This is dependent upon having spares readily available on-site and having personnel available to perform the repair. The historical method of detecting combustible gases involved sensors based on pellistor beads. Primary of these are catalyst poisoning and flame arrestor plugging, either of which prevent the sensing of gas. In the above example, it is easy to see how a covert pellistor failure could remain undetected for much longer than 10 hours as the only way to detect a failure is to periodically check with gas. The absence of covert failure modes is the intrinsic characteristic of infrared gas detectors that provides a significant advantage over catalytic beads.
All failure modes are overt and instantly signalled so that repair can be initiated immediately.
All infrared gas detectors use variations on the basic measurement scheme outlined in the illustration. In this scheme, there is an infrared source that illuminates a volume of gas that has diffused into the measurement chamber. The gas absorbs certain of the infrared wavelengths as the light passes through it whilst others pass through completely unattenuated. The amount of absorption is related to the concentration of the gas. The amount of absorption is measured by a set of optical detectors and subsequent electronics.