AU3605897A - Gas analyser and method - Google Patents

Description

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AUSTRALIA

Patents Act 1990 Original Complete Specification Standard Patent P/0o1011 Regulaion 3.21 Invention Title GAS ANALYSER AND METHOD The following statement is a full desciptioni of this invention, including the best method of performing known to me:- 't I 2 GAS ANALYSER AND METHOD TECHNICAL FIELD This invention relates to an apparatus being a gas concentration analyser and a method in which a semiconductor gas sensor exposed to an air flow and then to a gas or a vapour flow for purpose of measuring the concentration of gas or vapour either in which a gas sensor being of an array of sensors exposed to air flow and then to a flow of a mixture of different gases to select individual gases and their respective concentrations in the mixture.

BACKGROUND ART The metal-oxide semiconductor gas sensors e.g. n-type tin oxide semiconductor sensors Jo from Figaro Engineering Inc of Japan. customarilly used as detectors for reducible gases or organic vapours such as hydrogen, carbon monoxide. ammonia, hydrocarbons, ethanol. etc.

(In future gas(es) or vapour(s) will be reffered as gas(es) only.) When exposed to gas those sensors increase their electric conductivity from that in air or oxygen due to adsorption of gas molecules by the semiconductor surface.

In general such semiconductor devices are used as detectors being sturdy, cost e'fective.

resistant to poisoning, etc. However, in present form of application those sensors hardly can be used for analytical instrumentation or accurate measurements. In addition the sensors possess low selectivity to a designated gas.

Disclosure relating to the subject can be found, e.g. in U.S. Patent 3,631,436; .D U.K. Patent 1280 809. -o fl-, s Normally the sensor comprises a gas responsive semiconductor sensing element attached to a pair of electrodes for inducing an electric current through the semiconductor and a heater for warming up the semiconductor surface. The surface heated to predetermined temperature in order to increase the sensor sensitivity, to decrease response time. to expel moisture and Scontaminants and to stabilize the semiconductor resistance in air.

Normally the sensor output electric sigal voltage or current) reflects the semiconductor conductivity and it can be generated on a load resistor or an operational amplifier tied to one of thle S nsnr e.lectmrde with a ectnnrl iletrotmie cnnnererd to a nower nnource t nrnrldp.c an electric current through the gas sensing semiconductor. The output electric signal can be identified with a baseline and a response signal or a response, or a response voltage consisting of three parts. The output signal corresponding to the semiconductor resistance in air or a reference atmosphere (in future reffered as air only) when the semiconductor stabilized or conditioned to a stable mode of oxygen absorption constitutes the baseline.When air flow switched to a gas by means like three port valve the sensor semiconductor surface responds to the gas with adsorption of the incoming gas molecules having semiconductor electric conductance increased with a corresponding rise of the electric current which will produce the first part of the response signal with the rising in time transient voltage. The first part of the Jo response will be a transition from the baseline to a higher stable voltage plateau which corresponds to a stationary resistance of semiconductor in response to a given gas concentration when full adsorption of gas molecules occured. The fully established stationary plateau voltage is a second part of the response signal. When gas flow switched hack to air the sensor semiconductor surface responds with adsorption of the oxygen molecules and desorption of the gas molecules having semiconductor conductance decreased with a corresponding fall of the electric current through semiconductor. The output voltage will he falling in time and decaying to the baseline voltage when full desorption of gas molecules occured and the semiconductor conditioned into a stable state. This dimishing transient voltage is a third part of the response signal and which stipulates the sensor recovery time.It has been o known that the decay from the plateau to the baseline can take a long time especially after the sensor was exposed to a gas of a high concentration.

It also has been known that a stability and repeatability of the sensor's baseline depends on multitude of factors which precludes the baseline to be sufficiently reproducible. The same concemes the stationary output signal which in present art will be taken as a measure the gas Sconcentration with the consenquent inaccuracies. It is known as well that the established signal depends on the baseline.

It has been found that the sensor electric conductivity in the stationary mode versus gas concentration is a complex partly logarithmic function. This being a result of complexities connected with change of conductance induced by adsorpsion of gases on metal-oxide S s.micndrctor snrfare and hy svihnh uent nnn-linear diffiiuion sirfacr. reacrinnr Ji The mechanisms in relation to the electrical conduction of the device due to the adsorptivc and desorptive reactions of gases on the surface of said semicoductor as well as the diffusion are not fully understood yet.

It can be concluded that the semicoductor sensors are disadvantaged when used for analytical Sinstrumentation or accurate measurements. The sensors low selectivity to a designated gas are impending their application to select individual gases and their respective concentrations in the mixture.

One of the problem is that the semicoductor sensor characteristics of conductivity in the stationary mode under air and gas conditions are not sufficiently reproducible and stable. And S at present the semiconductor conductivity in this particular mode will be an indicator of the gas concentration. In addition the baseline and the stationary output signal are interdependable.

S The different disadvantage based on present methods of the gas concentration detection having the sensor stationary conductivity versus gas or vapour concentration as a complex i" non-linear partly logarithmic function. What is more the tifunction might vary from sensor to sensor presenting a cumbersome calibration problem in case of acurate measurements to leave alone the sensor replacement and recalibration.The other problem of the present art is a .need for a long time to establish a baseline especially after the sensor's heater was left without Spower even for a limited time and a slow recovery of the baseline after exposure to a gas.

I SUMMARY OF THE INVENTION.

The aim of the present invention to overcome of the disadvantages of the prior art.

It is an object of present invention to provide an apparatus and method in which the semiconductor gas sensors are coupled with means to acquire a derivative from the sensor electric output signal in a transient mode after air flow across the semiconductor S was changed to a gas flow. And consenquently to use the value of said derivative as an accurate measure of gas concentration.

Accordingly, the present invention provides a Gas Analyser intended for an accurate measurement of gas concentration, comprising

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three port valve with two inlets and one outlet which can be connected alternatively to either inlet wherein one inlet designated for a gas under test and the other being an air: a sampling pump or a similar means in communication with the valve for delivering a flow either gas or air to a semiconductor gas sensor: a said sensor containing a heater and a gas responsive semiconductor sensing element attached to a pair of electrodes which are electrically connected with an electronic means to generate an output voltage proportional to said semiconductor element conductance; a differentiator with its input coupled to said electronics means to produce on its output a signal corresponding to a derivative of said output voltage in time after said valve has been operated ,ta from air to gas: a peak detector or similar circuit which input connected to the differentiator output and its output electric signal represents a maximum value of said derivative; means for displaying and scaling the peak detector output signal which is taken as an accurate value of the said gas concentration: a logic unit to provide interface and timing to operate said heater and pump for 1 conditioning said sensor before measurement as well as to control said valve, the differentiator, the peak detector and the display: a test means to establish a state of the semiconductor element conditioning under air flow in time before measurement.

A op It is still the object of the invention to provide a method of gas concentration measurment based on the semiconductor gas sensor application in which a transformed electric signal derived from the sensor represents a near linear function of the gas concentration It is yet another object of of the present invention to achieve an accurate measurement of gas concentration without overwhelming dependence on a stable and repeatable baseline.

49 According to the further object of the invention the measurement time can be greatly reduced as the only short transient time from the output signal is needed for analysing. The recovery time of the sensor is diminishing as the semiconductor element has only a short exposure to a gas.

And still object of the invention is further decrease the recovery time of the sensor by introducing control means for interaction between the sensor heater and the pump to speed up the stabilizing of the semiconductor surface in air flow and therefore the conditioning of the sensor baseline. It has been known to stabilize the sensor by applying to the heater a pulsed S current e.g. FIGARO Engineering Inc sensor 203 or an Australian Patent Al-A-10970/88 1.

However, the combination of the pulsed current via the heater and a pulsed air flow through the semiconductor surface is a novel approach.

It is another object of the present invention to select individual gases and their respective concentrations in the mixture of different gases.

Jo To assist with understanding of the invention a reference will be made to accomnpaying drawings which show one example of the invention: FIG. I represents an example of the invention in a functional diagram form: FIG.2(a) is a typical response voltage Vo on the sensor output Vo after switclhing air h1ow to a gas with concentration C at a point of time ton and back to air flow at loff as shown on FIG.2(c): FIG.2(b) shows voltage Vd representing a derivative of the response Vo FIG.3(a) and are examples of the sensor heater and the sampling pump control signals.

It should be understood that there is no intention to limit the invention to the particular forms disclosed, but on the contrary. the intention is to cover all modifications and alternatives 0 falling within the scope of the invention as defined by the appended claims.

The gas analyser 10 shown in FIG.1 comprises three port valve 11 in position when air inlet 13 switched to outlet 14 and gas inlet 12 being disconnected. The valve 11 operated by a switching means 18. for example by a solenoid with an associated driver.The valve outlet 14 connected to the sampling pump 15 via its suction port 16. From pump pressure port 17 air or S c gas supplied with a constant flow rate into a chamber 19 containing the gas sensor 20 which semiconductor sensing element 22 attached to a pair of electrodes 23. The flow rate to the sensor can be adjusted by one or few vents 35 positioned before the sensor 20 The sensor semiconductor 22 is heated by the device heater 21 connected to power source 28 and to a driver 3 l.The logic unit 30 controls the heater 21 via driver 31 by a signal 38. tle pump with a driver 32 by a line 36 as well as the valve 11 by a signal In this instance the device output voltage Vo is generated on a load resistor 24 of value Ro tied to one of the sensor electrodes 23 whereas the second electrode connected to power source 28 C to produce an electric current Is through the element 22. Assuming that the sensor resistance has much greater value than the load resistor Ro the response voltage as a function of time can be described as Vo(t Is(t )*Ro Gs (t )*Ro*Vs where Gs(t) is the said semiconductor conductance in time domain and Vs is a constant voltage of the source 28. It can be seen that the response Vo I o J by design will be proportional to the device conductivity Gs. Therefore the change in the device conductivity transfers into direct change of the sensor output voltage.

The voltage Vo applied to an input of the differentiator circuit 25 when an electronic switch 26 switched ON simultaneously with the valve 11 and at this point of time the semiconductor 22 responds to the change of an air to a gas flow .The circuit 25 produces a signal 4 Vd Td*(dVoldt), where (dVo/dt) is a derivative of the transient part of the response Vo( t) S and Td is a circuit time constant The differentiator 25 output connected to an input of a peak detector 27 which output voltage i Vp=(Vd)max constitues a maximum or a peak value of the derivative see FIG.2).

According to present invention the peak (Vd)max= K*C, where K is a scale factor.

r represents an accurate measure of the gas concentration C.

This constitues a novel method of instrumentation with the semiconductor gas sensors in which a derivative of the sensor response Vo is used and where the peak value of the derivative is proportional to the gas concentration.

The peak detector connected to display unit 29 controlled from the logic unit 30 by a signal 9j[ 37. On the other hand voltages Vd and Vp applied to the unit 30 which identifies that the peak Vp at time t=tp (FIG.2 b was registered and indicaties the end of measurement through output 40. The end of measurement at t=toff can be chosen at any time after tp and at this point of time the valve 1 I( FIG.1 will be switched back to air by the signal 40. The beginning of measurement initiated by a comparator 33 via its output 41 to the unit 30. The i cnmpnrator tets the. ha hneline vnlt age Vb. prndurled by the. ennsr under air flow ;gninr :n 14 MM" -M,1 Fign TMT-- .7 "A N_ ,R 72 Ipl F Pr RV $h reference voltage 34 in the end of the sensor conditioning cycle. The cycle consists of the predetemined time intervals when the pump 15 is switched ON or OFF and the sensor heater 21 powered ON and fully or partially OFF. This time sequence algorithm delivered by the logic unit 30 to control the pump and the heater. On the other hand the logic unit controls the S comparator 33 to allow the baseline test in the end of the conditioning cycle (signal 39). If baseline conditioned the comparator output signal 41 will initiate beginning of measurement through the logic unit from which the control signal 40 via means 18 switches the valve I from air to gas and activates the electronic switch 26 connecting the sensor output Vo to the input of the differentiator circuit 25 for time of measurement between t=ton and toff which O has to be after occurance of the peak Vp at t= tp (FIG.2).

For those skilled in art it is apparent that functions of the logic unit 30. the comparator 33. the electronic switch 26. the differentiator 25. the peak detector 27 can be achieved hy means of a microcomputer or a microprocessor in future reffered as microcomputer only) with input multiplexer connected to an analog-to-digital converter. The microcomputer can he Sg'" programmed to time the sensor conditioning cycle, to mark beginning of measurement, to control the display 29 and to perform the mathematical calculation of the derivative (dVo/.lt).

to identify time tp of the peak Vd, etc. The microcomputer design can easy acconmodate ;multitude of inputs if the unique sensor replaced by an array of sensors subjected to a flow of a mixture of different gases. The computer can perform the matrix calculations having data from multitude of sensors with an end result of selection individual gases and their respective concentrations in the mixture of different gases.

Those skilled in art can assume that the purpose of described combination of the valve 11 followed by the pump 15 and the sensor chamber 19 after to generate air or gas flow through the sensor 20 in the chimney manner can be easily carried out by the positioning the sensor chamber 19 after the valve 11 and the pump 15 in the end of a flow chain. If the sensor enclosure design without a chimney the chamber construction can be with inlet and outlet on the top of the sensoretc.

It is understood that the pump 15 can be continuous or single action. The single action pump can be easilly accomodated as only a small amount of gas is needed to produce a tmsient part of re_,~pnnnp .ind a rflni1ring mepanlrment I o I i To further understanding of the invention reffering now to FIG.2 In FlG.2(a) is shown the typical output voltage Vo from sensor 20 (FIG. In time interval to ton the sensor chamber pumped with air and the baseline Vb part 1 of FIG.2 a) is beeing established and tested.When air flow switched over to gas at t=ton the output response Vo exhibits a feature of the second order electrical system. Before reaching the plateau Vm part 3 of FIG.2 a the rising slope 2 of the voltage Vo can overshoot with a ringing or be overdamped with a very slow response.The type of response depends on parameters of the system such as the sensor constants associated with the said semiconductor geometry and temperature- the pump rate.

the sensor chamber design. etc all of which are effecting the said gas diffussion into the co semiconductor surface.

It has been found that the best results are obtained for a near critically damped system.

And consenquently it is one of the object of the invention to aquire a near critically damped electrical system in which case the peak voltage Vp=(Vd)max (FIG2 b) representing a maximum value of a derivative of transient part 2 of the voltage Vo will be a linear function of gas concentration. At the same time it was found that the analyser will produce the most accurate measurement which has a minimum correlation with the baseline. For this end the analyser can be equiped with means to control the flow rate in which said gas passes through the semiconductor surface, e.g.by regulating the motor speed of the pump 15 or by having the sensor chamber 19 furnished with number of vents 35 thus to design the critically damped c response (FIG. On other hand for purpose of achieving the appropriate damping the temperature of said semiconductor 22 can be adjusted as well by changing the electric current of heater 21.

Reffering now to FIG.3 in which one example of an algorithm for conditioning of said sensor semiconductor surface. The conditioning achieved by powering the sensor heater ON with a maximum filament current and partially OFF with a minimum current or OFF with no current the sensor heater 21 and by switching ON and OFF the pump 15 (FIG.l). The time intervals normally, however, not exclusively bind by equation T1=2T2=T4 and with T2=T3 being from 10 to 20sec, e.g. for Sensor TGS 800 of FIGARO Engineering Inc. Japan. The time intervals affected by sensor characteristics, the pump rate, the sensor chamber geometry. etc.

^e 9- a 3 .1 r Jo The sensor conditioning will be considered complete at point of time t=ton by baseline test where the sensor output voltage Vo= Vb compared to a reference value Vob which is the sensor normal voltage under power and air exposure for a long time.If Vb is equal or below Vob the valve I 1(FIG. 1) will be switched from air to gas at t=ton. If the voltage Vh>Vob the cycle TI to T4 is repeated to reach the sensor conditioning. As it was mentioned the technical means for testing the baseline is said comparator 33 (FIG.1) with the reference voliage Xbo applied to input 34.The comparator is controlled by signal 39 from logic unit 30 to sample the voltage Vo at t=ton.

After measurement when the gas how switched back to air at t=toff iFIG.2 and 1) to flush S the sensor for the next measurement on the time interval from toff to t6 the similar ahlorithm can be used as for the sensor conditioning before the measurement at ton. The end of measurement (EOM)at the moment t=toff for a single sensor Analyser will occur after tlhe maximum value of the derivative Vp=Vdmax at t=tp was register by the peak detector 25. It is apparent that a Multisensor Analyser for analysing a mixture of gases. the EOM will he achieved having all peaks Vp1.%p2-- measured by a single processor nultiplexed to each sensor of the array or by a number of peak detectors in communication with sensorsIn this case the slowest peak will time the EOM and t=toff.

To identify individual gases and their concentrations Cl. C2 CN in a mixture of N Cascs data on Vp .Vp2...Vpn .where n is number of sensors and n is greater or equal N.

t coefficients of selectivity of each sensor to the individual gases in the mixture have to he computed by said microcomputer. As at present the semiconductor gas sensors have poor selectivity the additional data on Vpdl.Vpd2...Vpdn which are maximum values of a decav part of each sensor response V I. V2....Vn after t=toff (FIG.2) which are related to concentrations in more complex form than Vpl. can be introduced to a matrix calculation.And the data on the stationary values of the responses can be taken into account as well.Those additional data are enhancing an accuracy of the Multisensor Analyser.

For a single sensor as well as multisensor Analyser the time of the proposed method of measurement from moment ton to toff is exceptionally short as toff can coinside ith tp using only a fast rising slope of the sensor output voltage Vo. whereas the existing methods hazed on the mezmnrnment of the tatinnary output -ignal Vm whir-h i- cnniderhhly lnger in I-r order from 10 to a hundred times.Thus the present invention has a different advatage in minimising the sensor recovery time as far less molecules of the gas have been adsorped by the semiconductor surface during a short exposure to the gas.

Another unique feature of the present method of gas concentration measurement is obtaining a S linear function between voltage Vp and the gas concentration.The existing methods using Vm as a measure of gas concentration compell to calculate a complex non-linear partly logarithmic function which differs from sensor to sensor being a formidable obstacle for the semiconductor sensors application in analythical instrumentation.

THE CLAIMS DEFINING THE INVENTION AS FOLLOWS: 1. A Gas Analyser for measuring the concentration of gas, comprising three port valve with two inlets and one outlet which can be connected alternatively to either inlet by switching means wherein one inlet designated for a gas under test and the other being an air or a reference gas inlet; a sampling pump of continuous or single action in communication with the valve outlet for delivering a flow of gas or air through the valve to a gas sensor housed in a chamber which inlet tied with said pump pressure port a semiconductor gas sensor containing a heater and a gas responsive semiconductor sensing element attached to a pair of electrodes having one electrode tied to a power source and the second electrically connected with a load resistor or an operational amplifier to generate an electric current through said element and an output voltage Vo proportional to said semiconductor conductance; a differentiator with its input coupled to the resistor or the amplifier via an electronic switch which is open when said valve switched on air to produce on its output a signal corresponding to a derivative (dVo/dt) of said voltage Vo in time after the valve has been operated from air to gas and the synhcronised with valve electronic switch connected the differentiator to a source of the signal Vo; i -ai 'i c- r, Er

Claims (16)

1. A Gas Analyser for measuring the concentration of gas, comprising three port valve with two inlets and one outlet which can be connected alternatively to either inlet by switching means wherein one inlet designated for a gas under test and the other being an air or a reference gas inlet; a sampling pump of continuous or single action in communication with the valve outlet for delivering a flow of gas or air through the valve to a gas sensor housed in a chamber which inlet tied with said pump pressure port a semiconductor gas sensor containing a heater and a gas responsive semiconductor sensing element attached to a pair of electrodes having one electrode tied to a power source and the second electrically connected with a load resistor or an operational amplifier to generate an electric current through said element and an output voltage Vo proportional to said semiconductor conductance; a differentiator with its input coupled to the resistor or the amplifier via an electronic switch which is open when said valve switched on air to produce on its output a signal corresponding to a derivative (dVo/dt) of said voltage Vo in time after the valve has been operated from air to gas and the synhcronised with valve electronic switch connected the differentiator to a source of the signal Vo; i -ai 'i c- r, Er I UI a peak detector which input connected to the differentiator output and which output electric signal represents a maximum value of said derivative (dVo/dt); means for displaying and scaling the peak detector output signal which is taken as an accurate value of the said gas concentration; a logic unit to provide interface and timing to operate said heater, pump via corresponding drivers for conditioning said sensor before measurement as well as to control said valve switching means,electronic switch,the differentiator.the peak detector and the display; a test means to establish a state of the semiconductor element conditioning under air flow in J.o time before measurement.

2.A Gas Analyser as claimed in Claim I wherein said chamber constructed with one or few vents or wherein said pump or similar means coupled with a controller to adjust gas or air S* flow rate through said sensor in order to obtain the sensor output signal Vo with a near critically damped characteristics of transient response without overshoot or excessive damping in time after said valve was switched from air to gas.

3.A Gas Analyser as claimed in Claim 1 wherein said heater operated by an electric current controller to establish a temperature of said semiconductor element in order to obtain the sensor output signal Vo with a near critically damped characteristics of transient response without overshoot or excessive damping in time after said valve was switched from air to gas. i'

4. A Gas Analyser as claimed in Claim l,having said valve switched on air,wherein said sensor heater and said pump being powered ON and OFF,to speed up conditioning of said gas sensing semiconductor into a stable state and to reduce the sensor recovery time,by application of one or a few conditioning cycles where in the first time interval T1 the pump and the heater are ON, in the second interval T2 the pump is OFF and the heater is ON, in the third interval T3 the pump is ON and the heater power is fully or partially switched OFF, and in the forth time interval T4 which is preceding the measurement the pump and the heater are switched ON. f771 W. s Il 1 O L r i Gas Analyser as claimed in Claims land 4 wherein said signal Vo being subjected to a baseline test in the end of the fourth interval T4 of said cycle by the sensor test means consisting of a comparator in which first input tied to the sensor electric output and having the second input connected to a reference voltage corresponding to the normal baseline value whereas the said comparator output being of the logic unit input in order to identify a number of cycles for the sensor conditioning and to allow a beginning of measurment.

6.A Gas Analyser as claimed in Claims 1 and 4 wherein said cycle or its variations applied after the measurement upon which said valve switched back to air to provide a purge of said sensor chamber and to condition the sensor semiconductor. ,O 7.A Gas Analyser as claimed in Claim I wherein said differentiator and peak detector being of the analog electronic means such as a CR-circuit coupled with an amplifier and a sample-and- hold circuit or being of a microcomputer with an input analog-to-digital converter connected to said sensor output and said signal Vo derivative, its peak value Vp and said peak's time tp acquired with help of the computer program. S 8.A Gas Analyser as claimed in Claims 1, 3 and 7 wherein said logic unit. sensor test means. the electronic switch and the pump controller being of an assembly of electronic circuits or being of said microcomputer means.

9.A Gas Analyser as claimed in Claim 1 wherin the said differentiator input connected to the sensor electric output by an electronic switch only on a period of measurement. $o 10.A Gas Analyser as claimed in any one of the preceding Claims wherein the sensor chamber inlet coupled to said valve outlet whereas said chamber exhaust outlet connected to the sampling pump suction port.

11.A Gas Analyser as claimed in Claims 1, 8 andl0 wherein combination of said valve and pump can be substituted for air and gas pump with their pressure ports connected into common manifold via a T-piece or similar means.

12.A Gas Analyser as claimed in Claim 1 wherin said valve with the switching means being a T- or Y-piece connected to gas and air delivery lines whereas the switching from air to gas produced by blocking the air line.

13. A Gas Analyser for measuring a concentration of individual gases and for identifying a O rmnncitinn of aCes in a mixture nf diffe.rnt gaes- comprising S i .P T.FII.. J4 three port valve with two inlets and one outlet which can be connected alternatively to either inlet by switching means wherein one inlet designated for a gas mixture under test and the other being an air inlet; a sampling pump in communication with the valve outlet for delivering the gas mixture or S air through the valve to an array of the gas sensors in which individual sensors designated for different gases or group of gases having the said array housed in a chamber with its inlet manifold coupled with the said pump pressure port; semiconductor gas sensors with each sensor having a heater and a pair of electrodes whereas said electrodes electrically coupled with power source and resistors to generate electric signals: Jo differentiators with their inputs connected to said resistors and each of them performing a function of extracting a derivative from the electric signal of the corresponding sensor after the said valve has been switched from air to gases or visa versa causing an alternation of the medium pumped through the said sensors and change of the sensor signals; peak detectors connected to the differentiators in communication with amplifiers to produce voltages corresponding to maximum values of the said derivatives; computation means which input connected to each peak detector and each sensor output to identify said mixture composition and individual gases concentration; a display interfaced to said computation means; .a logic unit to provide interface and timing to operate heaters and the pump via o. o corresponding drivers for conditioning the said sensors before measurement as well as to S. control the said valve switching means,the differentiators, peak detectors and the display: "test means to establish the state of conditioning of the semiconductor elements under air flow in time before measurement.

14. A Gas Analyser as claimed in Claim 13 wherein said array chamber constructed with one or few vents or wherein said pump coupled with a controller to adjust gas or air flow rate through said sensors in order to obtain the sensor output signals with the near critically damped characteristics of transient responses without overshoot or excessive damping in time after said valve was switched from air to gas. r 'P 1 i-l o 1 l m q 1 d A Gas Analyser as claimed in Claim 13, having the said valve switched on air, wherein said sensor heaters and said pump being powered ON and OFF, to condition sensors of the said array before the measurement of gas concentration, with a cycle comprising in the first time interval TI having the pump and heaters ON, S in the second interval T2 the pump is OFF and heaters are ON, in the third interval T3 the pump is ON and heaters are fully or partially OFF, and in the forth time interval T4 which is preceding the measurement the pump and heaters being switched ON.

16. A Gas Analyser as claimed in Claims 13 and 15 wherein the sensor being subjected to a J number of said cycles,while the said signals baseline tested in the end of the fourth interval of the said cycle.

17.A Gas Analyser as claimed in Claims 13 and 15 wherein the said cycle applied after the measurement upon the said valve switched back to air to provide a purge the chamber of said array and to condition the sensors.

18.A Gas Analyser cs claimed in Claim 13 wherein any of said differentiators and peak detectors being of the analog electronic means comprising CR-circuits coupled to amplifiers and sample-and-hold circuits in communication with said computation means or being of a microcomputer with an input multiplexer connected to said output resistors followed by analog-to-digital converter connected to said output resistors having said sensor electric signals,their derivatives and peak values extracted by a program and computed to be supplied to said display via output port.

19.A Gas Analyser as claimed in Claim 13 and 17 wherein said logic unit together with computation and sensors test means, said electronic switches and the pump controller being of an assembly of electronic circuits or being of the said microcomputer. J 20.The method of measuring a gas or vapour concentration which comprises: generation of enforced gas or vapour flow via a semiconductor gas sensor; producing the electric signal proportional to said sensor conductivity; computing a derivative of said sensor output signal; extracting the maximum value of said derivative; i o ennditionings id ignnl htn'line p onr t mea iirenment. S 91" a P s' J i I

21.The -nethod as claimed in Claim 20 wherein the maximum value of the derivative of thesensor output signal represents a linear function of the gas concentration.

22.The method of gas or vapour analysis of a mixture of different gases or vapours by identifying a composition of gases or vapours and concentrations of individual gases or vapours comprising: generation of enforced flow of said mixture via an array of semiconductor gas sensors: producing the electric signals proportional to conductivities of said sensors: computing derivatives of each or selective electric signals: extracting maximum values of said derivatives; 0o computing stationary values of said output signals together with the maximum values of their derivatives in a manner to obtain a composition of gases or vapours and their concentrations; conditioning baselines of said electric signals prior to measurement. -v^ D 9- .t' s ~9 I I r I