Thermopile infrared gas detectors have many applications, from providing early warning systems for trace levels of atmospheric gases to analyzing several gases in an anesthetized patient in the operating room. All elements and compounds absorb light or energy at some point along the electromagnetic spectrum. To measure gasses, we are taking advantage of the unique and specific wavelengths where molecular absorption and resonance occur.

The generic NDIR (nondispursive infrared radiation) gas apparatus is a sample chamber, with an emitter on one side, and an IR detector on the other side, where your sample of gas is directed between them. IR energy from the source will radiate to the IR detector and produce a given response or voltage output. At a specific wavelength, a given gas will absorb the infrared energy as it travels from the source to the detector – and thereby reduce the detector response.

According to the Beer-Lambert law, the amount of energy absorbed by the gas is proportional to the concentration of the gas molecules in the IR path. CO2 is a great example of a gas measured by NDIR absorption at ~4.26 um in wavelength, but CO₂ is also a great example for flame detection. When hydrocarbon materials like wood or fossil fuels are burned, they produce heat and carbon dioxide. When the CO₂ molecules are excited by the heat, they resonate or emit energy at that same ~4.26 um frequency. So, a flame detection system can be filtered for that wavelength – it won’t see any ambient temperature CO₂ because it is not excited, but the hot/resonating CO₂ produced by a flame will emit energy trigger a response.

Heat or visible wavelength bands can also be used for flame properties. The resulting flame sensor system may be based on sensing single or combined CO₂, flash, and heat characteristics.

So these same gas absorption and resonance principles can be used for a number applications, whether it’s medical, refrigerants, emissions, environmental or even industrial and hazardous gasses.