Abstract: A mixture of H.sub.2 O, CO.sub.2 and CO are measured in the same sample chamber by infrared radiation passing through the chamber and absorption filters passing a narrow band in their respective absorption curves to an infrared detector. Background radiation is zeroed by grounding the detector response when an opaque segment is disposed in the path. A reference filter detects drift which is automatically corrected.

Abstract: A beam of infrared radiation encompassing the absorption band of the hydrocarbon alkanes is passed through an unconfined sample of test gas after which the beam is chopped before it impinges upon an interference filter. The filter restricts radiation passing therethrough to the spectral interval corresponding to the absorption band of the hydrocarbon alkanes. The filtered radiation impinges on an infrared detector which produces a signal that on being amplified, detected and demodulated is an analog signal of magnitude functionally related to the product of concentration and carbon number of the n-alkanes (flammability index) of a mixture of hydrocarbons irrespective of their individual carbon numbers. The apparatus is unitarily arranged in an elongated rod-like housing adapted for convenient hand-held use.