Infrared gas analyzer with automatic zero adjustment

Automatic zeroing apparatus for zeroing an infrared gas analyzer automatically upon the occurrence of preselected conditions to indicate zero in the absence of absorption of infrared radiation by a gas mixture being analyzed, the gas analyzer having a sample cell for containing a gas mixture to be analyzed, source means for directing infrared radiation through the sample cell over a range of wavelengths, detection means responsive to infrared radiation at at least one respective preselected wavelength to produce at least one respective detection signal systematically related to the infrared radiation at the respective preselected wavelength passing through the sample cell from the source means which respective wavelength corresponds to the absorption wavelength characteristic of a respective predetermined gas, and signal processing means responsive to said at least one detection signal for indicating the relative concentration of the respective predetermined gas in the mixture being analyzed, said signal processing means including reference means for producing a reference signal, and output means responsive to said at least one detection signal and said reference signal for outputting an output signal systematically related to the difference between said at least one detection signal and said reference signal, said automatic zeroing apparatus comprising:

• means for filling the sample cell with gas substantially nonabsorbent of infrared radiation at the respective characteristic wavelength;
• comparator means responsive to said output signal for producing an error signal when said output signal differs from zero;
• and
• gain control means responsive to said error signal for automatically controlling the signal level of said at least one detection signal to reduce said output signal substantially to zero with said substantially nonabsorbent gas filling the sample cell, said gain control means having a zero mode in which the analyzer is zeroed and a normal mode in which analysis is made and comprising mode control means for placing said gain control means in said zero mode, means for operating said mode control means periodically to place said gain control means in said zero mode periodically, means associated with said zero mode for operating said means for filling to fill said sample cell with said substantially nonabsorbent gas, means in said zero mode responsive to said error signal for adjusting the signal level of said detection signal by a factor to balance said reference signal to reduce said output signal substantially to zero, means for storing a representation of said factor, and means in the normal mode for adjusting the signal level by said factor according to said representation stored from the last preceding zero mode, such that after the zero mode said analyzer remains calibrated in the normal mode in the presence of an absorbing gas being analyzed.