摘要 |
<p>Pulse modulated infrared energy (12) is directed through a sample cell (14) 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 (16) 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 gasses. 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 (28) having a variable gain feedback (30) which provides a plurality of gain steps. Each gain step is associated with a different one of the gasses. 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 (14). The amplifier (28) may further include an analog to digital convertor (60) which stores a digital equivalent of a zero offset output voltage of the amplifier (28) 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 (28) to maintain zero stability.</p> |