gas is too high

A standard procedure for measuring the concentration of a gas with a sensor comprises firstly the activation of the sensor, i e setting the sensor under power, thereby causing the sensor to generate a sensor signal The sensor requires first an initiation period, during which the sensor signal in unstable At the end of the initiation period, i e within seconds to several minutes depending on the type of sensor, the latter reaches stable internal conditions (e g 400°C) and the sensor signal becomes relatively stable Then, the sensor is generally calibrated in order to assign a given sensor signal to a given concentration value This correspondence is then further used for every measurement in order to relate a sensor signal to a calibrated value

The problem with such sensors is that the actual concentration value i e the calibrated value, can only be obtained once the sensor has reached stable internal conditions At that moment, the sensor may have already been seriously damaged if exposed to high concentrations of gas Object of the invention

The object of the present invention is to provide a method for operating a sensor, wherein the sensor is protected against deterioration when exposed to too high quantities of the variable it measures This object is achieved by a method as claimed in claim 1

Summary of the invention

A method in accordance with the invention (first aspect) concerns the operation of a sensor, which is sensitive to a measurable variable According to the method of the invention, the sensor may be operated in a measuring mode in which it is activated so as to be able to generate, at the end of a required initiation period, a sensor signal that can be related to a calibrated value of the measurable variable However, it shall be appreciated that the sensor is deactivated during the initiation period if a signal parameter based on the sensor signal exceeds a protection threshold

The present method uses the unstable sensor signal generated during the initiation period of the sensor to predict, within the first seconds, a too high quantity of the measurable variable that might result in a deterioration of the sensor Indeed, although the sensor signal generated during the initiation period cannot be utilized for measurements or calibration as it is unstable, it shall be remarked that the variations of the sensor signal depends on the intensity of the measurable variable the sensor is exposed to Moreover, the sensor signal generated during the initiation period is reproducible The signal parameter, calculated from the sensor signal, has been introduced to characterize the variations of the unstable sensor signal A sensor parameter with a value beyond the protection threshold indicates that the sensor signal has varied in a way that indicates a too high intensity of the measurable variable In this case, the sensor is immediately deactivated It follows that by monitoring the sensor signal during the initiation period, with the help of the signal parameter, a too high quantity of the measurable variable can be detected, the sensor deactivated and thus protected from deterioration However, if the sensor parameter does not exceed the protection threshold, the calibrated value is obtained normally at the end of the initiation period A major advantage is that, since a sensor operated in such a way is protected against deterioration, it performs accurate and reliable measurements The sensor may be operated in the measuring mode for a certain measuring period, which includes the initiation period This implies that the measuring period should be at least as long as the initiation period in order to obtain a calibrated value if the signal parameter hasn't exceeded the protection threshold The duration of the measuring period should be defined on the basis of the use of the sensor The measuring period may be short if the calibrated value of the measurable variable is only to be known for a short duration, but it may also be very long, if the measurable variable has to be monitored e g for several hours

In a preferred embodiment, the sensor is operated alternatively in the measuring mode and in a rest mode, in which the sensor is deactivated for a certain period, hereinafter called rest period It follows that the sensor is operated cyclically, the duration of a cycle being equal to the measuring period plus the rest period Hence, a measurable variable may be monitored semi- contmuously and calibrated values determined regularly Preferably, the measuring period is restricted to the duration required to obtain a sensor signal that can be related to a calibrated value, in addition to the initiation period of course In this case, the sensor is exposed to the measuring variable during the shortest possible time interval Indeed, the sensor is activated and deactivated as soon as it has produced a stable sensor signal that can be related to a calibrated value, i e just at the end of the initiation period Besides, operating the sensor alternatively in the measuring mode and in the rest mode proves advantageous for portable detectors, as it permits to save energy while a calibrated value may still be obtained semi-continuously, i e every cycle For instance, the sensor may be a gas sensor in a portable gas detector to be worn by workers in hazardous areas in order to monitor the concentration of gas It will be understood that when the sensor is operated cyclically, i e alternatively in the measuring mode and in the rest mode, and if the signal parameter exceeds the protection threshold, then no calibrated value will be determined during the concerned cycle When the signal parameter exceeds the protection threshold, the sensor may be deactivated until the end of the measuring period and then only operated in the rest mode, thus ensuring a steady cycle duration (cycle duration = measuring period + rest period) However, the sensor may also be deactivated by directly operating the sensor in the rest mode when the signal parameter exceeds the protection threshold In this case, the cycles have irregular durations but a calibrated value may be obtained earlier after a deactivation

The signal parameter may be the absolute or the relative value of the signal during the initiation period In this case, a signal parameter exceeding the protection threshold means a too large variation of the sensor signal The signal parameter may also represent a variation per time unit of the sensor signal For example, the deπvate of the sensor signal may be calculated at a given elapsed time the faster the variation, the larger the derivative

As mentioned above, the measuring period may be very long, when the measurable variable has to be monitored e g for up to several hours In such a case, the sensitivity of the sensor may gradually drop if exposed for a long period to high quantities of the measurable variable To avoid this kind of drift, the sensor signal is advantageously monitored during the measuring period and moving-averages of the sensor signal are determined If the moving-average reaches a drift-threshold, the sensor signal is corrected According to a second aspect of the invention, there is proposed a method for calibrating a detector comprising a reference sensor and a measuring sensor The reference sensor is operable in the measuring mode described above (first aspect of the invention) Hence, the reference sensor will be deactivated if the sensor parameter exceeds the protection threshold The measuring sensor generates a measuring signal that can be related to a measured value of the measurable variable In this calibration method, the reference sensor is firstly operated in the measuring mode If the sensor parameter does not exceed the protection threshold, a calibrated value is determined at the end of the initiation period and compared to the measured value obtained with the measuring sensor If this measured value significantly differs from the calibrated value, then the measuring sensor may be adjusted so as to obtain a measured value essentially equal to the calibrated value

As the reference sensor is protected against high intensities of the measurable values, it does not deteriorate, and the calibrated value is extremely reliable Therefore, this calibration method permits a precise calibration of the measuring sensor, which is almost continuously active to monitor the measurable variable and thus deteriorates A particular advantage of this method is that it can be implemented on portable gas detectors having a reference sensor and a measuring sensor to perform an "in field" calibration, e g when worn by workers Advantageously the reference sensor is operated in the measuring mode only when the measured value is within a calibration range This ensures that a calibration will only occur when the quantity of the measurable variable allows a precise and reliable calibration, e g in a particularly sensitive interval of the sensor's measurement interval According to a third aspect of the invention there is proposed a method for calibrating a gas detector comprising a reference sensor and a measuring sensor The reference sensor is operable in the measuring mode described above (first aspect of the invention) Hence, the reference sensor will be deactivated if the sensor parameter exceeds the protection threshold The measuring sensor generates a measuring signal that can be related to a measured value of the measurable variable, i e the gas concentration This calibration method comprises the steps of

- establishing a constant gas concentration around the gas detector the gas concentration being within a calibration range, then - operating the reference sensor in the measuring mode to determine a calibrated value of the gas concentration, and - comparing the calibrated value to the measured value obtained with the measuring sensor, and then, if the measured value significantly differs from the calibrated value, adjusting the measuring sensor so as to obtain a measured value essentially equal to the calibrated value As the gas detector is equipped with a reference sensor operated in accordance with the first aspect of the invention, no exact concentration is needed to calibrate the gas detector, but a constant concentration is sufficient Indeed, as explained, the reference sensor always produces a reliable and accurate calibrated value, which serves to calibrate the measuring sensor It is clear that the calibration range is defined so that the concentration of gas established around the detector is preferably safe and permits a precise calibration

In a preferred embodiment, a constant gas concentration is obtained by placing a lid over the gas detector and then causing gas to diffuse within the lid This gas diffusion may be obtained by opening a bottle of gas, heating a sponge impregnated with a solvent, or sublimating a solid substance This is a very simple and efficient way of creating a constant gas concentration around the detector The lid can also be designed so as to cover a series of such gas detectors For example, the lid can be designed to cover a docking-station where a series of portable gas detectors are docked for maintenance

According to a fourth aspect of the invention, there is proposed a method for cleaning a reference gas sensor of a gas detector The gas detector also comprises a measuring gas sensor generating a measuring signal that can be related to a measured value of a gas concentration The reference gas sensor is operable in the measuring mode in order to determine a calibrated value of the gas concentration and will be deactivated as soon as the signal parameter exceeds the protection threshold According to the present cleaning method, if the measured value is below a cleaning threshold, then the reference gas sensor is operated in the measuring mode Next, if the calibrated value is also below the cleaning threshold a cleaning procedure of the reference sensor is started Generally the cleaning procedure consists in activating or heating the sensor for a certain time in order to evaporate or burn the sticky cohesion-due to the deposition of vapors-on the reference sensor In order to preserve the sensor, it is preferable to start a cleaning procedure only when the gas concentration is low Hence, by activating the cleaning procedure only when the calibrated value is also below the cleaning threshold, good conditions for cleaning are selected

Brief description of the drawings

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which Fig 1 shows the initiation curves up to 24 s of a gas sensor exposed to different concentrations of 2-propanol, Fig 2 shows the graph of Fig 1 between 0 and 3 s

Detailed description of a preferred embodiment

The method of the invention can be implemented on sensors having an initiation period required after activation before the sensor signal stabilizes Metal-oxide gas sensors belong to this type of sensors, as they need stable internal conditions, namely a given temperature, before they can generate a stable measuring signal Indeed, when the gas sensor is activated (i e set under power), it firstly requires an initiation period wherein it generates a sensor signal which vanes greatly At the end of the initiation period, the sensor reaches stable internal conditions (e g 400°C) and generates a stable sensor signal Then, the sensor is generally calibrated in order to assign a given sensor signal to a given concentration value This correspondence, usually realized as a calibration curve, is further used for every measurement in order to relate a sensor signal to a calibrated value Unfortunately such sensors deteriorate when exposed to relatively high gas concentrations (even within their measurement range), which results in a loss of sensitivity One solution to protect the sensor could be to deactivate it if a calibrated value indicating a too high gas concentration is obtained However, if the gas concentration is high, damages are caused to the sensor already during the initiation period, i e before the concentration value can be obtained

To avoid the deterioration of the sensor, the method of the invention proposes to monitor the sensor signal during the initiation period The reason for this will become clear at the sight of Fig 1 where the value of the sensor signal generated by a metal-oxide gas sensor is plotted versus time The vertical axis is graduated in units and the sensor signal values range from 0 to 255 The horizontal axis is graduated in seconds and indicates the time elapsed since activation of the sensor, l e beginning of the measuring mode Each curve corresponds to an exact concentration of 2-propanol (see legend) At 24 seconds, the sensor reaches stable internal conditions and generates a stable sensor signal, which can be related to a calibrated value It follows that this sensor has an initiation period of approximately 24 s As can be seen, although the sensor signal varies greatly, the way it varies depends on the concentration of 2-propanol and is reproducible Within the first seconds, namely within the first two seconds for most of the curves, the sensor signal reaches a peak The higher the concentration, the steeper the increase and the higher the peak Then the sensor signal decreases asymptotically to a stable value, for some concentrations with a second peak at 4-5 s In accordance with the invention, a sensor parameter based on the sensor signal has been introduced in order to characterize the variation of the sensor during this initiation period It will be understood that working on the sensor signal is possible whereas an absolute (calibrated) value cannot be obtained during the initiation period The sensor parameter can be the derivative of a curve at a given elapsed time It is clear from Fig 2 that the derivative of each curve at 0 5 s is different the higher the concentration, the larger the derivative Thus the variation of the sensor signal per unit of time about a given elapsed time (e g 0,5 , 1 or 1 ,5 s) may be used to predict a too high concentration of gas The signal parameter may also simply be an absolute value or a relative value of the sensor signal

On the basis of this observation and in order to protect a gas sensor exposed to high gas concentrations, there is proposed a method (first aspect of the invention) wherein the gas sensor can be operated in a measuring mode In this measuring mode the sensor is activated so as to be able to determine, at the end of a required initiation period, the calibrated value of the measurable variable, in this case the gas concentration However, the sensor will be deactivated if the signal parameter based on the sensor signal exceeds a protection threshold The protection threshold is defined so that the sensor is active until the end of the initiation period under safe conditions, i e for non- dangerous gas concentrations, and so that the sensor is deactivated if the gas concentration is dangerous for the sensor For example, in order to protect the sensor from a concentration of 2- propanol over 2 000 ppm, the sensor parameter could simply be equal to the value in units of the sensor signal (absolute value) and the protection threshold set to 232 units In Fιg 1 , the sensor signal at 24 s for 2 000 ppm equals 210 units However, at approximately 1 5 s it reaches a peak at 232 units As can be seen from Fig 1 and 2, this peak at 232 units (level 232 indicated by the dotted line) is only exceeded by curves plotted at higher concentrations of 2- propanol (5 000 and 10 000 ppm) By setting the protection threshold at 232 units, the sensor parameter will exceed the protection threshold every time the sensor is exposed to a concentration of 2-propanol superior to 2 000 ppm The deactivation will occur within the first seconds, whereas, if a calibrated value had to be obtained in order to detect a too high concentration, the sensor could only be deactivated after 24 s

It will be understood that after the initiation period the signal parameter is not needed anymore as the signal is stable and can be related to the calibrated value to detect a dangerous gas concentration Besides, in case of a sensor signal decreasing when the concentration of gas increases, the protection threshold will be exceeded when the sensor parameter becomes lower than the protection threshold It is also clear that the response of the sensor depends on the type of gas or vapor to be detected The protection threshold should be redefined for every type of gas

In a preferred embodiment of the present method, the gas sensor is operated cyclically, i e alternatively in the above-described measuring mode and in a rest mode, in which the sensor is deactivated for a rest period In this case, the measuring period is preferably restricted to the duration required to obtain a signal that can be related to a calibrated value, in addition to the initiation period of course For the sensor of Fig 1 , the initiation period is about

24 s as the signal is stable at 24 s Besides, the value of the sensor signal can be evaluated within a split second, therefore the measuring period can be set to

24 s This means that the sensor is deactivated as soon as a stable signal has been obtained, thus enabling the determination of a calibrated value of gas concentration Such a method proves very advantageous for battery operated gas detectors as deactivating the sensor permits to save energy Nevertheless, a concentration value is obtained regularly, normally once per cycle if the conditions are safe

As a continuation of the preceding example (protection threshold at 232 units), the measuring period of the sensor is set to 24 s (approx initiation period) and the rest period is set to 36 s It follows that under safe conditions, a calibrated value of the gas concentration will be obtained every minute (duration of a cycle) Now, if the concentration of 2-propanol reaches a level exceeding 2 000 ppm during the rest period, then during the following active period the signal parameter will exceed the protection threshold and the sensor will be immediately deactivated The deactivation of the sensor can be done by starting the rest mode as soon as the signal parameter exceeds the protection threshold It is also possible to deactivate the sensor and maintain it deactivated until the end of the initiation period, and then only start the rest period A gas sensor operated in accordance with the invention (first aspect) is protected against deterioration and the calibrated value of the gas concentration obtained is reliable Indeed, since the sensor doesn't deteriorate, the calibrated value never deviates Aware of this great advantage, the inventors have proposed a method for calibrating a gas detector having a reference sensor and a measuring sensor The reference sensor can be operated in the measuring mode and will be deactivated if the signal parameter exceeds the protection threshold The measuring sensor, which is generally continuously active, generates a measuring signal that can be related to a measured value so as to monitor the gas concentration

Most of the time, the reference sensor is inactive When a calibration of the measuring sensor is needed, the reference sensor is operated in the measuring mode, which can be done manually or automatically by software Under safe conditions, the reference sensor will be active during the entire measuring period and a calibrated value will be obtained Then, the calibrated value will be compared to the measured value obtained with the measuring sensor Finally, if the measured value significantly differs from the calibrated value, e g the measured value is significantly lower than the calibrated value due to the drift effect, then the measuring sensor is adjusted or connected to another calibration curve to produce results comparable with the reference sensor

Of course, if during the initiation period dangerous conditions are detected (signal parameter exceeds protection threshold), then no calibrated value will be obtained (unstable part of the sensor signal) and the calibration will be postponed until the sensor is re-operated in the measuring mode, manually or automatically

This calibration method can be fully automated and also be used as a tool for preventive maintenance If the measuring sensor reaches its limits of adjustment, then it should be replaced

It will be appreciated that a certain concentration of gas may be necessary to perform a precise calibration The actual gas concentration is known from the measured value Therefore, in the present calibration method, the reference sensor is preferably only operated in the measuring mode if the measured value is within a calibration range

A last remark about calibration concerns the improvement that can be realized over known calibration procedures when using gas detectors having a reference sensor operable in accordance with the invention Generally, in order to calibrate a gas detector, a certain amount of gas is injected in a closed calibration box or bottle in which the detector has been placed This conventional procedure is difficult and costly to automate, since a device must be developed in order to exactly establish a fixed gas concentration in the calibration box On the contrary, when using a gas detector having a reference sensor operable in accordance with the first aspect of the invention, no fixed concentration is needed, but only a constant concentration Therefore, to calibrate a gas detector with a reference sensor, it suffices to expose the sensor to a concentration in the preferred calibration range The precise and reliable calibrated values obtained with the reference sensor serve to calibrate the measuring sensor As a result, the inventors have proposed another method (fourth aspect of the invention) for calibrating a gas detector comprising a reference sensor and a measuring sensor The reference sensor is operable in the measuring mode and is deactivated as soon as the signal parameter exceeds the protection threshold The measuring sensor generates a measuring signal that can be related to a measured value of the gas concentration This second calibration method comprises the steps of

- establishing a constant gas concentration around the detector, the gas concentration being within a calibration range, then

- operating the reference sensor in the measuring mode to determine a calibrated value of the gas concentration, and

- comparing the calibrated value to the measured value obtained with the measuring sensor, and then, if the measured value significantly differs from the calibrated value, adjusting the measuring sensor so as to obtain a measured value essentially equal to the calibrated value The first Step is easily realized by covering the gas detector with an adapted lid, and releasing gas within the lid This can be done by opening a bottle of gas, heating a sponge impregnated with a solvent, or sublimating a solid substance (such as paraffin), preferably automatically A small electric fan can also be installed within the lid to assist the diffusion of gas, so as to reach a constant gas concentration more quickly It will be noted that the method will prove very advantageous for calibrating several portable gas detectors docked in a docking station for maintenance The lid will be adapted to the size of the docking station This second calibration method permits to calibrate easily and rapidly all the gas detectors

A last (fourth) aspect of the invention concerns a cleaning method for such gas detectors Indeed, although the reference sensor is generally protected by dust filters, a sticky film forms on the surface of the reference sensor due to deposition of certain vapors having a relatively low boiling point This reduces its measuring surface One way to clean the reference sensor is to heat it up at low gas concentrations to vaporize the pollution In that respect, the inventors have proposed a method for cleaning a reference gas sensor of a gas detector also comprising a measuring sensor If the measured value obtained with the measuring sensor is below a cleaning threshold, then the reference sensor is operated in the measuring mode Then, if the calibrated value is below the cleaning threshold, the cleaning procedure of the reference sensor (e g is heating) is activated

It is to be noted that when a gas sensor is operated in the measuring mode for a long measuring period at relatively high gas concentrations the sensitivity of the gas sensor may gradually drop Several experiments with a metal oxide sensor have shown gradual drops between 10 and 20%, after being exposed for 60 minutes at concentrations between 100 and 200 ppm of volatile organic compounds of conventional, high-solid and alkyd paints This kind of drift problem can be solved by monitoring the sensor signal (or the corresponding gas concentration value) during the measuring period and by determining moving-averages of the sensor signal When the moving-average reaches a predetermined drift-threshold, the sensor signal is preferably corrected by means of a formula based on the moving-average of the sensor signal and on the loss of sensitivity of the gas sensor Example

The present moving-average of the gas concentration during the previous i minutes can be expressed as: Yt = ^ δnCn

Wherein the sum of the weights is: ^T δn = 1

and

Yt is the present moving-average of past concentrations during (t-i) minutes, Cn is the concentration at minute (n), dn is the weight per delayed concentration Cn to express the influence of the past on the present moving-average Yt, i is the length of the interval to compose the moving-average.

The formula to correct the present concentration Ct by means of the moving-average may be e.g. of the following form:

Ccorrected(t) = Ct •^' (l + d ^• Yt)

wherein: d is a correction factor between 0 and 100% of the moving- average Yt.

Instead of a linear correction formula, a similar correction method based on the thresholds could be applied. In such a case, the correction factor may be: d 0 correction factor to be used when the moving-average Yt is less than Threshold (1 ), d 1 correction factor to be used when the moving-average Yt is between Threshold (1 ) and Threshold (2), d n correction factor to be used when the moving-average Yt is above threshold (n).

Claims

A method for operating a sensor sensitive to a measurable variable, comprising the operation of the sensor in a measuring mode in which the sensor is activated so as to be able to generate at the end of a required initiation period a sensor signal that can be related to a calibrated value of the measurable variable, wherein the sensor is deactivated during the initiation period if a signal parameter based on the sensor signal exceeds a protection threshold The method according to claim 1 , characterized in that, the sensor is operated in the measuring mode for a measuring period including the initiation period The method according to claim 2, characterized in that it comprises the operation of the sensor in a rest mode, in which the sensor is deactivated for a rest period, and in that the sensor is alternatively operated in the measuring mode and in the rest mode The method according to claim 3, characterized in that if the signal parameter exceeds the protection threshold, then the sensor is directly operated in the rest mode The method according to anyone of the preceding claims, characterized in that the signal parameter is based on one of an absolute value of the sensor signal, a relative value of the sensor signal, a variation per unit of time of the measuring signal The method according to anyone of the preceding claims, characterized in that the sensor signal is monitored after the initiation period and in that a correction of the sensor signal is carried out based on moving-average of the monitored sensor signal The method according to anyone of the preceding claims, characterized in that the sensor is a gas sensor and in that the gas sensor is operated in the measuring mode in order to determine the calibrated value of a concentration of gas A method for calibrating a detector comprising a reference sensor operable in accordance with the method defined in anyone of claims 1 to 7 and a measuring sensor generating a measuring signal that can be related to a measured value of the measurable variable, wherein the reference sensor is operated in the measuring mode, then if the calibrated value of the measurable variable has been determined by the reference sensor, this calibrated value is compared to the measured value obtained by the measuring sensor, and next if the measured value significantly differs from the calibrated value, the measuring sensor is adjusted so as to obtain a measured value essentially equal to the calibrated value The method according to claim 8, characterized in that the reference sensor is operated in the measuring mode only when the measured value is within a calibration range A method for calibrating a gas detector comprising a reference sensor sensitive to gas and operable in accordance with the method defined in anyone of claims 1 to 7 and a measuring sensor generating a measuring signal that can be related to a measured value of the gas concentration, the method comprising the steps of establishing a constant gas concentration around the gas detector, the gas concentration being within a calibration range, then operating the reference sensor in the measuring mode to determine a calibrated value of the gas concentration, and comparing the calibrated value to the measured value obtained with the measuring sensor, and then, if the measured value significantly differs from the calibrated value, adjusting the measuring sensor so as to obtain a measured value essentially equal to the calibrated value The method according to claim 10, characterized in that a constant gas concentration is obtained by placing a lid over the gas detector and then causing gas to diffuse within the lid The method according to claim 11 , characterized in that diffusion of gas is caused by opening a bottle of gas, heating a sponge impregnated with a solvent, or sublimating a solid substance A method for cleaning a reference gas sensor of a gas detector, the gas detector comprising a measuring gas sensor generating a measuring signal that can be related to a measured value of a gas concentration and the reference gas sensor being operable in accordance with the method defined in anyone of claims 1 to 6 so as to determine a calibrated value of the gas concentration, wherein if the measured value is below a cleaning threshold then the reference gas sensor is operated in the measuring mode, and then if the calibrated value is below the cleaning threshold a cleaning procedure of the reference gas sensor is started The method according to claim 13, characterized in that the cleaning procedure comprises heating the reference gas sensor to evaporate pollution deposited on the reference gas sensor A gas detector comprising a gas sensor operated in accordance with the method defined in anyone of claims 1 to 7