Combustible gases absorb infrared radiation at certain characteristic wavelengths.
A typical non-dispersive infrared detector passes a pulsed source of infrared energy through the sample, and measures the energy received by two detectors.
One “active” detector responds to wavelengths in the same band as the combustible gas, and the other detector responds to wavelengths in a reference band to compensate for changes in the source.
The flame temperature detector measures the heat given off by a flame as it burns combustible gas that diffuses into the flame from the sample.
The small, well-regulated flame heats the tip of a temperature sensor suspended directly above it. The signal produced by the sensor when no flammable vapors are present drives the LFL indicator up to 0% LFL. This failsafe technique is known as a "live" zero because a weakening or loss of flame caused by lack of fuel will generate a downscale malfunction alarm.