Gas analysis by measurement of the thermal conductivity

Types of gas analysis. Methods for the analysis of gases may be divided into two general classes, physical and chemical. In chemical methods, a constituent of the gas mixture undergoes chemical reaction and the amount of the constituent is determined by change in volume or by supplementary chemical methods such as titration. The physical methods of analysis differ in that no chemical change takes place in the gas. Instead, a determination is made of some physical property which is related to the composition of the gas mixture. Some of the physical properties which have been used for purposes of analysis are density, viscosity, refractive index, dielectric constant, and thermal conductivity. Physical methods of analysis have come into very wide use within the last decade. They are reliable and capable of a very considerable accuracy. Some of the methods are applicable to continuous recording devices, and it is in this field that their greatest commercial usefulness has been found. Many commercial operators who have been satisfied in the past with intermittent and perhaps infrequent chemical analyses have come to realize the value of continuous gas analysis records. In addition, the rapid development of chemical process industries during recent yea.rs has resulted in a real demand for equipment which will give accurate and continuous analyses of gas mixtures. The thermal conductivity method. Of the physical methods of gas analysis, that utilizing the measurement of thermal conductivity has found the widest application. The chief advantages of this method are: (a) the method is adapted to continuous recording and to the operation of relays, alarms or controls at definite predetermined gas concentrations. (b) The analysis of a gas is made within five minutes or less, so that a continuous record follows closely the actual gas composition at all times. This is especially advantageous in control or alarm processes. (c) Accuracy comparable to that of chemical methods can be obtained, the accuracy in any case depending on the precautions of analysis the operator is willing to take. (d) The equipment cost, although greater than the cost of equipment for intermittent chemical analysis, is well within reach of those requiring equipment for continuous recording and control. (e) Analyses are made with a very limited amount of attention on the part of the operator, and the equipment is adapted to commercial processes since highly skilled operators are not required. The chief limitations of the thermal conductivity method are: (a) An empirical calibration must be made on gases of known composition, since the dimensions of the measuring cell cannot be controlled with sufficient accuracy to permit calculations of the calibration. (b) The method as usually applied is applicable only to binary gas mixtures, although ternary or more complex gas mixtures may be analysed by special methods. (c) The method is applicable only to gas mixtures in which the thermal conductivity of the gas to be measured is appreciably different from the conductivity of the other constituents --Introduction, pages 1-3