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GSS - gas sensing solution application note

NON-DISPERSIVE INFRA-RED (NDIR) SENSORS by GSS

NDIR gas sensors work by detecting the amount of light that is absorbed by the target gas. Gases absorb light energy at different wavelengths, depending on the chemical bonds in the gas. The amount of energy absorbed by the target gas is proportional to the concentration, as described by the Beer-Lambert Law.

Electro-chemical sensors measure the current produced by a chemical reaction with the target gas. Gas diffusion is limited by a small hole, and the current is proportional to the gas concentration. They are similar to a battery and have a finite life, require a reactive gas and selective chemistry. They are the favoured option for low concentrations of toxic gasses. Thermal conductivity sensors use the thermal coefficient of the sample gas to infer a level of target gas, where a metal element is heated by constant power and the temperature change induced by the target gas is proportional to concentration, often arranged in a Wheatstone bridge configuration. They are frequently used for high concentrations of inert gases but are not selective. Pellistors are another type of sensor that uses a catalyst to burn a flammable gas, where increased heat is proportional to the target gas present. This type of sensor requires a flammable gas, is very cheap but high power and susceptible to poisoning of the catalyst, and not very selective.