DE102005009246A1 - Reducable or oxidizable gas or vapor concentration determining method, involves using electrical charge present in sensor as measure for concentration of gas or vapor and evaluating charge by evaluation circuit - Google Patents

Abstract

The method involves using an electrical charge present in a sensor (10.6) as a measure for concentration of a oxidizable or reducable gas or vapor present in surrounding atmosphere of the sensor, where the charge is varied in gas or vapor dependent manner. The charge is evaluated by an evaluation circuit. The sensor is charged by a voltage supply so that the charge is developed in an operating layer of the sensor. An independent claim is also included for a device for measuring gas or vapor concentration.

Description

Technical area:

The The invention relates to a method for determining the in one Metal oxide semiconductor gas sensor, for Example tin dioxide gas sensor, surrounding atmosphere contained Shares of oxidizable or reducible gases or vapors, consisting from an electrically heated metal oxide semiconductor gas sensor and a suitable electrical evaluation circuit, according to the preamble of claim 1 and a device for this purpose for carrying out the Process.

Since 1962, gas sensors (so-called TAGUCHI sensors) are known in which heated layers of certain metal oxides (eg, tin dioxide) are exposed to the air to be monitored for the purpose of detecting airborne gases or vapors. 1 , It is 1.1 , the substrate carrying the sensor layers, which can be made of ceramic or of silicon. 1.2 , is a heater, which can be made as an electrical resistance, for example, made of platinum or in the case of silicon sensors can also be made of polysilicon. The heater 1.2 Tempered the active layer to a certain temperature, which is typically between 300-450 ° C. The electrical resistance of the active layer is via a finger-like contact structure 1.3 tapped. The active layer 1.4 consists of metal oxide, and may have different grain sizes. As gas-sensitive metal oxide substances are known, such as tin dioxide, zinc oxide, tungsten oxide, indium oxide, gallium oxide and some other metal oxides.

at Fumigation with light oxidizable gases, e.g. Hydrogen penetrates the gas enters the metal oxide structure and reacts with the oxide, which is partially reduced.

in the As a result, the electrical resistance of the metal oxide decreases. Adsorb other gases or vapors first on the surface the active layer and then react with the metal oxides. Depending on affinity of gases or vapors to the metal oxide used as the active layer and its working temperature and also according to its grain size and distance the contact electrodes to each other, is at different gases a different response of the sensor active layer on the gas present was detected.

The change of the electrical resistance of the active layer as a function of the gas concentration basically follows a very strongly curved curve, 2 , For small gas concentrations, the change quotient R _S / R ₀ , 2.1 , very high. At higher gas concentrations, the curve is increasingly flatter and virtually asymptotic. Changes in the gas concentration have virtually no effect on the then very low electrical resistance of the gas sensor in the saturation case. The saturation occurs relatively early. For alcohol vapor, saturation is already reached at about 2000 ppm (vol.) For industrially offered sensor elements. Higher gas concentrations are no longer measurable, which adversely limits the application areas.

It It is known to add catalytically active substances to the metal oxides. Often For example, platinum (Pt) or palladium (Pd) are used.

The spectral sensitivity of the sensor compared to gas mixtures of different oxidizable gases changes with the presence of catalytic admixtures because of these the heated and the air exposed surface of the active layer at high Concentration of the catalyst efficiently with the atmospheric oxygen burn and therefore the deeper part of the active layer, where is measured by means of the electrode structure of the conductance, not more or less influence.

It is known that the characteristic curve (R = resistance of the active layer, R _(gas) ) in the case of sensors doped with catalytic admixtures, 2.2 ., Increasingly steep and that the asymptotic, very flat part of the curve begins at even lower gas concentrations.

It is known that the heated sensor active layer is electrically complex, and that not only the generally evaluated resistance of the active layer is a function of the gas supply. As an equivalent circuit is in the literature a circuit after 3 specified. It is

3.1

resistance in the mass of metal oxide particles

3.2

Capacity at the transitions between the metal oxide particles

3.3

Capacity at the transitions between Metal oxide particles and

contact

3.4

Contact resistance between the metal oxide particles

3.5

Contact resistance between the metal oxide particles and the

contact

• (See C. Diehl, 2000; J. Gutierrez, 1991; J.R. MacDonald, , 1987; Weimar, 1992).

When Consequence becomes clear that with fumigation the complex circuit formation electrically both in its resistance values and in the Sensor existing electrical capacity changes.

The resulting complex impedance of the sensor is additionally dependent on the type of gas offered. It has therefore been considered to take advantage of this fact to be able to conclude from changes in the sensor impedance on the Gastype, or to be able to preferentially detect desired types of gas. (see also P196 17 297.7, WO 97/41423; US 200 20 11851 ).

The technical solution to be solved Problem:

Of the Ohmic resistance of an oxide semiconducting gas sensor changes at normal air even with small gas offers by about an order of magnitude. influences from humidity (absolute humidity) and from the temperature of the active layer have in this steep part of the characteristic also considerable influence. A direct one conclusion The resistance of the active layer to the gas supply is with great uncertainties afflicted.

at Gas or steam offers in the order of about 2000-5000ppm (dependent from the gas / vapor), the resistance of the active layer is in the range very small steepness shifted. Will the offer be further elevated, the active layer is saturated and lets no further evaluations of the electrical resistance of the active layer more to.

The Time, which takes the resistance of the active layer to after fumigation with high gas or vapor concentrations again for normal air effective value is very high.

To Offer some vapors - such as Petrol vapor in a concentration> 1% vol. - the electrical resistance remains of the sensor a very long time (several days) lower than before fumigation.

Becomes the sensor over longer Store stored on the surface of the active layer sorbates. After switching on the sensor, the active layer resistance is in the Usually significantly reduced and reached only after a longer (days!) Einlaufzeit its for Normal air applicable resistance.

In Applications in which the sensor is stored under gas / steam could and in which a start of the measurements under gas might be necessary, is this behavior is disadvantageous.

The measurement of the sensor capacitance is not possible according to the state of the art without effort. Are known circuits according to 4 : It is

4.1

The heated sensor element (heater is not drawn)

4.2

frequency generator (preferably sinus generator)

4.3.

Nadir resistance or electrical inductance

4.4.

comparator and evaluation circuit (phase, vector, imaginary part)

The changing under gas Sensor capacity cause a phase rotation and act in addition like a frequency-dependent AC voltage divider. The under gas changing ohmic components act as voltage dividers and act together with the capacities of the sensor on the phase position. The evaluation of the won Measurements and the inference on the capacity shares in the equivalent circuit of the sensor active layer is with considerable technical and mathematical effort and costs possible.

In practical applications - about the scientific discussion - the sensor capacity is not as a gas concentration characteristic and practical Size used Service. In practical applications and in numerous patents, which deals with the use of oxidic semiconducting gas sensors employ, becomes common the electrical (ohmic) resistance of the active layer is evaluated as a gas-dependent variable.

The capacitances measured with the above-mentioned complex measurement methods on industrially manufactured semiconductor oxide sensors are dependent on the operating temperature and are given in the literature as follows, see 3 :

Technical task:

With inexpensive Methods should be the possibility be shown the disadvantages of using the ohmic Resistance or the conductivity to avoid a gas sensor as a guide and to move to another basic electrical quantity as a guide.

Technical invention Solution:

In front the description of circuit engineering approaches to the solution of the problem will be described below describes the physical principle underlying the invention, for the understanding the invention is a prerequisite.

The Mode of action of the gas-dependent dielectricity and thus gas-dependent different capacity of oxide semiconductor sensors is based on a change the permittivity of insulators by the adsorption of molecules from the gas or liquid phase. The relationship between adsorbed gas quantity and permittivity is at dielectric gas sensors are generally non-linear, so that on the gas concentration over a calibration curve can be closed. For higher occupancy, depending on the the sensor material, saturation effects occur.

at Polarization processes, which are caused by adsorption, one distinguishes between Volume effects and interface effects at the electrodes. The change the permittivity the volume is changed by the change the polarizability and the conductivity of the sensitive material caused.

The polarization and ion conduction mechanisms are not in phase because of their finite relaxation times r _i with the electric field. The dielectric function E * (w) is therefore frequency-dependent and complex-valued. At the interfaces of the sensor material to the electrodes polarization effects occur, which depend on the specific properties of the interface.

For the investigation the sensor properties in dependence from the gas concentration has been proposed, the sensor response to measure at one frequency. As sensor response become impedance or admittance and the quantities derived therefrom.

For the practical Use would be the capacitive component of the sensor response is important, that of an evaluation circuit is measured. The optimal frequency for such measurements can see the spectroscopic investigations. It is enclosed oxide semiconducting gas sensors normally between 10 kHz and 1 MHz.

There the dielectric function can not be measured directly is for the practical evaluation of their conversion into another representations, e.g. as impedance, necessary.

at dielectric sensors with insulating sensitive materials is essentially the polarizability of the sensor layer through changed the adsorption of gases. A change the ionic conductivity by the dissociation of adsorbed molecules or their reaction products is also possible. Both processes influence the permittivity of the material.

Not always does the adsorption of gases lead to a measurable change in the dielectric material properties. For molecules without a permanent dipole moment, such as N ₂ , O ₂ or CO ₂ , which are physisorbed in larger quantities, no change in the electrical properties of oxide semiconductors is observed.

at amorphous materials is generally not anisotropy of the dielectric Function expected. Likewise, with the applied measuring voltages It can be assumed that nonlinear field effects are negligible. A scalar description of the polarizability α of the individual molecules is therefore sufficient.

The polarizability α of a molecule is due to a number of different processes whose summation gives the total polarizability α _ges . These are the orientation polarization α _Δ and α _d , the electronic polarization α _el , that is the displacement of the outer electrical shell towards the atomic nucleus, and the ionic polarization α _ion , which are the displacements of the atoms of one molecule against each other.

The two latter polarization mechanisms show energetic reasons (resonance vibrations) only from the microwave range on a frequency dependence. In the low-frequency limit case up to 1 MHz, the contributions of the electronic and ionic polarizability are constant: α∞ = α el + α ion

aus ihrem elektrischen Dipolmoment p errechnet.The orientation polarization α _{Δ of} the polar groups becomes, according to Debye, assuming a free movement without mutual interaction

calculated from their electric dipole moment p.

Not only molecular dipoles show orientation polarization, but also ions that can jump locally between several neighboring defect sites. The contribution of this hopping mechanism for orientation polarization is described mathematically analogous to that of molecular dipoles and is therefore included in α _Δ . These localized charges do not contribute to electrical conductivity because their mobility is too low.

The static polarizability α _{ges of} a molecule at low frequency is the sum of the partial polarizabilities: α ges = α Δ + α∞ = α Δ + α el + α ion

In many cases, the electronic and the ionic portion α∞ of the total polarizability α _{ges is} considerably smaller than that of the orientation polarization α _Δ or α _d .

Neglecting the reciprocal interaction of the individual polarization mechanisms, the electrical susceptibility x is calculated from the simple summation of the polarizabilities or α _{i of} the individual molecules multiplied by their number Ni. This gives the static polarization of a solid and, with the definition of the dielectric displacement, the relative dielectric constant.

There the ionic and the electronic polarizability only a small Proportion of the dielectric constant have, in a pure physisorption of gases with low Dipole moment their change as well be low.

To The ideas of adsorption theories form the adsorbed molecules on the (inner) surface of the adsorbent is a mono- or multimolecular layer that has the character a liquid Has. To the estimate the static dielectric constant Therefore, the system adsorbate adsorbent as a heterogeneous dielectric be treated with separate phases.

According to the invention were approaches for calculating the dielectric constant looking for heterogeneous dielectrics, which the dielectric constants of the individual phases according to their volume fractions. The calculated capacities are due to the parallel electrical resistors with large loss factors afflicted.

Further are not classic surface capacitors, but to charge carriers, which at the contact transitions between the metal oxide grains the active layer as well as between the grains of the active layer and the Contacting structures occur.

If a DC voltage is applied to the sensor element, current flows through the resistors 8.5 , R3, 8.3 , R1, as well 8.4 , R2. The electrical charges at the contact junctions do not affect the current flow.

If an alternating voltage is applied to the sensor element, current flows through the resistors, as mentioned above, and through the capacitors, which form a capacitance according to the following expression:

Of the capacitive resistance goes to infinity at DC, and at infinite frequency to zero.

If no parallel resistors exist, two capacitors are connected in series. The capacity of both capacitors is then calculated as follows:

The resulting capacitor C _tot must be smaller than the smallest capacitor in the series in any case.

The phase shift of an applied sinusoidal voltage is calculated on each R / C element as a function of, inter alia, the frequency. If a very large frequency is applied, for example 1,000 KHz, the capacitance of the capacitor C1, which is relatively large with about 1000 pF, is 8.1 , relatively small and the phase shift is also small. The capacitor C2, 8.2 , is relatively small at 30-100pF and the capacitive resistance is relatively high and in relation to the parallel connected resistor R2 so that a well evaluable phase shift can be measured.

Around Being able to determine the capacitor C1 is a relatively low one Frequency to choose e.g. 5kHz, so that the capacitive resistance of C1 in relation to the ohmic resistance R1 of approximately 1: 1 stands to a maximum phase shift of the alternating current to effect. The current path then runs over the resistors R1, R2, R3 and over the capacitive resistance of C1.

at Change fumigation the mentioned capacitors in the sense that the capacity increases, whereas the resistances change in the sense that they become smaller under gas. Because both processes are inseparable connected to each other, is the evaluation of the in a gas sensor located electrical capacitances or electrical charges very elaborate and difficult, because usually only the way over the Determination of the frequency-dependent Impedance possible is.

Would alone the electric charge determines which abstract after applying a DC voltage in the mentioned electrical capacitors of an oxide semiconductor gas sensor saved would be the electric charge is a measure of the concentration of oxidizable or reducible gases conducted to the sensor.

Inventive solution of Task:

The solution the object is characterized in that the located in the metal oxide semiconductor gas sensor electrical charge as a measure of concentration in the ambient atmosphere the sensor existing oxidizable or reducible gases or fumes is used and evaluated.

In Another inventive embodiment of the method is the gas sensor charged to the voltage level of a DC voltage and after End of the charging process de-energized, after which the now in the sensor stored electrical charge in a suitable manner by measurement is determined.

In Another inventive embodiment of the method is using a controlled switching bridge of Gas sensor each charged in alternating polarity, wherein with the help of a program-controlled circuit of the gas sensor electrically is charged to a certain voltage potential, and the electrical Charge of the gas sensor over a fixed and two switchable electrical conductor polarity correct and pulse wise transferred to a reference capacitor until one of an electrical comparator detected voltage at the reference capacitor is achieved, according to which from the sizes frequency the recharge pulses, capacity of the reference capacitor and charging voltage of the reference capacitor the electrical charge of the gas sensor is determined, from which the Concentration of located in the ambient atmosphere of the gas sensor Gases or vapors is derived. Further advantageous embodiments of the invention are in the subclaims characterized.

The inventive method indicates the fundamental Advantage on that it is the use of electrical conductivity the active layer of a gas sensor avoids as a guide, but instead whose basic electric capacity or capacity used electrical charge of the active layer as a guide.

Of the electrical conductance is not used as a guide or possibly only in according to the invention new combination with the measured gas-dependent electric charge or the capacity of the gas sensor.

A patented device to the solution the task and to perform the method of determining in the one metal oxide semiconductor gas sensor, For example, tin dioxide gas sensor, surrounding atmosphere contained Part of oxidizable or reducible gases or vapors, with an electrically heated metal oxide semiconductor gas sensor and a suitable electrical evaluation circuit, is characterized that in a metal oxide semiconductor gas sensor electric charge as a measure of the concentration of in the ambient atmosphere the sensor existing oxidizable or reducible gases or fumes is used and evaluated.

Summary the drawing, in which show:

1 the well-known construction of a TAGUCHI gas sensor

2 the course of the resistance of the active layer of the gas sensor at normal air as a function of gas concentration

3 an electrical equivalent circuit diagram of the heated active layer of a gas sensor

4 an evaluation principle based on the circuit of 3

5 the characteristics in comparison of an oxide semiconductor gas sensor when exposed to gas / steam, namely the characteristic of the ohmic resistance of the active layer and the characteristic of the electrical charge of the active layer

6 a patented principle circuit for determining the capacity of an oxide semiconductor gas sensor

8th a representation of how to align in the short breaks between the individual discharges the charges on the resistors in several current paths

9 a representation of the electric charge, which varies in proportion to the gas supply, even with such offers, when the electrical resistance of the sensor is already in the asymptotic, saturated part of the characteristic curve

10 an electrical circuit diagram for detecting the energy stored in a semiconducting metal oxide gas sensor as a measure of the concentration of gases offered and

11 a schematic diagram according to the 10 ,

To determine the capacity of an oxide semiconductor gas sensor, a circuit can be used whose principle in 6 shown is the sensor 6.6 is via the actuated by a central evaluation and control unit switch S1, 6.1 , connected to a voltage, wherein the capacitors located in the sensor charge electrically. Then the switch is opened again.

Via the central evaluation and control unit, for a very short time, in the μsec range, the switch S2, 6.2 , closed and reopened. There is a charge transfer into the reference capacitor C1, 6.4 , This process is repeated at short intervals, with the voltage in the reference capacitor rising until it corresponds to a comparison voltage, which is an electrical comparator, comparator Comp1, 6.5 , states. The number of Umladeimpulse required until reaching the switching threshold is counted by the central control and evaluation.

is the electrical charge stored in the sensor is very small transhipment operations required to reach the switching threshold. Is that in the sensor stored electrical charge, however, large, little Umladevorgänge are required to reach the switching threshold.

Advantageous is that the discharge of stored in the sensor electrical Charge at very short intervals he follows. A complete one Discharge of the charge stored in the sensor is via the resistors connected in parallel are not possible because they are each are Schottky transitions, which have a certain threshold voltage, and so far not behave as ohmic resistors would do.

In the short pauses between the individual discharge processes, the charges are equalized via the resistors in several current paths, as in 8th by means of the paths 8.6 . 8.7 and 8.8 shown.

The Charge stored in the sensor can therefore be determined according to the invention be first by the sensor by means of a DC voltage electrically is loaded, then over discharge defined discharge pulses in a comparison capacitor becomes, being dependent on the size of the comparison capacitor and the chosen one Threshold after a number dependent on the charge of the sensor the electrical charge stored in the sensor is determined.

The The electrical charge of the sensor is now dependent on the atmosphere in which it is operated becomes. In normal air, which is free from oxidizable gases or vapors, the charge corresponds to an electric capacitor of the order of magnitude from about 20-100pF, depending on the design of the oxide semiconductor sensor.

at Fumigation of the sensor with oxidizable gas displaces this oxygen stored in the sensor with the result being that between each Schottky barrier electric field is reduced, which increases the resulting in electrical charge. For very large gas or steam offers increases the electric charge in the gas sensor such that it is a capacitor in the order of magnitude of up to 10,000pF. When gassing the sensor with reducible Gas, ozone or nitrogen oxides, the reverse effect occurs because the cargo is present opposite to these gases reduced in normal air.

It is advantageous that the electrical charge of the sensor is reasonably proportional to the gas supply, see 9 , and the characteristic in contrast to the resistance characteristic, 9.2 , An advantageous much lower curvature.

It is advantageous that the electrical charge, 9.1 , changes in proportion to the gas supply even with gas offers, if the electrical resistance of the sensor is already in the asymptotic, saturated part of the characteristic curve and no longer permits evaluation. It is therefore advantageous to use the energy stored in a semiconductive metal oxide gas sensor as a measure of the concentration of gases offered.

In the simplest way is a principle 11 suitable. The capacitance C of the gas sensor is connected via a switch S with a voltage U. When the switch is opened, the charge discharges after a time function. Time is a measure of capacity. A disadvantage of this simple solution that the discharge does not take place via constant parallel resistances to the internal capacitance, but that the resistive components of the sensor resistance in their value are dependent on the offered gas concentration.

Around To eliminate these disadvantages is proposed according to the invention, the gas sensor via a controlled Switch and a defined time, for example of <1msec, to one DC voltage U to create and then the gas sensor via a controlled, fast switch in a variety of very short, preferably in the μsec range, Discharging discharge pulses into a reference capacitor, until the voltage across the reference capacitor reaches a defined threshold reached.

Out the number of needed Discharge pulses and the size of the reference capacitor and from the threshold, the electrical capacity, ie determines the total stored in the sensor electrical charge become. After the end of the measuring process, the reference capacitor is connected via a discharged further controlled switch and thus for the next measurement cycle prepared.

It is useful, about a controlled switch through the gas sensor after the measurement a short circuit completely to unload.

Advantageous is that the gas sensor during the actual measuring process neither DC nor AC is flowing through, but that a static and only from the gas supply dependent electrical Charge exists very quickly in small amounts of charge in one Transfer reference capacitor is over before self-discharge the internal ohmic portions of the gas sensor can take place.

Preferred embodiments the invention:

oxide Gas sensors are not independent in their characteristics of polarity the connected measuring voltage, which is to be preferred solution the invention leads. In practice, the "switches" described below are implemented by transistor switches which from a programmatic microcontroller in their temporal Sequence precise to be controlled.

In the preferred solution of the invention, a circuit follows 10 for use. The sensor to be measured 10.6 is located in a switching bridge which consists of switches S1, S2, S3, S4, S5 that can be actuated electronically by a central evaluation and control unit (not shown), 10.1 . 10.2 . 10.3 . 10.4 . 10.5 , is formed.

The switches of the switching bridge are set after each measuring cycle in such a way that the sensor to be measured is always reversed, thus advantageously no electrolysis processes in the gas sensor 10.6 can take place.

The desk 10.1 For example, a time, eg, of <1 msec, charges the capacitance of the sensor from a positive voltage U to the electrical level of this voltage.

Depending on the position of the switching bridge, the respective positive pole of the charged sensor 10.6 via two electrical lines to two switches 10.7 or 10.7 ' guided. The switch leading to the positive pole of the sensor, 10.7 or 10.7 ' , transmits by multiple and very short time switch closing successively stored in the gas sensor electrical charge in a reference capacitor 10.8 , while the negative pole - practically third as the third leading away from the sensor to be measured - flows through the jumper as "ground" in the circuit.

Only the positive pole of the sensor 10.6 each facing switch 10.7 or 10.7 ' Immediately upon completion of the charge, the reference capacitor will be the result of a series of very short discharge pulses 10.8 Charge until the downstream electrical comparator 10:10 the achievement of a reference voltage 10.9 detects and generates an output signal.

The Number of discharge pulses until reaching this threshold a measure of the im Sensor stored electrical charge.

After each measurement cycle, the reference capacitor is closed by momentarily closing the switch 10:11 completely discharged.

The sensor capacitance is discharged by the sensor resistors connected in parallel with the capacitors, see 6 , In addition, the sensor capacitor is electrically discharged after each measuring cycle by brief short-circuiting (in 10 not drawn).

It will be complementary for the evaluation of the electric charge stored in the sensor according to the patent proposed, both the electrical charge of the sensor and the electrical Resistance of the sensor separated from each other metrologically in a suitable Way to determine, and these two readings in the evaluation to link together for more information from the complex circuit system of the Gas sensor to win.

Of the patented object the invention faces The prior art has a number of advantages.

5 shows the characteristics of an oxide semiconductor gas sensor when exposed to gas / steam, wherein 5.2 the characteristic of the ohmic resistance of the active layer and 5.1 is the characteristic of the electrical charge of the active layer.

It can be seen that the characteristic 5.2 , in the beginning is very steep and then expires relatively quickly in an asymptote, namely in the saturation. The characteristic 5.1 is significantly less steep in the beginning and changes with the gas supply into the range of very high gas concentrations, in which the resistance of the active layer is already saturated. The characteristic 5.1 , is not ideal linear, but allows a computational linearization with much less effort. Because in the steep area of the characteristic 5.2 even slight errors or environmental influences lead to large measured value deviations, this is when evaluating the characteristic curve 5.1 , not the case.

These Properties are for the practical application of oxide semiconductor gas sensors extremely advantageous. While the resistance of the active layer in its chemical structure massive fumigation under reaction with atmospheric oxygen, The gas molecules that influence the electrical charge diffuse very well quickly out of the active layer. The sensor is "faster".

About the Evaluable gas concentration in the range of very high concentrations is a very big advantage in numerous applications, especially in combination with the faster reset time after massive fumigation.

• a) Messung der Konzentration von Brenngasen (Methan, Propan, Butan) in der Luft: Bei Konzentrationen über ca. 5000ppm hinaus wird der gasabhängige Sensorwiderstand oxidischer Halbleiter-Gassensoren in den Bereich der Sättigung geschoben und die Kennlinie wird asymptotisch. Änderungen der Gaskonzentrationen ergeben keine nennenswerten, auswertbaren Änderungen des Widerstandswertes der Wirkschicht. Der Gas-Sensor ist daher nur im Bereich kleiner Konzentrationen, zum Beispiel im Bereich 1–100ppm, bis hin zu mittleren Konzentrationen, zum Beispiel im Bereich < 10.000 ppm, zu Messzwecken einsetzbar. Bei höherem Gasangebot wird der Sensor derart niederohmig, dass es mitunter Tage braucht, um in den Bereich normaler Werte zurückzukehren. Die Auswertung der elektrischen Ladung des Sensors erlaubt eine Ausdehnung des Messbereiches bis hin zu ca. 50% vol. Gasanteil. Die Rückstellzeiten der gemessenen Sensorladung nach Begasung sind mit einigen Minuten sehr kurz.
• b) Messung in Abgasen: Die Konzentrationen von Kohlewasserstoffen und/oder Stickoxiden im Abgas können sehr hohe Werte erreichen. Wird mit oxidischen Halbleiter-Gassensoren gemessen, arbeiten die Sensoren typisch im gesättigten, fast asymptotischen Bereich der Kennlinie. Vorteilhaft ist, dass Halbleitersensoren im Abgas sehr gute und stabile Resultate bringen, wenn nicht der Widerstand der Wirkschicht sondern die elektrische Ladung der Wirkschicht ausgewertet wird.
• c) Steuerung der Klima- und Lüftungsanlage in Fahrzeugen oder Gebäuden oder Industrieanlagen: Zum Beispiel in Automobilen ist das außerhalb oder innerhalb der Kabine befindliche Gasangebot sehr unterschiedlich und kann sehr hohe Konzentrationen annehmen. Es kommt darauf an, dass unter allen Umständen die Gaskonzentration sehr schnell gemeldet wird und dass nach Begasung der Gassensor sehr schnell seinen normalen Arbeitspunkt erreicht, was bei Auswertung des elektrischen Widerstandes der Wirkschicht nicht in allen Situationen der Fall ist. Die erfindungsgemäße Auswertung der elektrischen Ladung des Gassensors lässt einerseits die Messung auch höherer Gaskonzentration, wie in Tiefgaragen oder Tunnel, zu und gewährleistet eine sehr schnelle Rückstellung nach Begasung. Der Einfluss von Luftfeuchte auf die gemessenen Ladungsmengen im Gassensor ist deutlich geringer als der Einfluss von Luftfeuchte auf den elektrischen Widerstand der Wirkschicht.
• c) Messung der Gase, welche bei der Lebensmittelherstellung entstehen, wie beim Backen, Braten, Kochen oder Rösten: Die Zusammensetzung und Konzentration der in vorgenannten Prozessen abgehenden Gase sind abhängig vom Fortschritt des Herstellungsprozesses. Es wird gewünscht, diese Prozesse zu automatisieren, was eine schnelle und kostengünstige Analyse der abgehenden Gase/Dämpfe erfordert, deren Konzentrationen sehr hoch werden können. Die Auswertung des elektrischen Widerstands der Wirkschicht ist weitgehend unmöglich, weil sich der Arbeitspunkt des Sensors im asymptotischen, gesättigten Bereich befindet. Vorteilhaft wird hier die Lehre der Erfindung eingesetzt, um diese Applikation für oxidische Halbleitersensoren erst möglich zu machen.

For example, there are the following commercial applications:

• a) Measurement of the concentration of fuel gases (methane, propane, butane) in the air: At concentrations above about 5000ppm, the gas-dependent sensor resistance of oxide semiconductor gas sensors is pushed into the saturation range and the characteristic becomes asymptotic. Changes in the gas concentrations result in no appreciable, evaluable changes in the resistance value of the active layer. The gas sensor can therefore only be used in the range of small concentrations, for example in the range 1-100 ppm, up to medium concentrations, for example in the range of <10,000 ppm, for measurement purposes. With higher supply of gas, the sensor becomes so low that sometimes it takes days to return to the range of normal values. The evaluation of the electrical charge of the sensor allows an extension of the measuring range up to about 50% vol. Gas content. The reset times of the measured sensor charge after fumigation are very short with a few minutes.
• b) Measurement in exhaust gases: The concentrations of hydrocarbons and / or nitrogen oxides in the exhaust gas can reach very high levels. When measured with oxide semiconductor gas sensors, the sensors typically operate in the saturated, almost asymptotic region of the characteristic. It is advantageous that semiconductor sensors in the exhaust gas bring very good and stable results, if not the resistance of the active layer but the electrical charge of the active layer is evaluated.
• c) Control of the air conditioning and ventilation system in vehicles or buildings or industrial plants: For example, in automobiles, the supply of gas outside or inside the cabin is very different and can assume very high concentrations. It is important that under all circumstances, the gas concentration is reported very quickly and that after fumigation, the gas sensor reaches its normal operating point very quickly, which in evaluating the electrical resistance of the active layer is not the case in all situations. On the one hand, the evaluation of the electrical charge of the gas sensor according to the invention allows the measurement of higher gas concentrations, as in underground garages or tunnel, and ensures a very fast recovery after fumigation. The influence of air humidity on the measured charge quantities in the gas sensor is significantly lower than the influence of air humidity on the electrical resistance of the active layer.
• c) Measurement of the gases which are produced during food production, such as baking, roasting, cooking or roasting: The composition and concentration of the gases emitted in the aforementioned processes are dependent on the progress of the production process. It is desired to automate these processes, which requires a quick and inexpensive analysis of the outgoing gases / vapors whose concentrations can become very high. The evaluation of the electrical resistance of the active layer is largely impossible, because the operating point of the sensor is in the asymptotic, saturated region. Advantageously, the teaching of the invention is used to make this application for oxide semiconductor sensors possible.

It are several technical possibilities conceivable that stored in an oxide semiconductor gas sensor to measure electrical charge. Alternatively, the gas-dependent electrical capacity be determined by the sensor by an alternating current is flowed through, what if change the sensor capacity leads to evaluable effects.

all Measuring method is common that the electrical charge of the gas sensor as a measure of the the sensor surrounding gas concentration is determined in a suitable form, and that the electrical charge of the gas sensor is used as a measured variable in measuring, Control and regulating circuits is used.

Claims (11)

Method for determining the proportion of oxidizable or reducible gases or vapors contained in the atmosphere surrounding a metal oxide semiconductor gas sensor, for example, tin dioxide gas sensor, consisting of an electrically heated metal oxide semiconductor gas sensor and a suitable electrical evaluation circuit, characterized in that in the metal oxide -Halbleitergassensor electric charge is used and evaluated as a measure of the concentration of existing in the ambient atmosphere of the sensor oxidizable or reducible gases or vapors. Method according to claim 1, characterized in that that the gas sensor to the voltage level of a DC voltage charged and switched off after the end of the charging process becomes, after which the electric charge stored now in the sensor is determined in a suitable manner by measurement. Method according to claims 1 to 2, characterized that by means of a controlled switching bridge, the gas sensor in each case alternating polarity is electrically charged, and that with the help of a program-controlled Switching the gas sensor electrically to a certain voltage potential is charged, and that the electrical charge of the gas sensor over a fixed and two switchable electrical conductors polarity correct and pulse wise transferred to a reference capacitor until one of an electrical comparator detected voltage at the reference capacitor is reached, and that from the variables frequency of the recharging pulses, capacity of the reference capacitor and charging voltage of the reference capacitor the electrical charge of the gas sensor is determined, from which the Concentration of located in the ambient atmosphere of the gas sensor Gases or vapors is derived. Method according to claims 1-3, characterized that the gas concentration dependent electric charge of a Metal oxide semiconductor sensor as a sensor signal for the purpose of alarming, or controlling or regulating. Method according to claim 4, characterized in that that the electric charge determined by the gas concentration of a Metal oxide semiconductor sensor used in gas engineering applications and that of the gas concentration dependent electrical resistance the active layer of the metal oxide semiconductor sensor also determined is where both the electric charge and the electric Resistance in the evaluation are linked together in a suitable manner and for the evaluation of the sensor surrounding gas or vapor the atmosphere considered become. Method according to claim 1, characterized in that that the electric capacity of the gas sensor by the gas-dependent Impedance of the sensor is determined by the sensor of an alternating current suitable Frequency is flowing through, and putting the voltage on one in series switched electrical capacitor or resistor or electrical inductance as a measure of the gas concentration is being used. Method according to claim 1, characterized in that that capacity of the gas sensor by the gas-dependent Impedance of the sensor is determined by the sensor from an alternating current a suitable frequency is passed through, and by the resulting Phase shift between fed AC voltage and behind the measured AC voltage as a measure of the gas concentration becomes. Apparatus for determining in the one metal oxide semiconductor gas sensor, For example, tin dioxide gas sensor, surrounding atmosphere contained Part of oxidizable or reducible gases or vapors, with an electrically heated metal oxide semiconductor gas sensor and a suitable electrical evaluation circuit, characterized that in a metal oxide semiconductor gas sensor located electrical Charge as a measure of concentration in the ambient atmosphere the sensor existing oxidizable or reducible gases or fumes is used and evaluated. Device according to claim 8, characterized in that that the gas sensor to the voltage level of a DC voltage charged and switched off after the end of the charging process becomes, after which the electric charge stored now in the sensor is determined in a suitable manner by measurement. Device according to one of claims 8 or 9, characterized that by means of a controlled switching bridge, the gas sensor in each case alternating polarity is electrically charged, and that with the help of a program-controlled Circuit of the gas sensor electrically charged to a certain voltage potential is, and that the electrical charge of the gas sensor over a fixed and two switchable electrical conductors polarity correct and pulse wise transferred to a reference capacitor until one of a electrical comparator detected voltage at the reference capacitor is reached, and that from the variables frequency of the recharging pulses, capacity of the reference capacitor and charging voltage of the reference capacitor the electrical charge of the gas sensor is determined, from which the Concentration of located in the ambient atmosphere of the gas sensor Gases or vapors is derived. Device according to one of claims 8 to 10, characterized that the gas concentration dependent electric charge of a Metal oxide semiconductor sensor as a sensor signal for the purpose of alarm, or controlling or regulating.