Gas analyzer
BACKGROUND 
     The present invention relates to apparatus for determining the proportion of a given gas component in a gas mixture. 
     It is well known that when wishing to detect the presence of and/or determine the proportion of a given component of agas mixture, there can be used an apparatus or device which incorporates a piezoelectric crystal, e.g. a quartz crystal, having provided on its surfaces a thin substrate layer comprising one or more substances capable of reversibly and selectively adsorbing the gas component of interest, i.e. the relevant gas component, from a gas mixture, the crystal being so arranged as to enable the gas mixture being assayed to be brought into contact with the substrate layer. The substrate layer therewith adsorbs molecules of the relevant gas component from the gas mixture to an extent contingent on the concentration of said component in the mixture and increases in mass in dependence hereon, which results in a decrease in the vibration frequency of the piezoelectric crystal. The vibration frequency of the crystal is normally determined by incorporating the crystal in an oscillator circuit constructed to produce an output signal representative of the oscillating frequency of the oscillator circuit, and therewith also of the crystal, in relation to a reference frequency. The reference frequency is normally the oscillating frequency to another, similar crystal, which lacks the aforedescribed substrate layer but which, in all other respects, is arranged identically with the crystal provided with said substrate layer. An output signal representing the frequency difference of the two crystals will therefore constitute a measurement of the quantity of the relevant component present in the assayed gas mixture. Apparatus of this kind are described for example, in U.S. Pat. Nos. 3 164 004 and 4 399 686. Substances suitable for use as a substrate layer for the selective and reversible adsorption of many different gaseous components of a gas mixture are also known to the art. 
     One problem with gas analyzing apparatus of this kind is that the ability of the substrate material to adsorb the relevant gas component from a gas mixture is highly dependent on the temperature of the substrate material, such that with an unchanged concentration of the relevant gas component in the gas mixture the quantity of the component adsorbed decreases with increasing substrate layer temperatures. The sensitivity of the gas analyzer is thus correspondingly temperature dependent, and decreases with increasing substrate temperatures. In conjunction with gas analyzers for assaying respiration gas mixtures in order to determine the constituent proportion of anaesthetic gases of the type halogenated hydrocarbon compounds, such as halothane, enflurane, methoxy flurane and iso-flurane, said substrate layer being composed, for example, of different types of silicon oils, it has been found that the aforesaid temperature dependency has a form which can be expressed by the equation 
     
         S˜T.sup.-5/2 ·e.sup.1/T                     ( 1) 
    
     where S is the above-mentioned output signal representing the difference frequency of the crystals, and T is the temperature (°K.) of the substrate layer. Within a temperature range of 20°-45° C., which is an appropriate temperature range with gas analyzers of this kind, the change in analyzer sensitivity as a result of variations in temperature is in the order of 150%. 
     It will be realized that the aforesaid temperature dependency must be eliminated in some way. One possibility is to maintain the piezoelectric crystal and its substrate layer at a constant temperature. This can be effected by either cooling or heating the crystal. Cooling of the crystal results in the condensation of water vapor on the crystal, which creates problems, at least when analysing respiration gas mixtures, and renders an accurate assay totally impossible. When heating the crystal it is necessary to use temperatures of up to 50° C., at least when applicable to the aforesaid analysis of respiration or breathing gas mixtures, which results in a marked reduction in the sensitivity of the analyzer and also in far more rapid deterioration of the properties of the substrate layer, therewith reducing the useful life-span of the crystal. Both of these methods for maintaining the crystal and substrate layer at a constant temperature also have a considerable power consumption which, among other things, renders the use of a battery-driven gas analyzer impossible. A further difficulty in conjunction with maintaining the crystal and the substrate layer at a constant temperature resides in the fact that the crystal has a very low thermal capacity, and hence its temperature accompanies variations in ambient temperature extremely rapidly, i.e. in practice variations in the temperature of the analysed gas mixture. 
     Consequently, the object of the present invention is to provide an apparatus of the kind mentioned in the introduction with which the problem associated with temperature-dependent sensitivity is eliminated or, in all events, greatly reduced, without needing to maintain the crystal and its substrate layer at a constant temperature. 
     SUMMARY OF THE INVENTION 
     The most direct method of solving the aforementioned problem would be to measure the prevailing temperature of the piezoelectric crystal and correcting the analyzer output signal is dependence thereof. However, since the temperature dependency is complex, as made evident by the above equation (1), such a direct method is complicated and requires the provision of complex electronic circuitry. 
     The present invention is based on the discovery of a much simpler method of compensating for the temperature-dependent adsorption of the substrate layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The characteristic features of the apparatus according to the invention are set forth in the following claims, and the invention will now be described in more detail with reference to the accompanying drawing, in which 
     FIG. 1 is a circuit diagram illustrating schematically, and by way of example, an embodiment of the invention; and 
     FIG. 2 is a graph illustrating the result obtained with a test apparatus constructed in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The fundamental principle of the invention resides in arranging a temperature-dependent electric component in a manner such that the component is subjected to the same temperature variations as the crystal and its substrate layer, and supplying the temperature-dependent output signal from the oscillator circuit incorporating the crystal to a signal processing circuit, preferably an amplifying circuit, in which the aforesaid temperature-dependent electric component is incorporated in a manner such that the signal processing circuit produces an output signal which is substantially compensated for the temperature-dependent variations in the adsorbing capacity of the substrate layer. 
     As mentioned in the aforegoing, the temperature-dependent output signal of the oscillator circuit can be expressed by the aforegoing equation (1). Within the temperature range of 15°-50° C. this equation can be very well approximated by the equation ##EQU1## where S o  is the output signal of the oscillator circuit at the temperature T o  and a is a constant dependent on the properties of the substrate layer and the adsorbed gas component. 
     According to one preferred embodiment of the invention the temperature-dependent electric component used is a thermistor having a negative temperature coefficient (NTC), which is arranged so to be influenced by the temperature of the analysed gas in the same manner as the piezoelectric crystal provided with said substrate layer, and therewith subjected to the same variations in temperature as said layer. The circuit used for processing the output signal of the oscillator circuit is advantageously an amplifying circuit of the construction illustrated in FIG. 1, the output signal S of the oscillator circuit being applied to the input of said amplifying circuit. 
     The amplifying circuit includes an operational amplifier F which has a resistance feedback path through a resistor R o , from its output to one of its inputs. The output signal S from the gas-analyzer oscillator circuit applied to the input of the amplifying circuit is connected to the feedback input of the operational amplifier F through a resistor R 1 , and to the other input of the operational amplifier F via a resistive voltge divider comprising the resistors R 2  and R 3 . If the resistance values of the resistors R o , R 1 , R 2  and R 3  are selected so that 
     
         R.sub.o &gt;&gt;R.sub.1 and R.sub.2 &gt;&gt;R.sub.3 
    
     the amplifying factor V U  /V I  of the amplifying circuit can be expressed approximatively as ##EQU2## This gives ##EQU3## 
     According to a first embodiment of the invention the thermistor is coupled in the amplifying circuit in a manner to constitute the resistor R 1 , i.e. 
     
         R.sub.1 =R.sub.T 
    
     where R T  is the resistance of the thermistor. 
     Since the temperature-dependent resistance R T  of the thermistor can be expressed as ##EQU4## where R T .sbsb.o is the resistance at the temperature T o  and b is the characteristic temperature (°K.) of the thermistor, there is obtained ##EQU5## It will be seen from this expression that when the thermistor is selected so that 
     
         a=b 
    
     the output signal V U  of the amplifying circuit will be independent of the prevailing temperature T of the piezoelectric crystal and the substrate layer, i.e. independent of any variations of temperature in the crystal and its substrate layer, and proportional to the signal S o , i.e. the output signal of the oscillator circuit at a given temperature, this output signal being representative of the proportion of the relevant gas component in the gas mixture influencing the substrate layer. 
     Comparison tests are being carried out with a gas analyzer of the aforedescribed kind of assaying the proportion of the anaesthetic gas isoflurane in a respiration gas mixture, the substrate layer containing a silicon oil. The temperature-dependency of the sensitivity of the gas analyzer was determined firstly without endeavouring to compensate for temperature and secondly with a temperature compensating circuit of the aforedescribed kind illustrated in FIG. 1. In this case, the resistance values of the amplifying circuit were R T  =50KΩ at 25° C., R o  =1MΩ, R 2  =100KΩ and R 3  =5KΩ, and the characteristic temperature of the thermistor was b=3934° K. In the graph of FIG. 2 the curve A represents the sensitivity K of the analyzer in respect of the narcotic as in question as a function of the temperature T (°C.) in the absence of temperature compensation, whereas the curve B represents the sensitivity of the analyzer as a function of temperature when using the aforedescribed temperature compensating circuit illustrated in FIG. 1. As will be seen, the temperature dependency was reduced to about one tenth when using the temperature compensating circuit. 
     According to another embodiment of the invention, the temperature compensation sought for can also be achieved with the amplifying circuit illustrated in FIG. 1 by coupling the thermistor to form the resistor R 3 , i.e. 
     
         R.sub.3 =R.sub.T                                           (7) 
    
     In this case, the resistance values of the resistor are selected so that 
     
         R.sub.o &gt;&gt;R.sub.1 and R.sub.3 &gt;&gt;R.sub.2 
    
     and simultaneously 
     
         R.sub.o ·R.sub.2 &gt;&gt;R.sub.1 ·R.sub.3. 
    
     When the mutual resistance values of the resistors are selected in this way, the amplifying factor V U  /V I  of the amplifying circuit can be expressed approximately as ##EQU6## which when applying expressions (2), (5) and (7) gives ##EQU7## 
     The output signal V u  will also in this case be substantially independent of the prevailing temperature T of the piezoelectric crystal and the substrate layer, when the thermistor is chosen so that 
     
         a=b.