Abstract:
A gas measurement device measures gas using a gas sensor including a sense resistance exposed to the gas and a reference resistance not exposed to the gas. The gas measurement device applies a first current value and a second current value to the sensor. A detector functions to detect a first resistance variation and a second resistance variation of the sense resistance exposed to the gas with respect to the reference resistance as a function of the first current value and the second current value, respectively. The resistance variation dependent on relative humidity is then determined as a function of the first and second resistance variations and a first constant. The resistance variation dependent on gas content is then determined as a function of the first and second resistance variations and a second (different) constant.

Description:
PRIORITY CLAIM 
       [0001]    This application claims priority from Italian Application for Patent No. MI2014A001197 filed Jul. 2, 2014, the disclosure of which is incorporated by reference. 
       TECHNICAL FIELD 
       [0002]    The present disclosure relates to a gas measurement device and measurement method. 
       BACKGROUND 
       [0003]    A thermal conductivity detector (TCD) is well known in the state of the art. This is an environmental sensor device widely used for the measurement of the amount of gas in the environment. The operation is based on the fact that each gas has an inherent thermal conductivity and a filament (thermal resistor) changes its temperature as a function of the amount of gas that surrounds it. The most appropriate sensing element shape is that of a suspended thin finger, for which the temperature of the central part can locally reach even values of several hundred degrees. The feature that the finger is totally suspended allows for enhancing the amount of heat exchange with the gas in which it is immersed. The warming effect of the suspended finger is induced through an electrical stress of the sensor, that is by the flow of the current through the finger. The sensor is able to better discriminate the gases whose conductivity is much different than normal air (roughly nitrogen N 2  (79%), oxygen O 2  (19%), carbon dioxide CO 2  (0.04%), plus other gases with negligible quantities: for example the carbon oxide CO is a few ppm). 
         [0004]    When a current flows through the finger, the value of the resistance of the finger changes. The measurement of the resistance value allows for measuring the conductivity of the gas mixture which depends of the molar fraction of the gas of interest. 
         [0005]    However, it is difficult in principle to discriminate which gas is mainly responsible for the conductivity variation of the mixture of gas. For example, carbon dioxide CO 2  has a lower thermal conductivity than dry air, therefore if its percentage increases inside the mixture, this will raise the temperature of the sensor with a consequent increase of the value of the measured resistance. 
         [0006]    The TCD sensor operates in accordance with the thermodynamic equilibrium among heat generated by the current flow, heat exchange with the material of which the sensor is made (e.g. polysilicon crystalline), and heat exchange with the gas mixture surrounding it. The ambient temperature determines the equilibrium value of the sensor in standard dry air. To take into account and compensate the variation of ambient temperature a Wheatstone bridge as the sensor structure could be used. The reference branches of the bridge are of the same nature and positioned in the vicinity of the sensor so as to be sensitive to the same way to changes in ambient temperature, with the difference that these branches will not be exposed to the mixture of gas as is the sensor. 
         [0007]    The Relative Humidity (RH) is the amount of water vapor (gas) present in the environment compared to a saturated environment in the same conditions of pressure and temperature. The thermal conductivity of water vapor is much larger than the dry air therefore an increase in relative humidity produces a lowering of the temperature of the sensor with a consequent reduction of the value of the measured resistance. The contribution of the RH value of the measured resistance could be 1/10 compared to the change of resistance in the presence of carbon dioxide CO 2 , therefore, this is a parameter to measure and correct. Typically the correction is made by means of a dedicated sensor for the measurement of the RH. 
       SUMMARY 
       [0008]    One aspect of the present disclosure is to provide a gas measurement device of simple architecture. 
         [0009]    One aspect of the present disclosure is a gas measurement device for measuring gas by means of a gas sensor comprising at least one resistance exposed to at least one gas and at least one reference resistance not exposed to the gas, said gas measurement device comprising: a control device configured to manage the gas sensor so that the gas sensor receives at least a first current value and a second current value, a detector to detecting a first resistance variation and a second resistance variation of the resistance exposed to the gas with respect to the reference resistance as a function of the first current value and the second current value respectively, and a calculation circuit configured to calculate at least a first and a second equations wherein the first equation is given by the difference between the first resistance variation multiplied by a first constant and the second resistance variation while the second equation is given by the difference between the first resistance variation multiplied by a second constant and the second resistance variation, the first constant and the second constant having different values. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    For a better understanding of the present disclosure, a preferred embodiment thereof is now described, purely by way of non-limiting example and with reference to the annexed drawings, wherein: 
           [0011]      FIG. 1  shows a block diagram of a measurement apparatus comprising a gas sensor device and a gas measurement device according to the present disclosure; 
           [0012]      FIG. 2  shows a more detailed block diagram of the gas measurement device according to the present disclosure; 
           [0013]      FIGS. 3-5  show the waveforms of the resistance variations ΔR(Il), ΔR(Ih) as a function of the concentration of carbon dioxide CO 2  and the waveform of a resistance value as a function of the relative humidity RH; 
           [0014]      FIG. 6  shows the waveforms of the resistance variation value as a function of the different concentrations of CO 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1  shows a block diagram of a measurement apparatus comprising a gas sensor device  1 , that is a TCD sensor, and a gas measurement device  100  according to the present disclosure. 
         [0016]    The gas sensor device  1  comprises at least one variable resistance R 2  exposed to the gas and a reference resistance R 1  which is not exposed to the gas; the reference resistor R 1  has the value of the resistance R 2  at the condition of dry air and room temperature. The value of the resistance R 2  varies when exposed to the gas, the humidity and the temperature. Preferably, the gas sensor device  1  is a Wheatstone bridge including a couple of reference resistors R 1  and a couple of resistors R 2  exposed to the gas; the use of a Wheatstone bridge allows for minimizing the dependence on the ambient temperature. The four connecting nodes A-D of the terminals of the resistances R 1  and R 2  of the Wheatstone bridge  1  are connectible respectively with a variable current generator  200 , to ground GND and to the gas measurement device  100  able to receive the voltage signal at the output of the Wheatstone bridge  1 . 
         [0017]    The measurement device  100  ( FIG. 1 ) comprises preferably a temperature sensor  102  in the case wherein it is necessary to provide a temperature compensation of the signals at the output of the sensor device  1 . The measurement device  100  comprises preferably a multiplexer  101 , configured to receive the output signal of the sensor device  1  or the output signal of the temperature sensor  102 . The measurement device  100  comprises a device  103  configured to amplify the signal at the output of the multiplexer, an analog-to-digital converter  104  for converting the analog signals at the input into a digital signal at the output, a digital controller  105  for processing the signal deriving from the sensor device and an interface  106  for outputting the processed signal to the outside. 
         [0018]    The measurement device  100  is shown in more detail in  FIG. 2 . The device  103  is preferably a low noise analog front end comprising the cascade of two fully differential switched-capacitor amplifiers configured to amplify the signal at the output of the multiplexer  101  and to compensate the offset of the gas sensor  1  or the temperature sensor  102 . The low noise analog front end  103  makes use of both chopping and correlated double sampling techniques, which ensure offset canceling and low frequency noise filtering. 
         [0019]    A managing device  109  manages the devices  101 - 106  and the variable current generator  210 ; the managing device  109  manages the timing of the low noise analog front end  103 , the analog-to-digital converter  104  and the digital controller  105 . The managing device comprises a clock generator  111  configured to send two different clock signals at different frequency, for example 1 Mhz and 40 Khz, to a phase generator  110  which receives the output of the bit register  112 . 
         [0020]    When a gas having a concentration m is inside the gas sensor  1  at a relative humidity n, the managing device  109  is configured to effectuate the following steps:
       managing the variable current generator  210  to send a first current value Il to the gas sensor  1  and detect the resistance variation ΔR(Il) of the resistances R 2  with respect to the reference resistances R 1 ;   managing the variable current generator  210  to send a second current value Ih to the gas sensor  1  and detect the resistance variation ΔR(Ih) of the resistances R 2  with respect to the reference resistances R 1 ;   managing the digital controller  105  to calculate the resistance variation Δh depending only on the relative humidity variation by means of the following equation Δh=K 1 ×ΔR(Il)−ΔR(Ih) and the resistance variation Δc depending only on the gas concentration variation by means of the following equation Δc=K 2 ×ΔR(Il)−ΔR(Ih) wherein K 1  and K 2  are constants having different values. In this way the calculation of the above equations allow obtaining the indirect measure of the relative humidity alone, independently from the gas concentration, and of the gas concentration alone, independently from the relative humidity, and   managing the interface  106  to output the resistance variations Δh and Δc.       
 
         [0025]    In the case wherein the concentrations of a first and a second gases need to be measured, the digital controller  105  is configured to:
       manage the variable current generator  210  to send a first current value Il to the gas sensor  1  and detect the resistance variation ΔR(Il) of the resistances R 2  with respect to the reference resistances R 1 ;   manage the variable current generator  210  to send a second current value Ih to the gas sensor  1  and detect the resistance variation ΔR(Ih) of the resistances R 2  with respect to the reference resistances R 1 ;   manage the digital controller  105  to calculate the resistance variation Δc 1  depending only on the concentration variation of the first gas by means of the following equation Δc 1 =K 21 ×ΔR(Il)−ΔR(Ih)) and the resistance variation Δc 2  depending only on the concentration variation of the second gas by means of the following equation Δc 2 =K 22 ×ΔR(Il)−ΔR(Ih), wherein K 21  and K 21  are constants having different values. In this way the calculation of the above equations allow obtaining the indirect measure of the concentration of the first gas independently from the concentration of the second gas and vice versa, and   manage the interface  106  to output the resistance variations Δc 1  and Δc 2 .       
 
         [0030]    In  FIGS. 3-5  the waveforms of the resistance variations ΔR(Il), ΔR(Ih) are shown wherein on the X axis the variation of the concentration of the gas is indicated while on the Y-axis the variation of the sensor resistance is indicated at the condition for a relative humidity RH=0, RH=30% and RH=60%. The further waveform is the resistance value Δh=K 1 ×ΔR(Il)−ΔR(Ih) which is independent on the variation of the concentration of the gas and depends only on the relative humidity RH. 
         [0031]      FIG. 6  show the resistance values Δc=K 2 ×ΔR(Il)−ΔR(Ih) for gas concentrations m=0%, m=10% and m=20% which depend only on the concentration variation of the carbon dioxide CO 2  and are independent on the relative humidity RH. 
         [0032]    Preferably the constants K 1  and K 2  have respectively the values of 1.827 and 2.165. A method for calculating the appropriate value of the constants K 1  and K 2  is now described. 
         [0033]    The thermal conductivity of a gas mixture depends on the molar fraction of the gases of the mixture, on the conductivity of the gases and on the dynamic viscosity according to the Chapman-Enskog model. 
         [0034]    In first approximation, starting from the Chapman-Enskog model (“The mathematical theory of non-uniform gases: an account of kinetic theory of viscosity, thermal conduction and diffusion in gases” S. Chapman, T G. Cowling 1970, incorporated by reference) and obtaining a linear equation, the thermal conductivity of a gas mixture is linearly proportional to the temperature and the concentration of gases of the mixture. 
         [0035]    The resistance variation ΔR (that is the variation of the resistance R 2  with respect to the reference resistance R 1 ) is a linear function of both the concentration of the matters to be examined (the concentration of gas and the humidity or the concentrations of two gases) and the current flowing through the resistance R 2 , preferably, in the case wherein the sensor is a Wheatstone bridge, the resistance variation ΔR is a linear function of both the concentration of the matters to be examined and the current flowing through the bridge  1 . 
         [0036]    In fact, balancing and solving the equation for the thermoelectric equilibrium of the system comprising the bridge  1  and the gas mixture, the resulting temperature at the equilibrium is approximately a linear function of the concentrations of gas and humidity and of the current flowing through the bridge  1 . 
         [0037]    At the thermoelectric equilibrium it is necessary to consider the power dissipated by Joule effect on the resistance R 2 , P=R×I 2  wherein I is the current flowing through the bridge  1 , and the amount of the heat exchange due to the thermal conductivity of the gas mixture, 
         [0000]    
       
         
           
             
               Q 
               T 
             
             = 
             
               K 
               × 
               
                 A 
                 dx 
               
                
               Δ 
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               T 
             
           
         
       
     
         [0000]    where A is the surface of the resistance R 2 , dx is the thickness of the resistance R 2  and ΔT is the temperature variation; at the thermoelectric equilibrium it is obtained that the temperature variation ΔT is a linear function of the concentrations of gas and humidity and of the current flowing through the bridge  1   
         [0038]    The resistance variation ΔR depends on the temperature variation ΔT according to the ΔR=R 0 ×(1+αΔT) where a is the thermal coefficient of the resistance and depends on the material of the resistive bridge and R 0  is the resistance value at room temperature, therefore even the resistance variation ΔR, so as the temperature variation ΔT, is a linear function of the concentrations of gas and humidity and of the current flowing through the bridge  1 . The resistance variation ΔR as linear function of the concentrations of gas and humidity and of the current flowing through the bridge  1  can be represented by the following equation ΔR=(a×I+b)×m+(c×I+d)×n wherein m is the concentration of gas, n is the concentration of humidity, I is the current flowing through the bridge  1  and a, b, c and d are parameters depending on the balance of the system which are determined by effectuating four calibration measurements with known gas and humidity concentrations and currents. 
         [0039]    After determining the parameters a, b, c and d two measurements of the unknown mixture are effectuated with the unknown concentrations m and n and two different current values Il and Ih; solving said two equations and calculating the resistance variation as function of the current, that is ΔR(Il) and ΔR(Ih), the unknown values of the concentrations m and n are obtained. 
         [0040]    Considering the generic equation ΔR=K×ΔR(Il)−ΔR(Ih), exist only two values K 1  and K 2  of K which allow the m and n concentration components to become null. The equation becomes: ΔR(K)=(a×(K×Il−Ih)+b×(K−1))×m+(c×(K×Il−Ih)+d×(K−1))×n and setting equal to zero the m and n concentration components the values 
         [0000]    
       
         
           
             
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                       Il 
                     
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         [0000]    are obtained. In this way each one of the results Δc(K 2 ) and Δh(K 1 ) depends on the concentration variations only of one of two unknown concentrations.