Abstract:
The invention relates to a NDIR two beam gas analyser in which infrared radiation is guided by modulation in an alternating manner through a measuring chamber and a reference chamber and is subsequently detected, a measurement signal being produced due to the analysis which determines the concentration of a measurement gas component present in the measurement chamber. The detection and compensation of error effects, in particular modifications on the infrared radiation source or detector arrangement, is simplified as a phase imbalance is produced in the switching of the radiation between the chambers, and the measurement signal is detected in a phase-sensitive manner for modulating the radiation, a measurement signal vector (SF) comprising amplitude information and phase information (Φ F ) is obtained such that during calibration of the gas analyser for different known concentrations (K 1 , K 2 , K 3 , K 4 , K 5 ) of the measurement gas components, measurement signal vectors (S 1 , S 2 , S 3 , S 4 , S 5 ) having different amplitudes and phases are determined, vectors define a characteristic line ( 43 ), and when an unknown concentration of the gas component is measured, the unknown concentration of the measurement gas component is determined from the intersection point ( 45 ) of an obtained measurement signal vector (S F ) or the extension thereof with the characteristic line ( 43 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a U.S. national stage of application No. PCT/EP2010/056770 filed 18 May 2010. Priority is claimed on German Application No. 10 2009 021 829.7 filed 19 May 2009, the content of which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a method for determining the concentration of a measurement gas component in a gas mixture using a non-dispersive infrared (NDIR) two beam gas analyzer. 
         [0004]    The invention furthermore relates to a NDIR two beam gas analyzer. 
         [0005]    2. Description of the Related Art 
         [0006]    WO 2008/135416 A1 discloses a conventional method and a gas analyzer which serve for determining the concentration of a measurement gas component in a gas mixture. To this end, infrared radiation generated by an infrared radiation source is guided alternately through a measurement cell receiving the gas mixture and a reference cell containing a reference gas. The radiation exiting the two cells is detected using a detector array, where a measurement signal is generated and subsequently evaluated in an evaluation unit. Typical detector arrays include one or more optopneumatic detectors comprising single-layer or double-layer receivers. The radiation is switched between the measurement cell and reference cell using a modulator, which is typically a paddle wheel or chopper. If, for zeroing purposes, both cells are filled with the same gas, i.e., zero gas such as nitrogen or air, and the gas analyzer is optically balanced, the same radiation intensity always reaches the detector array with the result that no measurement signal (change signal) is generated. If the measuring cell is filled with the gas mixture to be examined, pre-absorption that is dependent on the concentration of the measurement gas component contained therein and of any cross gases that may be present occurs. As a result, different radiation intensities temporally sequentially reach the detector array in step with the modulation from the measurement cell and the reference cell, which detector array generates as a measurement signal a change signal with the frequency of the modulation and a variable that is dependent on the difference of the radiation intensities. 
         [0007]    The radiation intensity that is incident on the detector array is, however, not just dependent on the gas-specific absorption but also on other variables influencing the intensity of the infrared radiation. Influence variables of this type, such as dirt-, ageing- or temperature-related changes at the infrared radiation source or detector array cannot be readily identified and can lead to incorrect measurement results. 
         [0008]    It is necessary for this reason to calibrate the gas analyzer at regular intervals where, for example, the measurement cell is filled successively with zero gas and span gas, i.e., known concentrations of the measurement gas. 
         [0009]    For calibrating a NDIR two beam gas analyzer, DE 195 47 787 C1 discloses filling of the measurement cell with a zero gas and interruption of the radiation passing through the reference cell using an aperture. In this way, a one-beam functionality of the gas analyzer is achieved, which enables referencing, for example, to the intensity of the infrared radiation source, without the need to fill the measurement cell with a calibration gas. 
         [0010]    In the case of a NDIR two beam gas analyzer known from EP 1 640 708 A1, at least two dark phases are generated within the modulation period, during which the radiation passing through both the measurement cell and through the reference cell is interrupted. In this way, a harmonic with double the frequency is modulated onto the fundamental of the measurement signal. After a Fourier analysis of the measurement signal has been performed, measurement variables normalized by the two first Fourier components are determined and the concentration of the measurement gas component is determined by coordinate transformation of the normalized measurement variables. 
         [0011]    In the case of the NDIR two beam gas analyzer known from the already mentioned WO 2008/135416 A1, the detector array has at least two one-layer receivers, which each provide one measurement signal and are located one after the other in the beam path of the gas analyzer. The first one-layer receiver contains, for example, the measurement gas component and the at least one one-layer receiver arranged downstream contains a cross gas. The evaluation unit contains an n-dimensional calibration matrix corresponding to the number n of the one-layer receivers, in which calibration matrix measurement signal values, which are obtained at different known concentrations of the measurement gas component in the presence of different known cross gas concentrations, are stored as n-tuples. When measuring unknown concentrations of the measurement gas component in the presence of unknown cross gas concentrations, the concentration of the measurement gas component is ascertained by comparing the n-tuples of signal values obtained during the measurement with the n-tuples of signal values stored in the calibration matrix. Moreover, for example, if the cross gas concentrations are kept constant, the intensity of the generated radiation can be varied to ascertain the influence of transmittance changes, which are caused by ageing of the infrared emitter or dirt on the measurement cell, on the measurement result. 
       SUMMARY OF THE INVENTION 
       [0012]    It is an object of the invention to simplify detection of and compensation for error influences, such as dirt-, ageing- or temperature-related changes at an infrared radiation source or detector array. 
         [0013]    This and other objects and advantages are achieved in accordance with the invention by a method and NDIR two beam gas analyzer wherein a phase imbalance in switching of radiation between a measurement cell and a reference cell is produced, a measurement signal is detected phase-sensitively with respect to modulation of the radiation, where a measurement signal vector with amplitude information and phase information is obtained. In accordance with the invention, in the calibration of the gas analyzer, measurement signal vectors of different amplitude and phase, which define a characteristic curve, are ascertained for different known concentrations of the measurement gas component, and in the measurement of an unknown concentration of the measurement gas component, the unknown concentration of the measurement gas component is ascertained from the intersection point of the measurement signal vector, obtained in the measurement, or its extension with the characteristic curve. 
         [0014]    Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    For the purposes of further illustrating the invention, reference is made below to the figures of the drawing; specifically, the figures show in each case in the form of an exemplary embodiment, in which: 
           [0016]      FIG. 1  is a schematic block diagram of a NDIR two beam gas analyzer with a detector array which consists of two one-layer receivers, which are located one downstream of the other, and supplies two measurement signals in accordance with the invention; 
           [0017]      FIG. 2  is a graphical plot of a calibration matrix, in which measurement signal values, which are obtained with different known concentrations of the measurement gas component in the presence of different known cross gas concentrations, are stored as value pairs in accordance with the invention; 
           [0018]      FIG. 3  is a plan view of an arrangement of chopper, measurement cell and reference cell of the NDIR gas analyzer in accordance with the invention; 
           [0019]      FIG. 4  is a graphical plot of the power density distribution of the radiation introduced into the measurement cell and reference cell in accordance with the invention; 
           [0020]      FIG. 5  is a graphical plot of an alternative power density distribution of the radiation introduced into the measurement cell and reference cell in accordance with the invention; 
           [0021]      FIG. 6  is a graphical plot of a double lock-in amplifier for phase-sensitive detection of a measurement signal and its decomposition into an in-phase component and a quadrature component; in accordance with the invention; 
           [0022]      FIG. 7  is a graphical plot of a coordinate system (in-phase component and quadrature component) with a characteristic curve formed from different measurement signal vectors ascertained during a calibration of the gas analyzer for different known concentrations of the measurement gas component in accordance with the invention; 
           [0023]      FIG. 8  is a graphical plot of an exemplary rotation of the characteristic curve in the coordinate system for simplifying the measurement signal processing in accordance with the invention; 
           [0024]      FIG. 9  is a graphical plot of an exemplary measurement signal processing if the characteristic curve is linear; and 
           [0025]      FIG. 10  is a flow chart of the method in accordance with an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]      FIG. 1  shows an NDIR two beam gas analyzer, in which the infrared radiation  2  generated by an infrared radiation source  1  is split, using a beam splitter  3  (i.e., a “trouser chamber”), into a measurement beam path passing through a measurement cell  4  and a comparison beam path passing through a reference cell  5 . A gas mixture  6  with a measurement gas component, the concentration of which is to be determined, can be introduced into the measurement cell  4 . The reference cell  5  is filled with a reference gas  7 . A modulator  8 , arranged between the beam splitter  3  and the cells  4  and  5 , in the shape of a rotating chopper or paddle wheel is used to let through and block infrared radiation  2  alternately through the measurement cell  4  and reference cell  5 , with the result that radiation passes alternately through both cells  4  and  5 . The radiation, which emerges alternately from the measurement cell and the reference cell  5 , is guided, using a radiation collector  9 , into a detector array  10 , which in the exemplary embodiment shown consists of a first one-layer receiver  11  and a downstream, further one-layer receiver  12 . Each of the two one-layer receivers  11 ,  12  has an active detector chamber  13  or  14 , which receives the infrared radiation  2  exiting the cells  4 ,  5 , and a passive compensation chamber  15  or  16 , which is arranged outside the radiation  2 , with the detector chambers and compensation chambers being connected to one another through a connection line  17  or  18  having a pressure-sensitive or flow-sensitive sensor  19  or  20  arranged therein. Sensors  19  and  20  generate measurement signals Sa and Sb, on the basis of which an evaluation unit  21  ascertains, as measurement result M, the concentration of the measurement gas component in the gas mixture  6 . The measurement signal Sb of the second one-layer receiver  12  includes, in addition to the main signal component generated by the radiation absorption in the active detector chamber  14  of the second one-layer receiver  12 , also a smaller signal component from the first one-layer receiver  11 . The measurement signals Sa and Sb of the two one-layer receivers  11  and  12  therefore form a two-dimensional result matrix. If the detector array  10  consists of n one-layer receivers which are arranged one after the other, n measurement signals Sa, Sb, . . . Sn are obtained, which form an n-dimensional result matrix. If the first one-layer receiver  11  contains the measurement gas component and the downstream n−1 one-layer receivers are filled with different cross gases, the concentration of the measurement gas component can also be ascertained in the presence of these cross gases in different concentrations. 
         [0027]    The evaluation unit  21  contains a calibration matrix  22 , which corresponds to the abovementioned result matrix and which is shown in detail in  FIG. 2  and is used to explain further the mode of operation of the detector array  10 . 
         [0028]    Different cross gas concentrations with different concentrations of the measurement gas component are fed successively into the measurement cell  4 . For each available concentration, one value pair  23  of the signals Sa and Sb is measured, as is shown by way of example in the table which follows. Based on the recorded value pairs of the signals Sa and Sb and the associated known concentration values of the measurement gas component, the calibration matrix  22  is compiled, with intermediate values being formed by interpolation of the recorded or known support values. The calibration matrix  22  can also be stored in the evaluation unit  21  as a mathematical function describing it and the associated function parameters. A reduced measurement series according to the table can suffice for compiling the calibration matrix  22 . 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
               
               
                 Measurement gas 
                 Cross gas component 
                   
                   
               
               
                 component in ppm 
                 in ppm 
                 Sa 
                 Sb 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   0 (zero gas) 
                 0 
                 . . . 
                 . . . 
               
               
                   0 
                 5000 
                 . . . 
                 . . . 
               
               
                   0 
                 10000 
                 . . . 
                 . . . 
               
               
                   0 
                 15000 
                 . . . 
                 . . . 
               
               
                  500 
                 0 
                 . . . 
                 . . . 
               
               
                  500 
                 5000 
                 . . . 
                 . . . 
               
               
                  500 
                 10000 
                 . . . 
                 . . . 
               
               
                  500 
                 15000 
                 . . . 
                 . . . 
               
               
                 1000 (span gas) 
                 00 
                 . . . 
                 . . . 
               
               
                 1000 
                 5000 
                 . . . 
                 . . . 
               
               
                 1000 
                 10000 
                 . . . 
                 . . . 
               
               
                 1000 
                 15000 
                 . . . 
                 . . . 
               
               
                   
               
             
          
         
       
     
         [0029]    For real measurement situations, generally the cross gases and the fluctuation ranges of their concentrations that can be expected are known (for example, minimum 5000 ppm to maximum 15000 ppm), with the result that a corridor  24  can be defined in the calibration matrix  22 , within which the value pairs  23 , which are dependent on the concentrations of the measurement gas component and of the known cross gases, will normally fall. For variable concentrations of the measurement gas component, the value pairs  23  move in the direction designated  25  and, for the variable concentrations of the cross gases that can be expected, they move in the direction designated  26 . Therefore, if for successive measurements the value pair  23  moves in a direction that also has, in addition to a component in the direction  25 , a component in the direction  26 , the cross gas influence on the measurement result can be compensated for by ascertaining the direction component  26  and computationally moving the value pair  23  back by this component  26 . With the value pair that is corrected in this manner, the calibration matrix  22  gives the correct value of the concentration of the measurement gas component. The movement directions  25  and  26  can, however, be superimposed with additional movement directions which result from fluctuations of further measurement-specific and/or apparatus-specific parameters, such as the output of the infrared emitter  1  or dirt on the measurement cell  4 . This makes it difficult to distinguish between cross gas influences and other error influences and to correct the measurement result accordingly. 
         [0030]    In order to separate cross gas influences from other error influences such as dirt-, ageing- or temperature-related changes at the infrared radiation source  1  or detector array  10 , a fixed phase imbalance in the switching of the radiation  2  between the measurement cell  4  and the reference cell  5  is initially produced. 
         [0031]    As shown in  FIG. 3 , for this purpose, for example, the rotational axis  27  of the chopper or paddle wheel  8  can be offset with respect to the measurement cell  4  and the reference cell  5  in the direction of the arrow  28 . In accordance with the illustration in  FIG. 4 , the power density distribution  29  and  30  of the radiation  2  that is introduced into the cells  4  and  5  using the beam splitter  3  is symmetrical with respect to the axes  31  and  32  of the two cells  4  and  5 . The periodic change between allowing the radiation  2  to pass through the measurement cell  4  and interrupting it occurs using a small phase shift of for example 1° in phase opposition to the change between allowing the radiation  2  to pass through the reference cell  5  and interrupting it, with this small phase shift constituting the phase imbalance in the switching of the radiation  2  between the measurement cell  4  and the reference cell  5 . Finally,  FIG. 3  shows a light barrier  33  for detecting the current position of the chopper or paddle wheel  8 . 
         [0032]    As shown in  FIG. 5 , the phase imbalance in the switching of the radiation  2  between the cells  4  and  5  can, alternatively to the offset of the rotational axis  27  of the chopper or paddle wheel  8  (shown in  FIG. 3 ), be produced by introducing the radiation  2  into the measurement cell  4  and reference cell  5  by the beam splitter  3  asymmetrically with respect to the axes  31 ,  32  of the two cells  4  and  5 . Another possibility for producing the phase imbalance is by changing the distance between the two cells  4  and  5 . 
         [0033]    Due to the phase imbalance, the measurement signals Sa and Sb contain, in addition to amplitude information, phase information. While the measurement gas component and cross gases in the measurement cell  4  influence both the amplitude and the phase of the respective measurement signal Sa or Sb, intensity changes of the infrared radiation  2 , which affect the beam paths in both cells  4  and  5  in equal measure, affect only the amplitude of the respective measurement signal Sa or Sb. Such changes in intensity of the infrared radiation  2  which affect the beam paths in both cells  4  and  5  in equal measure can result in particular from dirt-, ageing- or temperature-related changes at the infrared radiation source  1  or detector array  10 . By separating the amplitude information and phase information of the measurement signals Sa and Sb, it is thus possible to distinguish between influences on the measurement result M owing to measurement and cross gases, on the one hand, and to changes at the infrared radiation source  1  and detector array  10 , on the other hand, and the measurement result M can be corrected accordingly. 
         [0034]    In order to separate the amplitude information and phase information, for example, each of the two measurement signals Sa and Sb can each be detected in the evaluation unit  21  using a double lock-in amplifier phase-sensitively with respect to the modulation of the radiation  2 , where a measurement signal vector with an in-phase component and a quadrature component is produced. This will be explained below for a measurement signal S as representative, which in each case represents one of the measurement signals Sa and Sb. 
         [0035]      FIG. 6  shows an example of the double lock-in amplifier  34 , which receives the measurement signal S as an input signal and a reference signal R from the modulator  8 , in this case, for example, the light barrier shown in  FIG. 3 . The lock-in amplifier  34  includes, if appropriate, a bandpass filter  35  and an amplifier  36  for pre-filtering and amplifying the measurement signal S. The bandpass-filtered and amplified measurement signal S is multiplied in a phase-sensitive detector  37  by the reference signal R and in this way demodulated in a phase-sensitive manner. To this end, the reference signal R can pass through a phase shifter  38  beforehand to make possible phase matching between the reference signal R and the measurement signal S. Subsequently, the demodulated measurement signal is integrated in a low pass filter  39  to obtain the in-phase component S x =S·cosφ. In order to obtain the quadrature component S y =S·sinφ, the bandpass-filtered and amplified measurement signal S is multiplied in another phase-sensitive detector  40  by the reference signal R, which has been phase-shifted beforehand in another phase shifter  41  by 90°, and subsequently integrated in a further low pass filter  42 . 
         [0036]      FIG. 7  shows, in the bottom part, various measurement signal vectors S 1 , S 2 , S 3 , S 4  and S 5  in a Cartesian coordinate system. The measurement signal vectors S 1 , S 2 , S 3 , S 4  and S 5  were ascertained in a calibration of the gas analyzer for various concentrations K 1 , K 2 , K 3 , K 4  and K 5  of the measurement gas component in the presence of known cross gas concentrations. The measurement signal vector S 1  was ascertained with zero gas and the measurement signal vector S 5  with span gas. The measurement signal vectors S 1 , S 2 , S 3 , S 4  and S 5  differ from one another in terms of amplitude and phase angle, where the vector component in the x-direction of the coordinate system corresponds to the in-phase component and the vector component in the y-direction corresponds to the quadrature component of the respective measurement signal vector. Thus, the measurement signal vector S 4  has the in-phase component S 4x =S 4 ·cosφ 4  and the quadrature component S 4y =S 4 ·sinφ 4 . The phase angle φ 4  results from the angle distance, viewed in the rotation direction of the chopper  8 , between the light barrier  33  supplying the reference signal R and the cells  4 ,  5 , from the phase shift φ by the phase shifter  38  and signal propagation times in the double lock-in amplifier  34  and from the measurement gas- and cross gas-dependent phase information produced by the phase imbalance in the switching of the radiation  2  between the measurement cell  4  and the reference cell  5  in conjunction with the radiation absorption in the measurement cell  4 . The measurement signal vectors S 1 , S 2 , S 3 , S 4  and S 5  define a characteristic curve  43  which can be stored as a table, where intermediate values of the characteristic curve  43  can be formed by interpolation of the recorded measurement signal vectors S 1 , S 2 , S 3 , S 4  and S 5 . The characteristic curve  43  can also be stored in the evaluation unit  21  as a mathematical function f(S x , S y ) describing it. 
         [0037]    The top part of  FIG. 7  shows the dependence of the concentration K of the measurement gas component on the amplitude (length) of the measurement signal vectors S. When measuring an unknown concentration K of the measurement gas component, with unchanged cross gas concentrations and on the proviso that no changes have occurred at the gas analyzer since its calibration, a measurement gas vector S is obtained, the head of which is located on the characteristic curve  43 . The current concentration of the measurement gas component is then determined in the evaluation unit  21  from the length of the measurement signal vector S. 
         [0038]    In the exemplary illustrated embodiment, one value of the in-phase component S x  is assigned bijectively (one-to-one correspondence) to each point on the characteristic curve  43 . As a result, it is also possible to use, rather than the length of the measurement signal vector S, its in-phase component S x  to determine the current concentration of the measurement gas component. In comparison, in the exemplary embodiment shown, it is not possible to use the quadrature component S y  because different points on the characteristic curve  43  within a partial region of the characteristic curve  43  have the same quadrature component. By setting the angle distance, viewed in the rotation direction of the chopper  8 , between the light barrier  33  and the cells  4 ,  5  or the phase shift φ by the phase shifter  38 , however, the characteristic curve  43  can be rotated in the direction of the arrow  44  about the origin 0 of the coordinate system until each point on the characteristic curve  43  has bijectively assigned to it one value of the quadrature component S y . Then the quadrature component S y  can also be used to determine the current concentration of the measurement gas component. 
         [0039]    If, owing to ageing-, dirt- or temperature-related changes at the infrared radiation source  1  or detector array  10 , the intensity of the generated or detected infrared radiation  2  changes with respect to the calibration state of the gas analyzer, this results during the measurement in a measurement signal vector S F , the head of which is located outside the characteristic curve  43 . As already explained, however, because of these intensity changes of the infrared radiation  2 , which affect the beam paths in the two cells  4  and  5  in equal measure, only the amplitude and not the phase of the measurement signal vector S F  is influenced. The measurement signal vector S F  can therefore be corrected in a simple manner, by being lengthened or shortened up to the characteristic curve  43  while keeping its phase angle φ F . From the intersection point  45  of the measurement signal vector s F  or its extension with the characteristic curve  43 , it is then possible, as already explained above, to ascertain the unknown concentration of the measurement gas component. The length of the uncorrected measurement signal vector S F  with respect to the length of the measurement signal vector S F , which has been corrected up to the point  45  on the characteristic curve  43 , is a measure of the quality of the measurement signal S F  and can be output by the evaluation unit  21  together with the measurement result M. 
         [0040]    In measurement practice, however, not only the concentration of the measurement gas component in the measurement cell  4  but also that of the cross gases is variable, with the result that, on account of the previously explained separation of amplitude information and phase information of the measurement signal, a distinction is made only between the influence of the measurement and cross gases on the measurement result M, on the one hand, and the influence of changes at the infrared radiation source  1  and detector array  10  on the measurement result M, on the other hand. The distinction between the influence of the measurement gas on the measurement result M and the influence of the cross gases on the measurement result M is made by generating the two (or more) measurement signals Sa and Sb, which are evaluated using the calibration matrix  22  after correction in a correction unit  46  of the evaluation unit  21  according to the method described in connection with  FIGS. 6 and 7 , as was explained in connection with  FIGS. 1 and 2 . 
         [0041]    As already mentioned, the characteristic curve  43  can be stored in the correction unit  46  of the evaluation unit  21  as a table or a mathematical function f(S x , S y ). In order to simplify the function f(S x , S y ) and to reduce the computational complexity for correcting the measurement signal vector S F , it is possible, as shown in  FIG. 8 , by way of setting the angle distance, viewed in the rotation direction of the chopper  8 , between the light barrier  33  and the cells  4 ,  5  or the phase shift φ by the phase shifter  38 , for the characteristic curve  43  to be rotated in the direction of the arrow  47  about the origin 0 of the coordinate system until the measurement signal vector S 1 , obtained for the zero gas, or alternatively the measurement signal vector S 5  for the span gas, coincides with one of the axes of the coordinate system, in this case, for example, the y-axis. 
         [0042]      FIG. 9  shows the special case where the characteristic curve  43  is exactly or approximately linear. By setting the angle distance, viewed in the rotation direction of the chopper  8 , between the light barrier  33  and the cells  4 ,  5  or the phase shift φ by the phase shifter  38 , it is possible here, too, for the characteristic curve  43  to be rotated in the direction of the arrow  48  about the origin 0 of the coordinate system until the characteristic curve  43  is parallel to one of the axes of the coordinate system, in this case for example the x-axis. For each point on the characteristic curve  43 , the quadrature component is then S 1y . In the case of a measurement signal vector S F  with the in-phase component S Fx  and the quadrature component S Fy , the in-phase component S Fx  can be corrected in a simple manner by S Fx corr= S 1y ·(S Fx /S Fy ). 
         [0043]      FIG. 10  is a flow chart of a method for determining the concentration of a measurement gas component in a gas mixture using a non-dispersive infrared (NDIR) two beam gas analyzer, where infrared radiation is guided alternately, by modulation, through a measurement cell receiving the gas mixture and through a reference cell containing a reference gas and subsequently detected with a measurement signal being generated, and a concentration of the measurement gas component is determined by evaluating the measurement signal. The method comprises producing a phase imbalance while switching the infrared radiation between the measurement cell and the reference cell, as indicated in step  1010 . 
         [0044]    The measurement signal is detected phase-sensitively with respect to the modulation of the infrared radiation to obtain a measurement signal vector with amplitude information and phase information, as indicated in step  1020 . 
         [0045]    Measurement signal vectors of different amplitude and phase are ascertained for different known concentrations of the measurement gas component during a calibration of a gas analyzer, as indicated in step  1030 . Here, the measurement signal vectors define a characteristic curve. 
         [0046]    An unknown concentration of the measurement gas component is ascertained in the measurement of an unknown concentration of the measurement gas component from an intersection point of a measurement signal vector, obtained in the measurement, or its extension with the characteristic curve, as indicated in step  1040 . 
         [0047]    Thus, while there have shown and described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.