Patent Publication Number: US-6710347-B1

Title: Device for measuring gas concentration

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to gas analyzers for determining the presence and concentrations of gas components in a sample. The invention is especially adapted for use in the area of environmental measurements, such as engine emissions, although it may have other applications. 
     Using gas cells for calibration of optical gas benches has been described in the prior art. U.S. Pat. No. 5,060,505 describes an infrared-based device to measure gas concentration in which gas cells enclosing an amount of the component gas to be detected are selectively positioned in the optical path. While this device is very effective in eliminating the necessity for utilizing gases for calibration, it requires mechanical means to place and remove the gas cell in the optical path. The required mechanical means cannot be effectively used for measuring gas emissions of moving vehicles since they are quite sensitive to vibration. 
     SUMMARY OF THE INVENTION 
     An apparatus for detecting at least one component gas in a sample includes a source for providing radiation along at least one optical path in a pre-selected spectral band. The spectral band has at least one absorption line of the component gas to be detected. The apparatus further includes at least one optical detector positioned in the at least one optical path for detecting radiation in the pre-selected spectral band and for producing at least one detection output. A sample chamber is positioned in the at least one optical path between the source and the at least one detector and adapted to contain a quantity of sample gas including the component gas to be detected. A gas cell enclosing an amount of component gas to be detected is permanently positioned in the at least one optical path in series with the sample chamber. A control includes an algorithm for determining a mathematical relationship between the at least one detector output and the concentration of a sample gas filling the sample chamber. 
     A method for detecting at least one component gas in a sample, according to an aspect of the invention, includes providing radiation along at least one optical path in a pre-selected spectral band. The spectral band has at least one absorption line of the component gas to be detected. The method further includes detecting radiation in the at least one optical path in the pre-selected spectral band. The method further includes positioning a sample chamber in the at least one optical path. The sample chamber is adapted to contain a quantity of sample gas including the component gas to be detected. The method further includes fixedly positioning at least one gas cell enclosing an amount of the component gas to be detected in the at least one optical path in series with the sample chamber. The method further includes determining a mathematical relationship between radiation detected in the at least one optical path and the concentration of a sample gas filling the sample chamber. 
     The major advantage of the present invention is that gas cell(s) can be permanently embedded in the optical paths of the device. Therefore, gas analyzers with no moving parts can be effectively implemented. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of an apparatus for detecting at least one component gas in a sample, according to the invention; 
     FIG. 2 is a flowchart of a calibration method, according to the invention; and 
     FIG. 3 is a flowchart of a method of detecting a component of a gas in a sample, according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the preferred embodiment, illustrated in FIG. 1, an apparatus  18  for detecting at least one component gas includes a gas-sampling cell  1 . A sample gas is transferred through gas-sampling cell  1  through an inlet pipe  2  and exhausted through an outlet pipe  3  of gas-sampling cell  1 . A radiation source  4  generates electromagnetic energy directed to the sample gas in the gas-sampling cell  1 . The energy from the radiation source is preferably in the range of infrared, near infrared, visible or ultraviolet wavelengths. The emitted energy is limited to specific bands by an optical filter  5 , such as an interference filter, which is well known in the art. These limited bands are chosen to cover certain absorption bands of at least one gas component present in the sample gas. The partially absorbed energy leaves the gas-sampling cell  1  through two optical channels  11  and  12 . The gas channel  11  directs the energy to an optical detector  8 , which measures the energy directly. Such detectors are well known in the art. The reference channel  12  directs the energy via a gas cell  6  to a second optical detector  7 . The gas cell includes at least one gas component present in the sample gas that is being measured by the device. The gas cell is permanently present in the second channel thereby avoiding the necessity for additional mechanical components to selectively place and remove the gas cell to and from the channel. 
     The apparatus is calibrated using a calibration procedure  20 , illustrated in FIG.  2 . While calibration procedure  20  can be carried out at any time, it may, advantageously, be carried out during manufacturing of apparatus  18 . Zero gas (nitrogen) is used at 22 to obtain and store the zero voltage Vzg of the gas channel  11  measured by the detector  8 , the zero reference voltage (gas cell out) Vzr and the span voltage Vr (gas cell in) in channel  12  as measured by the detector  7 . Multiple gas concentrations are used at 24 to obtain the span voltage Vi for each concentration Ci in the gas channel  11 . Modulation values Mi for each concentration Ci in the gas channel  11  are calculated at 26 using the expression: 
     
       
           Mi =( Vzg−Vi )/ Vz   
       
     
     A function f (e.g., polynomial of 4 th  order) is applied at 28. Function f relates the gas concentrations to the calculated modulation values according to: 
     
       
           Ci=f ( Mi ) 
       
     
     The reference span modulation obtained from the reference channel  12  is calculated and stored at 30 using: 
     
       
           Mr =( Vzr−Vr )/ Vzr   
       
     
     An “equivalent reference concentration” Cr may optionally be calculated and stored at 31 using: 
     
       
           Cr=f ( Mr ) 
       
     
     Once the apparatus is calibrated, the gas cell can be used in apparatus  18  instead of calibration gas stored in gas bottles using a component gas detection method  40 , illustrated in FIG.  3 . During the zeroing process  42 , the zero gas channel voltage Vzg′ is measured in channel  12  and the voltage of the reference channel Vr′ is measured in channel  11 . The zero voltage of the reference channel Vzr′ is calculated at 44 by relating the zero voltage obtained in the manufacturing phase using: 
       Vzr ′=( Vr′/Vr ) Vzr   
     The modulation of the reference channel corresponding to the gas cell is calculated at 46 using: 
     
       
           Mr ′=( Vzr′−Vr ′)/ Vzr′   
       
     
     The Calibration Coefficient K is calculated at 48 using: 
     
       
         
           K=Mr′/Mr 
         
       
     
     Each modulation reading of the gas sample in the gas channel is adjusted at 50 by the Calibration Coefficient K: 
     
       
           Mg′=K *( Vzg′−Vg ′)/ Vzg′   
       
     
     The corrected gas concentration is calculated at 52 by: 
     
       
           C=f ( Mg ′) 
       
     
     Where f is the function determined in the manufacturing phase. 
     In a different embodiment of the invention, a sampling cell with a single optical path can be implemented. In this case, both the sampling cell and the gas cell are serially placed in the optical path between the radiation source and the optical detector. During manufacturing, two zero-voltage values are read and stored, one with the gas filter present in the optical path and the other without the gas cell, thereby providing a calibration reference based upon the gas concentration enclosed in the cell. 
     The radiation source generates electromagnetic energy in specific energy bands. It can cover the whole range utilized for spectroscopy; i.e., visible light, near infrared, infrared or ultraviolet radiation. Usually, heated filaments are used for the infrared and visible ranges. Gas discharge lamps can be used for the visible and the ultraviolet bands. For all radiation sources, the presence of gas in the source can serve either as an additional passive gas cell or, in the case of gas discharge lamps, as an active radiation source in bands determined by the molecular structure of the enclosed gas. 
     Changes and modifications in the specifically described embodiments can be carried out without departing from the principles of the invention which is intended to be limited only by the scope of the appended claims, as interpreted according to the principles of patent law including the doctrine of equivalents.