Twin beam infrared absorption analyzer

Claim 1

In a selective twin beam infrared analyzer for monitoring gas for a component of maximum limited concentration, a pair of electric radiators connected to a source of stabilized electric power for producing twin beams, the analyzer having a zero drift tendency, and cells for containing reference and sample gas respectively in said beams, means for modulating the beams, a detector for translating the modulated beams passing through the cells to electrical voltage, an amplifier receiving the voltage as input, a phase-independent rectifier for rectifying the output of the amplifier, and an electrical measuring instrument connected to the rectifier for indicating the output thereof as a measure of concentration of the component, the improvement comprising the sample cell being of a length such that absorption is between 40% and 60% at the maximum limit of component concentration, said amplifier having a variable gain control and rectifier having a sensitivity threshold voltage greater than the output voltage of the amplifier due to zero drift thereof and prior to rectification, and at zero concentration of the component, the measuring and reference beams being so matched with respect to amplitude and phase that the output current of the amplifier is zero; a fixed resistor, a switch for selectively shunting the resistor across the radiator for the measuring beam to reduce the intensity of same before passing through the sample cell, the the resistance of the resistor being of the value to reduce the intensity of the measuring beam by the amount corresponding to the attenuation of the matched measuring beam passing through the sampLe cell when the latter contains sample gas at the maximum limit of concentration, for checking the functioning of the analyzer; means for varying the gain of the amplifier; a temperature-dependent resistor selectively connected across the measuring instrument; and a switch ganged with the first mentioned switch for shunting the temperature- dependent resistor across the measuring instrument when the measuring beam is reduced in intensity by closure of the first mentioned switch, for compensating the effect of the ambient temperature on the percentage absorption of the measuring beam in the sample cell.