DE4317879B4 - Apparatus for detecting inorganic-organic compounds, anesthetic gases, pesticides, insecticides and halogenated hydrocarbons in gas streams by means of semiconductor gas sensors - Google Patents

Abstract

contraption for the detection of inorganic-organic compounds, anesthetic gases, Pesticides, herbicides, insecticides and halogenated hydrocarbons in gas streams by means of semiconductor gas sensors, with at least one as a phthalocyanine sensor (6) formed semiconductor gas sensor, at least one other Semiconductor gas sensor (5), at least one upstream in a gas stream the at least one phthalocyanine sensor (6) and the at least one another semiconductor gas sensor (5) arranged electrically activatable catalyst (4), and a central control unit (9), by means of which the temperature of the at least one electrically activatable catalyst (4) periodically variable is the signals of the at least one phthalocyanine sensor (6) and the at least one further semiconductor gas sensor (5) evaluable and, upon detection of the target gas, the determined concentration displayable in a display unit (10) and / or via an alarm transmitter (11) audible alarm can be triggered is, characterized in that the at least one further semiconductor gas sensor as Metal oxide sensor (5) is formed, that the at least one phthalocyanine sensor (6), the at least one metal oxide sensor (5) and the at least one electrically activatable catalyst (4) arranged in a chamber (7) ...

Description

The This invention relates to an apparatus for detecting inorganic-organic compounds, Anesthetic gases, pesticides, herbicides, insecticides and halogenated Hydrocarbons in gas streams by means of semiconductor gas sensors according to the preamble of the claim 1.

It is known gases of this kind with the help of flame photometers, infrared absorption analysis or with gas chromatographs or mass spectrometers. The costs these devices are very high and forbid for economic reasons for the masses Commitment. Inexpensive detectors that are easy to use and reliable and Long-term stable the problem of detection of target gases mentioned above triggers, will hardly offered.

Height Concentrations of the target gases can with Catalytic bead sensors (Pellistors) are detected. The responsiveness begins approximately at a concentration of 500 ppm of the target gas in the air.

district heating power station allowed to not in concentrations e.g. > 150 ppm into the atmosphere be delivered. This concentration range is with pellistors not reliably monitored.

at Most chlorinated organic compounds will have a sensitivity of significantly <1 ppm required.

To comes that pellistors basically all detect oxidizable gases and thus no security across from False alarm at random present oxidizable gases, such as alcohol, carbon monoxide, etc. exists.

It is also known to detect the target gases with the help of metal oxide sensors (eg SnO ₂ ) Zinndioxid sensors. The problem here is caused by a humidity and air temperature caused zero-point drift, which makes the detection of small concentrations impossible.

Also the broadbandness of the sensor is more oxidizable in any way Gases very disadvantageous. Last, especially for some missions not be ruled out that the measurement at the same time high concentration must be made of nitrogen oxides.

It It is known that the simultaneous presence of oxidizable and reducible gases can significantly falsify the measurement result, so in such situations metal oxide sensors generally not applicable are.

halogenated Hydrocarbons can be decomposed if these gases over a heated noble metal wire or surface, e.g. made of platinum. The molecule is decomposed into a hydrocarbon fragment and a Radical of the halogen. It is further known that the life of the Platinum wire considerably extended when coated with an oxide layer of base metals is.

Further It is known that with a copper phthalocyanine sensor element, the presence of radicals extraordinarily sensitive, wherein phthalocyanine gas sensors depending on preparation technique no cross-sensitivity to oxidizable gases or Have humidity.

From the DE 30 38 155 C2 Metal oxide sensors are known, which are designed as thin-film gas sensors and which are stabilized in a special way.

From the EP 0 305 963 A1 a device for detecting harmful gases in gas streams by means of semiconductor gas sensors is known, which can be used for the detection of inorganic-organic compounds, anesthetic gases, pesticides, herbicides, insecticides and halogenated hydrocarbons. Here, in addition to a phthalocyanine sensor already another, different type of sensor is provided. The device further includes an electrically activatable catalyst, by means of which the gas mixture to be analyzed is thermally treated before it is fed to the downstream sensors. In addition, the aforementioned known device is controlled by a central control and regulating unit, by means of which the temperature of the electrically activatable catalyst is periodically variable and by means of which the measurement signals generated at the sensors of the device can be evaluated. The control and regulation unit includes a display unit on which the determined concentrations are readable.

outgoing from the above-described prior art is the invention the task is based, a device of the beginning and above to create described genre, by means of the most diverse Basic load of the monitored gas flows and therefore the one in the monitoring sensors used meaningful and reliable Evaluations regarding the target gases are possible.

These The object is achieved by the specified in the characterizing part of claim 1 features solved. According to the invention can be independently from a possible base load of the sensors of the device different gases that period of time that elapses between the time in which the at least one electrically activatable catalyst in Operation is set, and the time passes in which the signal at least one of the metal oxide or phthalocyanine sensors changed by a predefinable amount is, capture and as a measure of concentration of the target gas. Essential for the accuracy and reliability Of these measurements is also that the quantity of the at the sensors of the device according to the invention passing out Gas flow can be detected, which ensures in the case of the invention thereby is that the gas flow is sucked through the chamber, in the the for the measurement predetermined arrangement of at least one phthalocyanine sensor, the at least one metal oxide sensor and the at least one electrically activatable catalyst are arranged.

advantageous Further developments of the invention will become apparent from the dependent claims 2 to 7th

The The invention will be described below with reference to embodiments closer to the drawing explained.

3 shows the typical behavior of a tin dioxide gas sensor when offered with different types of target gas. This gives a clear reaction when it comes to oxidizable gases ( 3 ). If reducible gases, such as nitrogen oxides or ozone, are offered, the sensor resistance increases significantly and the sensor, which strictly speaking consists of anreduced tin dioxide (SnO _(2-x) , becomes high-ohmic, if a tin dioxide sensor has been fully oxidized and all oxygen In the crystal lattice vacancies are occupied by an oxygen atom, the ability of the metal oxide sensor to detect gases that release oxygen atoms is irreversibly lost.

Becomes a mixed gas, e.g. from carbon monoxide or another oxidizable Gas, and e.g. Nitric oxide or another reducible gas, Although there is a significant change in the response of the sensor, However, the very strong of the mixing ratios of the offered gas mixture depends and from the reactivity the offered types of gas. In any case, the results are for metrological Purposes unusable.

Providing tin dioxide sensors with halogenated hydrocarbons results in a complex reaction sequence:
Since tin dioxide sensors usually contain a reaction accelerator in the form of, for example, palladium and these sensors are usually operated at a temperature between 300 and 400 degrees C, there is a dissociation of the molecule into a hydrocarbon fragment and into a radical. The hydrocarbon fragment is subsequently oxidized on the surface of the tin dioxide sensor, whereby the electrical conductance of the tin dioxide increases.

In adsorb and react to a complex chemical reaction Halogen radicals with the sensor surface. The chemical processes in the The scope of the present invention is not critical are, are currently not completely elucidated. Observations, however, show that it is a permanent loss responsive Sensor mass comes and in continuous operation under gas supply of the sensor is significantly limited in its lifetime.

Phthalocyanine sensors work against it after a completely other principle. In particular, there are no chemical reactions on the surface. Rather, molecules from the surface absorbed and affect the electrical conductivity of the phthalocyanine. After some time the trapped molecules desorb, whereby the desorption time is essentially a function of the surface temperature is.

In the preferred solution is used copper phthalocyanine, which is used as a thin film applied to a planar silicon substrate has been.

Phthalocyanine sensors react in particular to gases such as ozone, nitrogen oxides, sulfur dioxide and restricted also on halogenated hydrocarbons. Thus, e.g. both refrigerants as well as the target gases are detected in concentrations of> 20 to 50 ppm.

It It is known that the sensitivity of phthalocyanine sensors to these Target gases can be increased by a factor of 1000 or more, if one the molecule by a heated platinum helix already described above disassembled, its surface for extension the lifetime may be coated with a metal oxide.

4 shows the typical behavior with respect to various gases in tin dioxide sensors when the above-mentioned catalyst or "cracker" - ie the arrangement of a hot platinum wire - turned on or off know that in pure oxidizable gases, such as carbon monoxide, alcohol, methane, etc., the resistance of the sensor immediately drops significantly and that the condition of the cracker has a low and virtually negligible effect.

( 4a )
15 indicates the sensor value.

4b shows the same sensor, but fumigated with NO ₂ .
16 shows the increase in the sensor resistance, which breaks down when switching on the cracker NO ₂ and there is a slight decrease in the effect.

4c shows a mixed offer of NO ₂ and CO.
17 The "masking effect" shows at the beginning of each change of state a kind of settling without a definite effect 17 arises in this form at stoichiometric gas mixture. Depending on the gas mixture, the resistance may increase or decrease.

4d shows that a tin dioxide sensor is also suitable for the detection of HKWs in principle.
18 shows that the resistance of the sensor immediately decreases. Using the cracker, free radicals that can increase the resistance of the sensor again.

Of the Course of this waveform is very much dependent on the composition of the offered HKWs and its internal structure, thus the relationship of Hydrocarbons and halogens.

5a to 5d show the same target gases, but in the context of a phthalocyanine sensor.

5a shows through the curve 19 in that there is no sensitivity to gases such as CO, alcohol, methane, etc. Also, the use of the cracker shows these gases expected no change in behavior.

5b shows that the sensor is very sensitive to NO ₂ .

When using the cracker shows the course of the curve 20 that parts of the NO ₂ offer are broken down by the cracker.

5c shows the sensor when it is offered a gas mixture of NO ₂ and CO. The curve 21 hardly differs from the curve 5b ,

When offering typical target gases shows 5d with the curve 22 in that the sensor detects in each case and that the cracker significantly increases the sensitivity.

5e shows the curve 23 , if HKW and NOx are offered at the same time. The sensor reacts, whereby after switching on the cracker in the periods 24 it comes to a further reduction in the sensor resistance, reasonably comparable to the curve 5d ,

From what has been said:
By providing target gases in the gas mixture or alone, using a cracker, both tin dioxide sensors and phthalocyanine sensors behave according to a typical pattern.

While the curve 23 However, with pure nitric oxide, the tin dioxide sensor behaves significantly differently when there is little difference from the offer of pure nitric oxide.

It can be concluded from this that, when a phthalocyanine sensor and a metal oxide sensor (SnO ₂ ) are observed at the same time, the presence of target gases can be perfectly detected in concentrations of about 25 ppb. According to the invention, it is therefore proposed to reuse the gas mixture of an apparatus to be monitored 1 supply.

By a pump 3 possibly contaminated air passes through an inlet 1 in a sensor housing 7 in which a sensor for the detection of oxidizable air components 5 which preferably has tin dioxide as the sensitive layer, and a sensor for detecting radicals 6 containing phthalocyanine as a sensitive substance.

Both sensors are heated, the control of the surface temperature of the individual sensors by a central re gel and measuring device 9 with taken over. This central control and measuring device is in practice a microcontroller.

In the sensor housing in front of the sensors is a heated wire helix 4 made of a transition metal, preferably platinum.

The wire helix can according to 6 Also be designed such that on one side of a temperature-resistant ceramic 6.1 is a continuous coating of platinum Pt. On the other side of the ceramic substrate 6.1 is a meandering heating structure 6.2 applied, which is also made of platinum, for example. To protect against chemical reactions of platinum, especially in the presence of chemical radicals, is the Heizmäander 6.2 through a glass layer 6.3 passivated. This is advantageous high chemical reactivity coupled with a very high life of the heater.

The heater gets its energy from a switching amplifier 8th that from the microcontroller 9 is controlled. The measurement results are displayed via a display module 10 and, if necessary, an acoustic alarm sounds 11 triggered.

The aspirated gas leaves the assembly via the outlet 2 ,

In the evaluation, the invention makes use of the observation that immediately after switching on the cracker and in the presence of the target gas, a change in the sensor values is immediately apparent. It therefore becomes a detection method 7 proposed:
Before activating the catalyst, the sensor value is located 7.6 at a certain resistance level, which results from the type of sensor, the basic load with different gases and the sensor temperature.

If there is a target gas at the time 7.2 When the catalyst is switched on, in particular the phthalocyanine sensor reacts immediately with a resistor reduction. If the sensor value is a predetermined value 7.3 rushes through, the catalyst is turned off 7.4 ,

The timespan 7.7 is a measure of the concentration of the target gas.

Advantageous is achieved by the invention that the provocation of the sensor element is minimized with quite chemically aggressive radicals.

The Most semiconductor sensors absorb and desorb the relevant ones Target gases. If in the present case the catalyst is switched off again, the sensor will desorb the previously adsorbed molecules again and accordingly become high impedance again.

8th shows in principle the connections. It is 8.3 the time before switching on the catalyst. According to the previously adsorbed coverage of the sensor with target gas molecules becomes a gradient 8.1 be determined. The reverse gradient 82 , after switching on the catalyst at the time 8.5 is in its steepness dependent on the desorption gradient 8.1 , The invention therefore proposes the gradients 8.1 and 8.2 to consider together and to consider in the evaluation. This can be the resulting angle 81 , and 8.2 can be added together or the gradient 8.1 from the gradient 8.2 be subtracted.

in principle This can be done by the method proposed above is used and a fixed level jump in gas concentration-dependent, variable time is evaluated. The other method would be here the change of the Sensor resistance in a fixed time (delta R / delta t) to evaluate.

According to the invention both methods are considered to be in principle equivalent in this invention proposed.

Most characteristics of sensor elements are not linear but strongly curved. If, due to any gas concentration present at any given time, the sensor is in the strongly curved region of its characteristic curve, the level jump caused by the catalyst will have a lower value in the presence of the target gas (cf. 9 ).

The central control and evaluation unit should therefore be set up that dependence of the level jump from the current static sensor value is compensated.

The periodic activation of the catalyst 4 may be at freely selectable but constant intervals, with the activation time of the catalyst 4 also freely selectable, however, is constant, as in 2 is shown.

Claims (7)

Device for detecting inorganic-organic compounds, anesthetic gases, pesticides, herbicides, insecticides and halogenated hydrocarbons in gas streams by means of semiconductor gas sensors, with at least one phthalocyanine sensor ( 6 ) formed semiconductor gas sensor, at least one further semiconductor gas sensor ( 5 ), at least one in a gas stream upstream of the at least one phthalocyanine sensor ( 6 ) and the at least one further semiconductor gas sensor ( 5 ) arranged electrically activatable catalyst ( 4 ), and a central control unit ( 9 ), by means of which the temperature of the at least one electrically activatable catalyst ( 4 ) is periodically changeable, the signals of the at least one phthalocyanine sensor ( 6 ) and the at least one further semiconductor gas sensor ( 5 ) are evaluable, and upon detection of the target gas, the determined concentration in a display unit ( 10 ) and / or via an alarm transmitter ( 11 ) an acoustic alarm can be triggered, characterized in that the at least one further semiconductor gas sensor as metal oxide sensor ( 5 ) is formed such that the at least one phthalocyanine sensor ( 6 ) containing at least one metal oxide sensor ( 5 ) and the at least one electrically activated bare catalyst ( 4 ) in a chamber ( 7 ) are arranged, in which the gas flow through a gas inlet opening ( 1 ) and out of the gas flow through a gas outlet opening ( 2 ) by means of a pump ( 3 ) and that the at least one electrically activatable catalyst ( 4 ) by means of the central control unit ( 9 ) can be switched off as soon as, within a predefinable time period after starting up the at least one electrically activatable catalyst ( 4 ) the signal of at least one of the metal oxide ( 5 ) or phthalocyanine sensors ( 6 ) is changed by a predefinable amount. Apparatus according to claim 1, wherein the evaluation of the sensor signals takes place by the catalyst ( 4 ) is activated periodically by heating and the resulting change in the sensor signals is used to determine the target gas type and its concentration. Apparatus according to claim 1 or 2, wherein the catalyst ( 4 ) as a one-sided platinum coated ceramic substrate ( 6.1 ), wherein the other side has a meander-shaped heating structure ( 6.2 ) having a glass layer ( 6.3 ) is chemically passivated. Device according to one of claims 1 to 3, in which the periodic activation of the catalyst ( 4 ) at freely selectable, but constant intervals, wherein the activation time of the catalyst ( 4 ) also freely selectable, but is constant. Device according to one of claims 1 to 4, wherein the central control unit (a) carries out the signal evaluation such that in the time before the activation of the catalyst ( 4 ) detected sensor gradient ( 1 in 8th ) is subtracted from the sensor gradient which occurs immediately after the catalytic converter ( 4 ), wherein the resulting difference is used for determining the concentration of the target gas, wherein the evaluation is carried out by a table or mathematical formula stored in the central control and evaluation unit (a). Device according to one of Claims 1 to 5, in which target gases are assumed to be present if, after switching on the catalytic converter ( 4 ) the phthalocyanine sensor ( 6 ) immediately changed its resistance and at the same time as Zinndioxidsensor ( 5 ) trained metal oxide sensor also briefly reduces its resistance. Apparatus according to claim 5 or 6, wherein the central control unit ( 9 ) takes into account a static sensor value.