NON-DISPERSIVE INFRARED FLUID ANALYZER WITH COMPENSATION FOR ABSORPTIVE AND MECHANICAL EFFECTS OF AMBIENT CONDITIONS

Alternately chopped analytic and reference beams of infrared energy are passed through sample and reference fluids, respectively, and then sequentially through a first detector, a filter, and a second detector. Each detector is of the variable capacitor type responsive to pressure pulses, producing an output signal that is a function of the difference in the energies absorbed in the two beam paths. The first detector responds to absorption by the sample fluid due both to the component of interest and to an ambient condition (icluding the presence in the sample of an interferent having an absorption band overlapping that of the component in a spectral region where both are weakly absorbent). The second detector responds to absorption by the sample fluid due to the ambient condition alone. The filter limits the absorptive response of the second detector to an amount that will compensate for the similar response of the first detector to said ambient condition. The net output signal of the two detectors is a function of the difference of the individual outputs and responsive to component concentration. The two detectors are physically identical, and the signal output of the second detector may be adjusted to eliminate mechanical effects, such as vibration, due to other ambient conditions, without affecting the absorptive sensitivity of the instrument. A modification of more limited application, using a single beam, is also disclosed.