Abstract: The photometer comprises a light source, a reference cuvette containing a reference gas, a measurement cuvette containing a gas component whose concentration is to be measured accompanied by a carrier gas, a light modulator for effecting antiphase modulation of the infrared light passing through the two cuvettes, and a detector arrangement comprised of two gas-filled absorption chambers arranged one behind the other as well as a transducer for converting pressure changes induced in the absorption chambers into a measurement signal. The carrier gas consists of a plurality of component gases whose relative proportions fluctuate. The gas in the absorption chambers includes a gas component identical to the gas whose concentration is to be measured in the measurement cuvette, accompanied by an inert gas.

Abstract: The concentration of gas is determined by monitoring the coloration of a reagent test paper, or the like, whose color changes progressively or otherwise in response to continued exposure to such gas. The concentration is determined by determining the length of time which the reagent test paper must be exposed to the gas for the coloration of the test paper to reach a preselected threshold value.