Abstract:
The disclosure relates to an operating method for a gas sensor, in particular a gas sensor for detecting asthma. According to said method, nitrogen monoxide or nitrogen dioxide is detected in a measuring phase and the gas sensor is heated by a heating device in a desorption phase in order to accelerate desorption. The heating process is continued until the temporal alteration of the measuring signal of the gas sensor falls below a threshold value.

Description:
This application is a 35 U.S.C. §371 National Stage Application of PCT/EP2012/050294, filed on Jan. 10, 2012, which claims the benefit of priority to Serial No. DE 10 2011 003 291.6, filed on Jan. 28, 2011 in Germany, the disclosures of which are incorporated herein by reference in their entirety. 
     The disclosure relates to an operating method for a gas sensor, in particular for a gas sensor for detecting nitrogen monoxide or nitrogen dioxide, and to a gas sensor for carrying out the method, in particular for the determination of carbon monoxide or carbon dioxide. 
     BACKGROUND 
     Measuring the concentration of nitrogen monoxide (NO) in respired gas is an important means for optimizing the treatment of asthmatic conditions. One promising technology for the detection of nitrogen monoxide is an NO 2  sensor based on suspended gate FET technology. The structure of these sensors is known, for example, from the documents DE 19 814 857 or DE 19 956 744. Field effect transistor-based gas sensors have the advantage of simple production by using standard processes (CMOS) as well as a low energy demand in operation. Another advantage of these sensors is that they comprise a sensitive layer for the gas detection. The material of the sensitive layer may in this case be selected almost freely, and it is therefore possible to produce a range of different gas sensors on the same basis. 
     For the measurement of nitrogen dioxide, a layer of a porphyrin dye or phthalocyanine, in particular copper phthalocyanine, for example, has been found to be particularly promising. Nitrogen dioxide is adsorbed on this layer, and leads to a potential change and therefore to a measurable signal. If the nitrogen dioxide disappears from the ambient air, then the nitrogen dioxide bound on the surface of the copper phthalocyanine layer is desorbed and the signal falls off. One difficulty in this case is that the desorption of the nitrogen dioxide takes place very slowly. For instance, the t90 time at room temperature is more than 1 h. 
     SUMMARY 
     It is an object of the present disclosure to provide an operating method for a gas sensor, in particular for detecting nitrogen monoxide or nitrogen dioxide, which makes it possible to deal with the aforementioned problem in an improved way. It is another object of the disclosure to provide a corresponding gas sensor for detecting nitrogen monoxide or nitrogen dioxide. 
     In respect of the method, the object is achieved by a method having the features described herein. In respect of the gas sensor, the solution consists in a gas sensor having the features disclosed herein. Additional advantageous configurations are further described herein. 
     The operating method according to the disclosure for a gas sensor is expediently to be used with a gas sensor which alternates during its operating period between a measurement phase, in which it is exposed to the gas to be measured, and a regeneration phase in which the gas to be measured is desorbed. In other words, the measurement of the gas takes place discontinuously. This is achieved, for example, in the case of an asthma sensor in which the measurement takes place only during an expiration process. During the expiration phase, the gas sensor is exposed to nitrogen dioxide which is formed in the air from nitrogen monoxide, and during the regeneration phase the nitrogen dioxide is desorbed again. 
     According to the operating method disclosed herein, the gas sensor is to be operated during the measurement phase at room temperature or while being heated slightly, i.e. at temperatures of between 35° C. and 60° C. During the regeneration phase, on the other hand, the sensor is heated and brought to a temperature which is from 40° C. to 100° C. In a particular configuration, the sensor is even brought to a temperature of between 50° C. and 130° C. above the measurement temperature, in particular at least 100° C. above the measurement temperature. 
     Since the nitrogen oxide sensitivity of the sensitive material decreases with temperature, the effect thereby achieved is that a nitrogen oxide sensitivity which is as high as possible is advantageously obtained. Furthermore, the energy consumption of the sensor is relatively low as a result of this, since little electrical energy is required for heating during the measurement phase. On the other hand, the effect of the heating of the sensor carried out during the regeneration phase is that the desorption of the gas, for example nitrogen dioxide, takes place substantially more rapidly than would be the case at the temperature of the measurement phase. The sensor is therefore converted substantially more rapidly into a defined state in which a new measurement, which is independent or only slightly dependent on the history of the measurements, is possible. In other words, the sensor is heated out during the regeneration phase. 
     The proposed heating out is particularly advantageous in the case of a sensor such as the respired air sensor, since this entails not a continuous measurement but only an intermittent, discontinuous measurement. For sensors which carry out a continuous measurement, on the other hand, heating out is less advantageous. In a leak sensor, for example, the measurement values are considered continuously and heating out leads to a significant variation in the measurement value which, however, does not in fact correspond to any change in the quantity to be measured. In the case of continuously measuring sensors, this leads to difficulties in the signal evaluation. 
     It is particularly advantageous for the operating method to be used with a field effect transistor-based gas sensor (GasFET). This type of gas sensor allows problem-free measurement at room temperature together with problem-free electrical heating for the regeneration phase. At the same time, a GasFET allows very sensitive measurement and economical of nitrogen dioxide in its vicinity. 
     The gas sensor according to the disclosure is preferably a GasFET. It comprises a heater which allows heating of the sensor. The gas sensor according to the disclosure furthermore comprises a control device, which is configured in such a way that it carries out heating of the gas sensor in a regeneration phase. 
     In an alternative configuration, the gas sensor is configured in order to read out the electrical conductivity of a sensor layer as the measurement signal. In another alternative, the gas sensor is configured in order to read out changes in a mass or in a viscoelastic effect with a mass-sensitive transducer, for example QMB, SAW; CMUT, cantilever or FBAR, as the measurement signal. 
     In an advantageous configuration, the change in the measurement signal of the gas sensor as a function of time is determined in particular during the regeneration phase, i.e. during the heating out. This may, for example, be done in an analog-electronic or digital manner. In particular, the control device of the gas sensor is configured in order to calculate the change in the measurement signal as a function of time. 
     The change in the measurement signal as a function of time which is determined is used in an advantageous configuration by ending the heating out when the magnitude of the change in the measurement signal as a function of time falls below an establishable threshold value. In other words, when the establishable threshold value is fallen below, the heater is switched off and the gas sensor returns to room temperature or ambient temperature. 
     In this case, account is taken of the fact that initially rapid desorption of nitrogen dioxide from the sensitive layer of the gas sensor usually takes place after the end of a measurement. This desorption then slows down noticeably. The heating out accelerates the desorption considerably. When the magnitude of the change in the measurement signal as a function of time reaches the establishable threshold value, then a nitrogen dioxide surface condition of the sensitive layer of the gas sensor, established by the threshold value, has been reached. 
     Owing to the fact that the heating out is ended at this time, the temperature of the gas sensor decreases and the subsequent desorption of nitrogen dioxide slows down significantly. Therefore, an almost invariant starting point is defined for the subsequent measurement since the preloading with nitrogen dioxide is substantially established by the threshold value. In this way, the measurement accuracy for the subsequent measurements is improved significantly. 
     To this end, for example, the control device may be configured in order to monitor the change in the measurement values as a function of time and to switch off the heater when the threshold value is reached. 
     The change in the measurement value as a function of time which is established during the regeneration phase is used, according to another advantageous configuration, in order to increase the accuracy of the measurement value for the preceding measurement phase. 
     To this end, the value of the change as a function of time, in particular directly after the end of the measurement phase, is incorporated into the evaluation. Thus, besides the absolute value of the measurement signal at the end of the measurement phase, the change in the measurement value as a function of time after the end of the measurement phase is jointly taken into account. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred but in no way restrictive exemplary embodiments of the disclosure will now be explained in more detail with the aid of the figures of the drawing. The features are in this case represented in a schematized fashion. 
         FIG. 1  shows a measurement system and 
         FIG. 2  shows a measurement curve of a gas sensor. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a measurement system  10  for respired gas analysis. The measurement system  10  is contained in a housing, which is not shown in  FIG. 1 . The measurement system  10  comprises a main circuit board  11 , on which the further elements are mounted. The further elements include a gas channel  12 , which comprises an inlet opening  15  and an outlet opening  14 . A pump unit  13  is accommodated in the gas channel  12 . By means of the pump unit  13 , air from outside the measurement system  10  can be drawn into the gas channel through the gas inlet  15 . In this case, the air passes over a nitrogen dioxide sensor  16 . 
     The nitrogen dioxide sensor  16  is a field effect transistor-based gas sensor. It comprises a sensitive layer  17  and a heater  18 . Furthermore, the gas sensor  16  also comprises the typical electronic components for a field effect transistor. 
     The gas-sensitive layer  17  is in this case separated as a so-called suspended gate from the rest of the elements of the gas sensor  16  by a narrow air gap. In this exemplary embodiment, copper phthalocyanine is used as the material for the gas-sensitive layer  17 . 
     The pump unit  13 , the heater  18  and the electrical connections in the region of the sensitive layer  17  are connected to a control and evaluation device in the form of a microprocessor  19 . 
     The microprocessor  19  controls the pump unit, so that a measurement can be carried out at a given time. Furthermore, the microprocessor  19  acquires measurement values from the gas-sensitive layer  17 . Lastly, the microprocessor  19  controls the heater. 
     The microprocessor  19  is configured in order to carry out the following method during operation. When a measurement is imminent, the microprocessor  19  adjusts the heater  18  to an optimal value for a gas measurement. This optimal value may correspond to the ambient temperature, which means that the heater  18  is switched off. The optimal value for the temperature may also lie above the ambient temperature. In that case, the heater  18  is controlled accordingly in order to set this temperature in the gas sensor  16 . The ambient temperature may be jointly measured in order to compensate for the effect of minor variations in the ambient temperature on the gas sensitivity, so as to increase the measurement accuracy. 
     The measurement values for the nitrogen dioxide measurement per se are then recorded and evaluated. Once the measurement phase is finished, the regeneration phase begins. In the regeneration phase, the nitrogen dioxide is desorbed from the surface of the sensitive layer  17 . This causes reversal of the excursion of the measurement value of the sensitive layer  17  which occurred in the measurement phase. The measurement value in this case exhibits a change as a function of time, which is picked up and determined by the microprocessor  19 . 
     During the regeneration phase, the microprocessor  19  regulates the heater  18  to an optimal temperature for heating out the gas-sensitive layer  17 . The temperature used for this may, for example, be 100° C. or 150° C. or even more. 
     Subsequently, during the desorption step, the microprocessor  19  compares the change in the measurement signal as a function of time with an established threshold value. When the change in the measurement signal as a function of time falls below this threshold value, the desorption has taken place to a sufficient extent and the heating-out phase is ended in response to this, i.e. the microprocessor  19  turns the heater  18  off. The sensor  16 , or the gas-sensitive layer  17 , is now in a defined state. In order to minimize further changes of this state before the next measurement phase begins, the heater  18  may now be switched off. 
     The heating out up to a predeterminable threshold value of the change in the measurement signal as a function of time is illustrated in  FIG. 2 .  FIG. 2  shows the profile of a measurement signal of a gas sensor  16  over a measurement period of a few hours. In this case, nitrogen dioxide and air without nitrogen dioxide are alternately brought into the vicinity of the sensor. The sensor signal of the gas-sensitive layer  17  exhibits corresponding excursions. An end of a respective heating-out phase is in this case always reached when the change in the measurement signal as a function of time, symbolized in  FIG. 2  by the tangents  23 ,  24 , reaches the threshold value. 
     The effect advantageously achieved by this is that the desorption of the nitrogen dioxide is accelerated by the heating-out step, without thereby incurring the disadvantage of a reduced sensitivity at high sensor temperatures during the measurement phase. The measurement per se during the measurement phase can be carried out at the optimal temperature in relation to the desired properties of sensitivity and response time as well as other criteria. Independently of the optimal measurement temperature, the desorption takes place at the optimal temperature for the desorption. 
     The change in the measurement signal as a function of time during the desorption of the nitrogen dioxide is dependent on the desorption rate of the nitrogen dioxide. This is determined by the heating-out temperature and the amount of nitrogen dioxide remaining on the surface of the sensitive layer  17 . The higher the temperature and the higher the residual amount of nitrogen dioxide, the greater is the desorption rate and therefore the change in the measurement signal as a function of time. From this change as a function of time, it is therefore possible to deduce the residual amount of nitrogen dioxide on the sensor surface. This is taken into account by the microprocessor  19  when calculating the nitrogen dioxide concentration in a preceding measurement.