Gas Analyzers

instruments for determining the qualitative and quantitative composition of gas mixtures.

Gas analyzers may be hand operated or automatic. The most common analyzers of the former type are based on the absorption principle: the components of a gas mixture are absorbed one after another by different reagents. Automatic gas analyzers continuously measure some physical or physicochemical property of a gas mixture as a whole or of its separate components.

Automatic gas analyzers may be divided into three groups according to their principle of operation. The first group includes instruments using physical methods of analysis, including auxiliary chemical reactions. These gas analyzers, called volumetric-manometric or chemical gas analyzers, measure changes in the volume or pressure of a gas mixture resulting from the chemical reactions of its separate components. The second group includes instruments using physical methods of analysis, including auxiliary physicochemical processes (such as thermochemical, electrochemical, photocolorimetric, and chromatographic processes). Thermochemical instruments are based on the thermal effect of the reaction of catalytic oxidation (combustion) of a gas; they are used chiefly to detect concentrations of inflammable gases (for example, dangerous concentrations of carbon monoxide in the air). Electrochemical instruments allow the determination of the concentration of a gas in a mixture according to the electroconductivity of a solution absorbing the gas in question. Photocolorimetric instruments are based on the change in the color of certain substances when they react with a component of a gas mixture under analysis; they are used mainly to measure microconcentrations of toxic impurities in gas mixtures—hydrogen sulfide and nitrogen oxides, for example. Chromatographic instruments are most widely used to analyze mixtures of gaseous hydrocarbons.

The third group of gas analyzers consists of instruments based on purely physical methods of analysis (thermoconductometric, densimetric, magnetic, optical, and ultraviolet). Thermoconductometric instruments are based on the change in the thermoconductivity of gases; they may be employed to analyze two-component mixtures (or multicomponent mixtures if the concentration of only one component changes). Densimetric instruments are based on the change in the density of a gas mixture; they are used chiefly to determine the quantity of carbon dioxide (whose density is 1.5 times that of the atmosphere) in a mixture. Magnetic gas analyzers are used mainly to measure the concentration of oxygen in a mixture: oxygen has a great magnetic susceptibility. Optical gas analyzers are based on the change in the optical density, the absorption spectra, or the emission spectra of a gas mixture. Ultraviolet gas analyzers are used to determine the quantity of halogens, mercury vapors, and certain organic compounds in gas mixtures.