DE102008028682A1 - Method for measuring gas concentrations by means of a metal oxide gas sensor, sensor device for carrying out the method and use thereof - Google Patents

Abstract

The invention relates to a method for operating a gas sensor (16) located in a measuring space (36). In order to operate cost-effective gas sensors in a sensitive environment with greater reliability and lower cost, it is proposed to operate the gas sensor (16) in a small measuring volume (14) in at least two operating modes alternately discontinuous. By catalytic cleaning in one of the operating modes and / or filtering out only over a longer period of time changing signal components can then be a baseline-independent survey.

Description

The The invention relates to a method for measuring gas concentrations by means of a metal oxide gas sensor located in a measuring chamber and a sensor device for detecting substances in a fluid with a measuring space and a metal oxide sensor in the measuring space. It also concerns the invention advantageous uses of such a sensor device.

to Detection of different gases and vapors In the industry today, inexpensive thick film metal oxide sensors used. However, metal oxide sensors change theirs Base resistance with operating time and operating temperature. A possible use of these gas sensors in fire alarm and surveillance equipment is thus problematic as a by a control or software set alarm threshold with the Operating time is exceeded. A use of metal oxide sensors therefore always involves complex calibration and Maintenance work.

Examples of sensor devices with metal oxide gas sensors are in WO-01/65211 A2 as well as the EP 0 722 789 A1 disclosed. In this case, elaborate calibration work using external gas sources is described. The known sensor devices are therefore labor-intensive and require external gas reservoirs.

Out Th. Becker, St. Mühlberger, Chr. Bosch von Braunmühl, G. Muller, A. Meckes and W. Benecke: "Gas Mixture Analysis Using Silicon Micro Reactor Systems" Journal of Microelectromechanical Systems 9 (2000) 478-484 ; and from Th. Becker, St. Mühlberger, Chr. Bosch von Braunmühl, G. Müller, A. Meckes and W. Benecke: "Microreactors and Microfluidic Systems: An Innovative Approach to Gas Sensing Using Tin-Oxide-Based Gas Sensors"; Sensors & Actuators B77 (2001) 48-54 It is known to operate gas sensors in conjunction with microreactors. A microreactor (also called a microstructured reactor or microchannel reactor) is a device for performing chemical or physical reactions in an environment having typical widths of less than about 1 mm. Microreactors have z. B. microchannels or are designed as such. Such microchannels typically have a flow area of the order of less than about 1 mm ² .

to Detection of different gases and vapors Nowadays in the industry relatively easy and less expensive manufacturable Dickschichtmetalloxidsensoren used. To the power consumption To reduce these sensors, the current trend of research however in the direction of thin-film technology. The sensitive ones Layers of the sensors here only have a nominal layer thickness from a few 100 nm.

One big disadvantage of metal oxide (MOX) gas sensors is the Drift of the sensor base resistor. By grain size growth the base resistance of the polycrystalline sensor materials increases slowly during operation. This effect will in particular due to the necessary high sensor temperatures (300 ° C-500 ° C) driven.

The drift of the sensor resistance is in 10 clarified in more detail. 10 FIG. 12 shows the measurement results of the measurement of a sensor base resistance of various types of prior art MOX gas sensors during a constant temperature operation (at about 400 ° C.) over a period of several months. In particular, for all four MOX gas sensors measured, the base resistance initially increases more strongly and does not remain the same thereafter.

A change in the basic resistance detected during a measurement Thus, on the one hand by the presence of a reactive gas or on the other hand caused by the grain size growth become.

The drift of the base resistor is particularly problematic for use in automatic fire alarm systems or other automatic monitoring systems, as described below with reference to 11 is explained. 11 shows a graph of a sensor resistance with a base resistance 100 over a longer period of time. The basic resistance 100 rises, as before by the 10 gradually explains. There are two fire situations 102 and 104 located. A fire alarm threshold 106 has been fixed based on the initial base resistance. In case of fire, the sensor resistance decreases to some extent. At the first fire situation 102 is the base resistance 100 still relatively low, so that by lowering the sensor resistance of the fixed fire alarm threshold is exceeded and a fire alarm is triggered. But, as with the second, later fire situation 104 , the base resistor 100 higher, the lowering of the sensor resistance in case of fire is no longer sufficient to fall below the fixed fire alarm threshold; the fire is not recognized.

For the reasons mentioned above, when using MOX gas sensors, regular calibration of the base resistor under zero-gas conditions is required. However, in open systems, as they are particularly desirable for monitoring tasks, this is only with great effort perform.

in the Generally, such a review is with human resources Expense and the need for external reference gas reservoirs connected.

This elaborate testing prevents use of MOX gas sensors in application areas with increased Safety requirements. For example, the aircraft manufacturer uses Airbus only sensors that constantly perform a self-test without additional resources (gas reservoirs, Staff) are needed. To date, no procedure It is known that a determination of the Basiswidertand of MOX gas sensors without additional reference gas reservoir allows.

task The invention is a method for operating a MOX gas sensor to provide such that the MOX gas sensor via a longer and more accurate and reliable.

These The object is achieved by a method according to the attached Claim 1 solved.

A Sensor device with which perform such a process is, is specified in the secondary claim.

advantageous Embodiments of the invention are the subject of the dependent claims.

advantageous Uses of the invention are specified in the further independent claim.

With the method according to the invention is the measurement of reactive gas concentrations using MOX gas sensors without Relation to the sensor base resistor allows.

According to the invention to avoid influencing the measurement by a drift of the Sensor base resistor used two basic principles of operation. On the one hand, in a measuring room in which the MOX gas sensor is operated is catalytically decomposed existing reactive gases to a zero gas atmosphere in the measuring room, which is used to re-calibrate the MOX gas sensor can be used. This is especially true for a small one Measuring chamber volume feasible, which is why the measuring space preferably is designed as a micro-chamber. On the other hand, the sensor signal processed so that only slowly over a longer Periodically varying signal components filtered out be and only in smaller periods of time, the implementation of measuring cycles, changing signal components for the measurement is observed.

• a) Ersetzen eines in dem Messraum befindlichen Gases mit dem zu untersuchenden Gas,
• b) Einschließen des zu untersuchenden Gases in dem Messraum und
• c) Vermessen der Gaskonzentration über Erfassen des elektrischen Widerstandes des Metalloxid-Gassensors während der Einschließphase in Schritt b),

und wobei weiter zur Verringerung oder Vermeidung von Messfehlern durch eine Sensordrift:

• • nach Schritt c) und vor dem erneuten Schritt a) der Schritt d) katalytische Umsetzung von Gasen in dem Messraum in nicht-reaktive Produkte und Kalibrieren des Basiswiderstands für darauffolgende Mes sungen auf einen daraufhin erhaltenen elektrischen Widerstandswert des Metalloxid-Gassensors durchgeführt wird und/oder
• • von einem von dem Metalloxid-Gassensor (16) während der Messzyklen erhaltenes primäres Sensorssignal sich nur langsam über eine Vielzahl von Messzyklen hinweg ändernde Signalanteile herausgefiltert werden und nur der verbleibende, auf Änderungen im Rhythmus der Messzyklen basierende Signalanteil zur Vermessung herangezogen wird.

The invention provides a method for measuring gas concentrations by means of a metal oxide gas sensor located in a measuring space, wherein a measuring cycle has at least the following steps a) to c) and several such measuring cycles are carried out successively:

• a) replacing a gas in the measuring space with the gas to be examined,
• b) enclosing the gas to be examined in the measuring space and
• c) measuring the gas concentration by detecting the electrical resistance of the metal oxide gas sensor during the containment phase in step b),

and further to reduce or avoid measurement errors by a sensor drift:

• After step c) and before the renewed step a) the step d) catalytic conversion of gases in the measurement space into non-reactive products and calibration of the base resistance for subsequent measurements to a subsequently obtained electrical resistance of the metal oxide gas sensor is carried out and / or
• • one of the metal oxide gas sensor ( 16 ) obtained during the measuring cycles primary sensor signal are filtered out only slowly over a plurality of measuring cycles away changing signal components and only the remaining, based on changes in the rhythm of the measuring cycles signal component is used for the measurement.

By the multiple execution of several measuring cycles in a row On the one hand there is the possibility of between measuring cycles or as part of the measurement cycles, a calibration based on a cleaned Perform the measuring room. On the other hand, through the repetition the measuring cycles given a certain period, within which should change the sensor signal. This period can be for Signal processing are used by longer-term Changes are filtered out.

Both basic procedural principles can basically alternatively be performed. In a particularly preferred Embodiment be the two principles of the method carried out jointly, one with the first procedural principle Calibrated base resistor during signal processing after second process principle is used, so a larger one and possibly more accurate output signal.

In a preferred embodiment, the procedure for determining the base resistance is such that the sensor is placed in a small chamber volume (in FIG Area of at most a few cm ³ , preferably less than 1 cm ³ , more preferably less than 500 mm ³ ) is integrated, which can be separated from the environment time discrete. Activating a catalyst also introduced into the volume, the existing gases are converted into non-reactive products both on the sensor surface and through the catalyst. For example, gases are burned to CO ₂ and H ₂ O. The gas volume after the combustion phase no more reactive gas shares (zero gas concentration) and then measured sensor resistance corresponds to the base resistance. A sensor calibration can thus be carried out automatically.

at a further proposed approach is via a appropriate signal processing a measurement independent of Base resistance achieved.

It result in longer replacement cycles for the sensors. Furthermore, false alarms are avoidable. The sensor function is - too over longer periods and maintenance intervals - guaranteed. The maintenance intervals can be reduced. Besides, they are the uses and applications of gas sensors, as in particular metal oxide sensors further extended.

Especially is an employment in fire detection systems in aerospace vehicles possible.

embodiments The invention will be described below with reference to the attached Drawings explained in more detail. It shows:

1 a schematic cross section through the structure of a microreactor system with a gas sensor;

2 a graph of a gas measurement using this microreactor system;

3 a graph showing the change in the chamber temperature and the chamber humidity when switching on and off a pump;

4 a graph showing the change in the chamber humidity and the sensor resistance when switching on and off the pump;

5 two graphs showing the sensor response (relative resistance change) of a MOX sensor when exposed to a) humidity and b) of NH ₃ at different sensor operating temperatures;

6 a circuit diagram of an RC network, which is used for signal processing for measuring concentration levels of reactive gases at a MOX gas sensor with base resistance R _{S_0} ;

7 a graph showing as simulation results the effects of a principle of the circuit of 6 performed signal processing shows;

8th the comparison of two graphs comparable to those of 7 that illustrates the effects of better matching a base resistor;

9 a circuit diagram of another embodiment of an RC network, which is used for signal processing for measuring concentration levels of reactive gases on a MOX gas sensor and having an adaptable to a variable base resistance load resistor;

10 a multi-trace graph illustrating the drift of a sensor base resistance of different types of MOX gas sensors of the prior art during a constant temperature operation over a period of several months; and

11 a schematic representation of the drift problem in a method according to the prior art for operating a MOX gas sensor with fixed alarm thresholds.

in the Below is an embodiment with reference to the accompanying figures an operating method for a gas sensor, in which a micro-chamber for measuring gas concentrations with used as the metal oxide gas sensor formed gas sensor so is that the measurement is not affected by a sensor baseline drift.

In a preferred embodiment, a sensor device 10 with an advanced microreactor system 12 (hereinafter also referred to as EMRS) used for gas analysis.

This in 1 closer illustrated microreactor system 12 has a micro chamber 14 , a MOX gas sensor 16 , a catalyst 18 and a pump 20 ,

The micro chamber 14 is made, for example, in microtechnology. The micro chamber 14 has thin gas inlets 22 and gas outlets 24 on, the microchannels 26 have as flow channels. The microchannels 26 Due to their very small flow cross-section have a high flow resistance, so that the micro-chamber 14 - Even without end elements such as valves or the like - only by the thin gas inlets 22 and gas outlets 24 separated from the environment.

The MOX gas sensor 16 is in the thus closed micro-chamber 14 integrated. He is about a not shown and only by heating connections 58 indicated heating device heated.

The catalyst 18 is also by means of a non-illustrated and only by heating connections 58 indicated heating device designed to be heated and also in the micro-chamber 14 built-in.

The pump 20 is designed for example as a micropump and designed such that the air in the micro-chamber 14 aspirated and replaceable by fresh air with unknown concentrations of reactive trace gases.

• 1) in einem durchströmten Aktivzustand 28 (Pumpe an) – Pumpphase-;
• 2) in einem Ruhezustand 34 (Pumpe aus) – Ruhephase oder Einschlussphase-.

For the purpose of gas analysis, the microreactor system 12 operated alternately in two different operating modes:

• 1) in a flowed through active state 28 (Pump on) - Pumpphase-;
• 2) in a dormant state 34 (Pump off) - resting phase or enclosure phase-.

In a pumping phase (active state 28 ) Fresh air with unknown trace gas concentrations from the environment in the micro-chamber 14 sucked.

This is followed by a hibernation 34 , The pump 20 is switched off, which is why the sucked fresh air in the micro chamber 14 is included. In this confining phase, the volume of air drawn in is calculated using the built-in MOX gas sensor 16 analyzed.

After completion of the gas measurement, the possibly still existing reactive gas by heating the also integrated catalyst 18 catalytically decomposed or burned. In this way, a zero gas sample can be generated synthetically before each gas exposure. After that is the microreactor system 12 ready for a new measuring cycle.

• a) Einschalten der Pumpe 20;
• b) Ausschalten der Pumpe 20;
• c) Abfragen des Sensorsignals des MOX-Gassensors 16;
• d) Beheizen des Katalysators 18.

The measuring cycle therefore has the following steps:

• a) switching on the pump 20 ;
• b) Switch off the pump 20 ;
• c) Querying the sensor signal of the MOX gas sensor 16 ;
• d) heating the catalyst 18 ,

After the end of this measurement cycle, a new measurement cycle can be performed again with the beginning of step a). Such measuring cycles can be carried out periodically one after the other in an automated measuring method. For this purpose, the sensor device 10 with a controller 50 for controlling the individual components of the microreactor system 12 Mistake. The controller 50 is a calibration device 51 assigned for automatic calibration.

The control 50 is with a lead 52 to the pump 20 connected in order to control these periodically. The pump 20 with the gas inlets 22 and the gas leaks 24 and the controller 50 thus form a flow device 62 by which the flow of gas and in particular moist air through the micro-chamber 14 and thereby through the measuring room 36 can be adjusted and, in particular, can be changed periodically. In the preferred embodiment shown here, this is done simply by switching the pump on and off 20 , The control 50 is on with a signal line 54 to the MOX gas sensor 26 connected and with a line 56 to the heater of the catalyst 18 connected. The control 50 has a - for example, realized as a software - self-test device 60 on that over the signal line 54 obtained sensor signal in response to the control of the flow device 62 and in particular depending on the switching state of the pump 20 correlates and thereby performs a self-test. Such a self-test function will be explained in more detail below.

In a periodic execution of the above-mentioned measuring cycles results from the switching on and off of the pump 20 a periodic gas exchange process, which is reflected in a periodic waveform, which in the following also as "breathing" of the microreactor system 12 referred to as.

Starting from the periodically discontinuous mode of operation of the microreactor system 12 There are special possibilities of a baseline-independent gas measurement, which are explained in more detail below. In addition, there is the possibility of a self-test function.

First procedure for baseline-independent Gas measurement:

Gas detection with the help of the microreactor system and periodic re-calibration by zero-gas generation:

For illustration of the microreactor system-based gasification method is shown in 2 the course of the electrical resistance of the MOX gas sensor 16 plotted over several EMRS measurement cycles as a function of time. As a rectangle schematically indicated are the respec gen times - active states 28 - in which the microreactor system 12 by activating the pump 20 Forcibly flows through fresh air.

In the measuring example of 2 In each of the six illustrated pump cycles, moist fresh air is sucked in from the environment. Except for very small periodic increases in sensor base resistance during the pumping phases - active state 28 - As expected, no sensor response is observed.

However, with the start of the third pumping phase, the microreactor system becomes 12 Air sucked in with a small amount of NO ₂ . In response, as expected, a large increase in resistance is observed. At the end of the inclusion phase, the built-in catalyst becomes 18 activated by electric heating and the sucked NO ₂ in not or much less detectable N ₂ and O ₂ reacted. By catalytic conversion of the reactive gas sample so artificial zero gas can be generated. After that is the microreactor system 12 ready for a new measuring cycle.

By zero gas exposure and measurement of the MOX sensor resistance at the beginning of the subsequent fourth pumping phase, a re-calibration of the sensor base line can thus be obtained relative to which the possibly subsequently aspirated reactive gas can be evaluated. The calibration device 51 is software designed to automatically perform such calibration at the end of each measurement cycle.

A possibly occurring slow drift of the sensor baseline during of EMRS operation may be due to frequent periodic Re-calibration measurements are tolerated.

2 thus provides the gas measurement using a microreactor system 12 dar. The curve 30 gives the electrical resistance of the MOX gas sensor 16 as a sensor signal again. The time in seconds is entered on the x-axis. Next is the pump 20 applied pump voltage 32 reproduced in volts. With the beginning of the third pumping phase, NO _{2 is} sucked into the EMRS. During the other pumping phases, moist fresh air is sucked in. At the end of each of the measuring cycles, the microchamber is catalytically cleaned of any reactive gas present.

Implement a self-test function:

As further observation falls in the 2 on, that even in the absence of reactive trace gases, the resistance 30 in the pumping phase - active state 28 - and the inclusion phase - resting state 34 - slightly different values. As will be shown below, this is "breathing" of the microreactor system 12 due to the fact that with each intake cycle more or less moist air is sucked in and that then in the measuring room 36 - ie in the reactor interior of the micro chamber 14 - set different moist concentrations in the two different operating modes. These naturally occurring humidity variations can be used to self test the EMRS integrated MOX gas sensor 16 be used.

Since H ₂ O molecules can not diffusively penetrate dense surface coverings due to their size, the absence of a different MOX sensor response in the two operating modes indicates the absence of passively acting surface coatings. With regard to this diffusive inhibition, H ₂ O behaves similarly to a large number of other reactive gases, such as those described in US Pat. B. occur in the event of a fire or other accident in the area. An exception in the field of reactive gases is formed by H ₂ alone, which due to its small size can also penetrate relatively thick, passively acting SiO ₂ and Al ₂ O ₃ layers. Since atmospheric moisture is present in all regions of the earth, the proposed H ₂ O-based process is universally applicable and, in particular, is superior to possible H ₂ -based processes.

• 1) Die in 2 dargestellte Mikrokammer 14 ist aus Keramik oder einem anderen schlecht wärmeleitenden Material hergestellt.
• 2) Der Messraum 36 ist durch die Gaseinlässe 22 und Gasauslässe 24 mit einem hohem Strömungswiderstand mit der Außenwelt verbunden. Diese Gaseinlässe 22 und Gasauslässe 24 behindern den diffusiven Gasaustausch mit der Umgebung so stark, dass bei abgeschalteter Pumpe 20 die Luft im Inneren der Mikrokammer 14 – also in dem Messraum 36 – effektiv von der Außenluft getrennt ist.
• 3) Im Inneren der Mikrokammer 14 wird der durch die Heizeinrichtung beheizbare MOX-Gassenor 16 betrieben. Durch die Beheizung und die thermische Isolierung des Messraumes 36 kommt es deshalb in dem Ruhezustand 34 des Mikroreaktorsystems 12 zu einem Hitzestau in dem Messraum 36.

The physical backgrounds are explained in more detail below. The sensor device 10 is characterized in particular by the following three facts:

• 1) The in 2 illustrated micro-chamber 14 is made of ceramic or other poor thermal conductivity material.
• 2) The measuring room 36 is through the gas inlets 22 and gas outlets 24 connected to the outside world with a high flow resistance. These gas inlets 22 and gas outlets 24 hinder the diffusive gas exchange with the environment so much that when the pump is switched off 20 the air inside the micro chamber 14 - in the measuring room 36 - is effectively separated from the outside air.
• 3) Inside the micro chamber 14 becomes the heated by the heater MOX-Gassenor 16 operated. By the heating and the thermal insulation of the measuring room 36 therefore it comes in the idle state 34 of the microreactor system 12 to a heat accumulation in the measuring room 36 ,

• – Nach Ansaugen einer frischen Probe mit endlicher aber ansonsten unbekannter Luftfeuchte wird bei Beginn des Ruhezustandes 34 das eingeschlossene Luftvolumen über die Umgebungstemperatur erhitzt. Durch den in dem Ruhezustand 34 des Mikroreaktorsystems 12 auftretenden Hitzestau wird die Absorption der eingetragenen Feuchte an den Redaktorinnenwänden reduziert und die Aufnahmefähigkeit der eingeschlossenen Luft für H₂O-Moleküle erhöht. Im Ruhezustand 34 sieht der eingeschlossene MOX-Gassensor 16 deshalb eine gegenüber der Außenluft erhöhte Luftfeuchte.
• – Im darauf folgenden durchströmten Zustand – Aktivzustand 28 – wird die feuchte Luft aus dem Reaktorinneren abgesaugt und durch kältere und weniger feuchte Außenluft ersetzt.

From the thermal and gas kinetic separation of the interior of the reactor from the outside world, the following consequences arise for the two operating modes:

• - After aspiration of a fresh sample with finite but otherwise unknown humidity is at the beginning of the resting state 34 the trapped air volume is heated above ambient temperature. By in the resting state 34 of the microreactor system 12 occurring heat accumulation reduces the absorption of the registered moisture at the inner editor walls and increases the absorption capacity of the trapped air for H ₂ O molecules. At rest 34 sees the enclosed MOX gas sensor 16 Therefore, compared to the outside air increased humidity.
• - In the subsequent flow state - Active state 28 - The moist air is sucked out of the interior of the reactor and replaced by colder and less humid outside air.

An MOX gas sensor 16 with reactive surface therefore sees - depending on the operating mode - two different humidity levels that can be used for the sensor test.

The self-test device 60 Therefore, for example, is designed such that it monitors a constant change of the sensor signal when switching between the two operating modes. If, however, the sensor signal remains the same when the operating mode is changed, then the self-test device outputs 60 a fault signal. The control 50 then reports a function control of the MOX gas sensor 16 ,

The mechanisms described are by the in the 3 . 4 and 5 Illustrated as graphs results.

3 shows the change in the chamber temperature and the chamber humidity when switching on and off the pump 20 , The gradually decreasing humidity is due to the settling of the previously opened microreactor system 12 due. 3 in particular, shows the periodic variation of the chamber temperature 40 in the two operating modes and a concomitant change in the relative humidity in the chamber interior. A curve for the chamber humidity 42 was here using one in the microreactor system 12 placed wet sensor detected directly.

4 shows the change in the chamber humidity 42 and the sensor resistance 30 when switching on and off the pump 20 , 4 proves that the humidity changes measured with the built-in humidity sensor (not shown) are also from the micro chamber 14 integrated MOX gas sensor 16 be detected.

5 shows the senior response (the relative resistance change) of the MOX gas sensor 16 when exposed to a) humidity - left-hand illustration - and b) NH ₃ - right-hand illustration - at different sensor operating temperatures. The parameter in both figures is the applied vapor or gas concentration. The left illustration in 5 ( 5a ) shows that the moisture sensitivity of a MOX gas sensor 16 itself strongly depends on the sensor operating temperature. In particular, it is observed that at sensor operating temperatures less than about 100 ° C no water vapor sensitivity is observed. The right representation of 5 ( 5b On the other hand, at sensor operating temperatures where significant water vapor sensitivity is observed, MOX gas sensors 16 also have a high sensitivity to other reactive gases (here NH ₃ ).

Overall, in the 3 . 4 . 5 shown data that at sufficiently high sensor operating temperature, the "breathing" of the MOX gas sensor - that is dependent on an air flow and / or the moisture content change of the sensor signal (in this case the resistance 30 ) - can be interpreted as an indication of a functioning reactivity of the sensor surface.

The microreactor system used here 12 is preferably manufactured in microtechnology using manufacturing techniques known in the semiconductor device industry. The volume of the measuring room 36 is preferably below 1 cm ³ , for example in the range of less than about 200 mm ³ .

The alternating, in particular periodic operation offers but also the following advantageous procedure for a baseline-independent gas measurement:

Second method for baseline-independent gas measurement:

In addition to the method described above, the sensor base reference after every single gas measurement to redefine and thus re-calibrate, offers the microreactor system 12 and the sensor device provided therewith 10 Another way to perform gas measurements without reference to the specific sensor base resistance.

By periodically switching between inclusion phase - resting phase 34 - and pumping phase - active state 28 - the microreactor system 12 Almost rectangular changes in concentration can be generated. After completion of an inclusion phase, reactive gas enters the microchamber 14 sucked, so there is a strong, abrupt change in resistance. Resistance changes based on such measurements of reactive gases change as the cycles of measurement change.

The sensor device 10 has a signal processing device 64 on, by means of which over long periods slowly changing signal components can be filtered out, so that essentially only those in the order of the frequency of the measuring cycles and / or the operating phases changing signal components are considered for the gas measurement.

In one embodiment, the signal processing device 64 an RC network 66 on how it is in 6 is shown in more detail. 6 shows a block diagram of a circuit for signal processing. In this case, a base voltage U _B via a voltage divider with the load resistance R _L and the sensor resistance R _{S is} applied. A voltage picked off at the sensor resistor R _S is applied to the RC network as the primary sensor signal U _S (t) 66 transfer.

The RC network 66 has a differentiated as a differentiating first RC element 68 with a capacitance C _diff and a resistor R _diff , a driver amplifier 69 and a second RC element connected as an integrator 70 with a resistance R _int and a capacitance C _int . Next is in 6 the one by the first RC element 68 flowing input current I _into and through the second RC element 70 flowing output current I _out shown.

The aforementioned abrupt change in resistance becomes capacitive coupling to the RC network 66 transfer. In the presence of a concentration gradient, as occurs in particular with a change from reactor-internally generated zero air to sucked reactive gas, the resistance change occurring at the sensor resistance R _S by the first RC element 68 transmit, wherein at its output substantially the time derivative U _drive (t) of the primary sensor signal U _S (t) is formed. Behind the following driver amplifier 69 (unity gain buffer) is the second RC element 70 arranged, which integrates the differentiated output signal again, whereby the output signal U _out (t) is formed.

The first RC element 68 acts as a high pass filter. By this high pass filter property of the first RC element 68 U _out (t) is not a 1: 1 image of the primary sensor output signal U _S (t) that includes the baseline information. Rather, the output signal U _out (t) is a processed sensor signal which contains only the metrologically relevant alternating component.

The only condition for this signal processing method, which filters out the sensor drift, is that the sensor base resistance compared to the Per odendauer of the microreactor system 12 only drifts very slowly. However, this condition is generally always met.

In the 6 The circuit shown only shows a practical example with which this signal processing method can be carried out. It will be appreciated that similar procedures may be practiced using a variety of filtering techniques known in the art. Conceivable are both hardware implementations and software implementations.

undThose in the RC network 66 occurring voltage _signals U _drive and U _out result, for example, from the de primary generated sensor input signal U _S (t) by solving the following two coupled differential equations:

and

For example, these two solutions can also be generated digitally from successive discrete values of the primary sensor signal U _S (t).

7 shows accordingly digitally obtained simulation results. In this case, the primary voltage signal U _S , the differentiated output signal of the first RC element, ie the time derivative U _drive and the secondary sensor output signal produced by integration - output signal U _out - are shown. One recognizes the separation of the sensor base line, which has resulted in the present case by a special choice of the voltage divider resistors. In the present example, the load resistance R _{L was} based on the actual base resistance R _{S_0 of} the MOX gas sensor 16 set. As a gas-induced resistance change has been assumed for the purpose of the present simulation ΔR _S = 0.5 × R _{S_0} .

In 8th a comparison of such simulations for different adaptations of the load resistance R _L to the actual base resistance R _{S_0 is} shown. It is the primary sensor output signal U _S , the differentiated output signal U _drive and the sensor output signal U _out restored by integration under conditions of poor adaptation of the load resistance R _L to the sensor base resistor R _{S_0} . As a gas-induced resistance change ΔRs = 0.5 × R _{S_0 has again been} assumed in the simulation.

The comparison shows that baseline unab pending output signals U _{out can} also be obtained at significantly smaller (x 0.1) and larger (x 10) sensor base resistances R _{S_0} . However, it can also be seen that in the case of poor adaptation (R _L not equal to R _{S_0} ), smaller sensor output signals U _out are obtained. Serious deviations from the condition R _{S_0} = R _L reduce the primary sensor signal U _S and the secondary sensor signal U _out .

This Disadvantage can in a preferred embodiment of the method be compensated for by the one described in the first method Possibility of a periodic or at least some the zero cycles measured in the microreactor system Use.

9 shows one opposite the circuit of 6 Modified circuit with which this possibility is available. The circuit of 9 is different from that of 6 essentially in that the load resistance R _{L is} controllable, for example as a transistor T, is executed. The load resistance R _L is then, for example, depending on the zero gas measurement adjustable so that it corresponds to the current sensor base resistance R _{S_0} . With such a circuit, the condition R ₁ = R _{S_0} (or at least about the same) can be maintained even in the case of a drifting sensor base line and thus the relative size of the gas response can be maintained.

9 shows a variant of the circuit of 6 with adjustable R ₁ (transistor T). In the case of a drifting sensor baseline and especially when using a microreactor system 12 For example, the condition R _L ≈ R _{S_0} or R _L = R _{S_0 can be made} at the end of each confinement phase after the generation of zero gas. Also indicated is the possibility of an integrator reset S _reset at the end of an inclusion phase.

In summary, it can be stated that by using a micro-chamber 14 Gas concentrations using metal oxide gas sensors 16 Measure without influencing a sensor baseline drift.

It is a method for operating a in a measuring room ( 36 ) located gas sensor ( 16 ). In order to cost-effective gas sensors ( 16 ) in sensitive environment with a greater reliability and lower cost to operate, it is proposed that the gas sensor ( 16 ) in a small measuring volume ( 14 ) operate alternately intermittently in at least two modes of operation. By catalytic cleaning in one of the operating modes and / or filtering out only over a longer period of time changing signal components can then be a baseline-independent survey.

10

sensor device

12

Microreactor system (EMRS)

14

microchamber

16

MOX gas sensor

18

catalyst

20

pump

22

gas inlet

24

gas outlet

26

microchannel

28

active state

30

resistance

32

pump voltage

34

hibernation

36

measuring room

40

chamber temperature

42

chamber humidity

50

control

51

calibration

52

management to the pump

54

signal line

56

management to the heater of the catalyst

58

Heating connections

60

Self-testing device

62

Flow device

64

Signal processing device

66

RC network

68

first RC element

69

driver amplifier

70

second RC element

100

base resistance

102

first fire situation

104

second fire situation

106

firmly set fire alarm threshold

U _B

base voltage

R _L

load resistance

R _S

sensor resistance

U _s

primary sensor signal

C _diff

capacity of the first RC element

R _diff

resistance of the first RC element

C _int

capacity of the second RC element

R _int

resistance of the second RC element

U _drive

time Derivation of the primary sensor signal

U _out

output

T

Transistor (adjustable load resistance R _L )

S _reset

Integrator reset

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• WO 01/65211 A2 [0003]
• - EP 0722789 A1 [0003]

Cited non-patent literature

• -Becker, St. Mühlberger, Chr. Bosch von Braunmühl, G. Muller, A. Meckes and W. Benecke: "Gas Mixture Analysis Using Silicon Micro Reactor Systems" Journal of Microelectromechanical Systems 9 (2000) 478-484 [0004]
• - Th. Becker, St. Mühlberger, Chr. Bosch von Braunmühl, G. Muller, A. Meckes and W. Benecke: "Microreactors and Microfluidic Systems: An Innovative Approach to Gas Sensing Using Tin-Oxide-Based Gas Sensors"; Sensors & Actuators B77 (2001) 48-54 [0004]

Claims (23)

Method for measuring gas concentrations by means of a measuring chamber ( 36 ) metal oxide gas sensor ( 16 ), wherein a measuring cycle has at least the following steps a) to c) and a plurality of such measuring cycles are carried out successively: a) replacing one in the measuring space ( 36 ) gas with gas to be examined, b) enclosing the gas to be examined in the measuring space ( 36 ) and c) measuring the gas concentration by detecting the electrical resistance of the metal oxide gas sensor during the containment phase in step b), and further to reduce or avoid measurement errors by a sensor drift: • after step c) and before the renewed step a) Step d) catalytic conversion of gases in the measurement space ( 36 ) in non-reactive products and calibration of the base resistance (R _{S_0} ) for subsequent measurements to a subsequently obtained electrical resistance of the metal oxide gas sensor ( 16 ) and / or of one of the metal oxide gas sensor ( 16 ) received during the measuring cycles primary sensor output signal (U _S ) are filtered out only slowly over a plurality of measuring cycles away changing signal components and only the remaining, based on changes in the rhythm of the measuring cycles signal component (U _out ) is used for the measurement. Method according to Claim 1, characterized in that, for filtering out the signal components which change only slowly, the signal from the metal oxide gas sensor ( 16 ) Obtained primary sensor output signal (U _S) is first differentiated with respect to time and then integrated again so that such a processed output signal (U _out), although variations from one measurement cycle to the next indicative, but only slowly changing over a plurality of measuring cycles, signal components other than eight stay. A method according to claim 2, characterized in that the sensor signal (U _drive ) is amplified between differentiating and integrating. Method according to one of the preceding claims, characterized in that the primary sensor signal (U _S ) by means of a circuit, in particular a voltage divider, with a depending on the determined in step d) base resistance (R _{S_0} ) adjustable or variable load resistor (R _L ; T). Method according to one of the preceding claims, characterized in that the filtering out by means of an analog high-pass filter ( 66 ; 68 ) is carried out. Method according to one of Claims 1 to 4, characterized in that the filtering out takes place by means of digital signal processing on the basis of successive discrete values of the primary sensor signal (U _S ). A method according to claim 6, characterized in that the digital signal processing in dependence on the base resistance (R _{S_0} ) determined in step d) takes place. Method according to one of the preceding claims, characterized in that for carrying out step a) a flow of ambient air through the measuring space ( 36 ) is turned on. Method according to one of the preceding claims, characterized in that the metal oxide gas sensor ( 16 ) in the interior of a microreactor ( 12 ) is operated. A method according to claim 8 and 9, characterized in that for replacing the gas to be examined at least one pump ( 20 ) is switched on to the gas to be examined by flow channels ( 22 . 24 . 26 ) of the microreactor ( 12 . 14 ), which have such a high flow resistance that a substantial gas exchange can take place only by pump action, wherein the inclusion of the gas to be examined by switching off the pump ( 20 ) he follows. Method according to one of the preceding claims, characterized in that it is performed automatically controlled becomes. Method according to one of the preceding claims, characterized in that the measuring cycles are repeated periodically become. Method according to claim 12, characterized in that the gas sensor ( 16 ) is re-calibrated periodically by means of repeated execution of step d). Method according to one of the preceding claims, characterized in that for the catalytic purification in step d) a catalyst arranged in the measuring space is heated. Sensor device ( 10 ) for detecting substance concentrations in a gas with a measuring space ( 39 ), and a metal oxide gas sensor ( 16 ) in the measuring room ( 36 ) characterized by a catalyst ( 18 ) for catalytic cleaning in the measuring room ( 36 ) located gas having calibration ( 51 ) for automatically calibrating the metal oxide gas sensor ( 16 ) and / or a signal processing device ( 64 ) for filtering out signal components changing only slowly over a plurality of measurement cycles from one of the metal oxide gas sensors ( 16 ) received during the measurement cycles primary sensor signal (U _S ). Sensor device according to claim 15, characterized in that the signal processing device ( 64 ) supplies as output signal (U _out ) only the remaining signal component based on changes in the rhythm of the measuring cycles for measuring the gas concentration. Sensor device ( 10 ) according to one of the preceding claims, characterized in that a flow-through device ( 62 ; 20 . 22 . 24 . 26 ) for preferably cyclically changing a flow of gas to be examined through the measuring space ( 36 ) is provided. Sensor device according to one of claims 15 to 17, characterized in that the measuring space ( 36 ) as an interior or microchamber ( 14 ) of a microreactor ( 12 ) is formed, which with at least one flow channel ( 22 . 24 . 26 ) is provided for introducing and / or discharging gas to be examined. Sensor device according to claim 18, characterized in that a pump ( 20 ) to the at least one flow channel ( 22 . 24 . 26 ) and that the flow channel ( 22 . 24 . 26 ) is formed capillary with such a high flow resistance, that without switching on the pump ( 20 ) Substantially no gas exchange takes place. Sensor device according to one of the preceding claims, characterized in that a controllable heating device of the catalyst ( 18 ) is provided. Sensor device according to one of the preceding claims, characterized in that the signal processing device ( 64 ) a differentiator ( 68 ) for differentiating the primary sensor signal (U _S ) in time and an integrator ( 70 ) for integrating one of the differentiator ( 68 ) supplied signal (U _drive ) over time is provided. Sensor device according to one of the preceding claims, characterized in that the signal processing device ( 64 ) for picking up the primary sensor signal (U _S ) as a function of the voltage applied by the calibration device ( 51 ) calibrated base resistor (R _{S_0} ) adjustable load resistance (R _L ; T). Use of a sensor device ( 10 ) according to one of claims 15 to 22: • for monitoring the air in a room for people, in particular in public spaces and / or in the cabin of a vehicle, in particular an aircraft or spacecraft, or • in fire alarm systems, in particular in air or spacecraft.