Thermal Conductivity Gas Detectors
All gases possess varying degrees of ability to conduct heat. In order to have a means of comparing the gases’ abilities, a coefficient has been assigned to each gas according to its capacity to retain heat. These coefficients and a controlled environment provide a way to determine the amount of a particular type of gas present in a gas sample. The unit of measure is percentage concentration of gas.
A gas analyzer consists of two parts: a sensor assembly and a control unit. The sensor assembly contains two gas chambers, one for the measured gas and the other for the reference gas. The entire sensing assembly is temperature-controlled to ensure a constant temperature, and there is a thermistor in each of the chambers for sensing the temperature of the respective gases. By knowing the properties of the sensor assembly, the conductivity coefficients of each gas, and the temperature reading of each gas chamber, the concentration of the measured gas can be calculated and displayed on the control unit.
In most applications, the sample gas must be properly conditioned before entering the sensing unit’s gas chamber. This conditioning usually consists of reducing and/or regulating the pressure of the sample, filtering the sample of various contaminants, and maintaining the temperature within a standard range.
A sample panel, used for most applications, contains all the necessary filters, pumps, pressure and flow regulators, and valves for proper conditioning and ease of calibration and normal operation. The panel also provides the necessary piping for transporting the sample from the process to the analyzer, as well as for disposing of the analyzed sample by venting to atmosphere or returning it to the process.