The principles of operation and the applications of combustibles analyzers will be discussed in this section. These instruments are designed to detect the presence and measure the concentration of combustible gases and vapors on a continuous basis. The methods of detecting the presence of combustible gases and vapors can utilize the phenomena of catalytic combustion, electrical resistance, luminosity, thermal conductivity, infrared (IR) absorption, or gas ionization. Of the above methods, the most widely used is catalytic combustion, where a change in the resistance or temperature of the sensing filament is caused by the catalytic combustion of the flammable gases, and this change is measured to detect the concentration of combustibles. The second most widely used and a much newer technique is infrared. As will be seen, both techniques have both advantages and disadvantages. Most suppliers offer a variety of designs, so that the user might select the best choice for his application. The selection process usually considers cost, robustness, selectivity, poison resistance, speed of response,The most commonly used combustibles detectors are the catalytic filament units, which use a self-heated platinum wire as the catalytic surface to initiate combustion. A special portable variation of this unit is one that can be pinpointed at leaks by pointing a sample probe at the seals on manholes, tanks, or other containers that are likely to leak. In some instruments, two filaments are provided: a catalytic combustion filament for low ranges, and a thermal conductivity filament for higher ranges. When the goal of the measurement is the detection of total hydrocarbons, or if the presence of lead, silicone, chlorinated compounds, or sulfur compounds could otherwise poison the catalytic filament, infrared and flame or photoionization analyzers should be considered.are extremely wide. In detecting the presence of such vapors or gases, their LELs are usually of most interest, and, in order to maintain safety, flammable gas and vapor concentrations must be kept below those limits. Since air is usually the diluent and is almost always present, all concentrations above LEL are usually dangerous.