Non-dispersive infrared gas analyzer

摘要

Pulse modulated infrared energy is directed through a sample cell containing a gas mixture. The gas mixture may include a selected one of a plurality of gases. Each gas exhibits a characteristic absorption in the infrared wavelengths. A detector responsive to infrared energy at a preselected wavelength develops an AC signal having an amplitude proportional to the infrared energy passing through the sample cell at the preselected wavelength and having a frequency corresponding to the preselected pulse modulation frequency. The preselected wavelength is common to the absorption characteristic of each of said gases. The AC signal is processed to develop a DC signal having an amplitude determined from the AC signal. The DC signal is applied to an amplifier having a variable gain feedback which provides a plurality of gain steps. Each gain step is associated with a different one of the gases a single surrogate gas may be used to calibrate each gain step. The DC signal is amplified by the appropriate gain step to develop a second DC signal proportional to the concentration of the gas and the mixture contained in the sample cell. The amplifier may further include an analog to digital convertor which stores a digital equivalent of a zero offset output voltage of the amplifier when its input is grounded. The digital signal is reconverted to an analog signal and a current proportional to such signal is normally applied to the input of the amplifier to maintain zero stability.