Detectors 159
Detectors 159 : Particle Detectors: (4.17) Infrared Open-Path Detector: Rain, fog etc. in the measurement path can also reduce the strength of the received signal, so it is usual to make a simultaneous measurement at one or more reference wavelengths. The quantity of gas intercepted by the beam is then inferred from the ratio of the signal losses at the measurement and reference wavelengths. The calculation is typically carried out by a microprocessor which also carries out various checks to validate the measurement and prevent false alarms. The measured quantity is the sum of all the gas along the path of the beam, sometimes termed the path-integral concentration of the gas. Thus the measurement has a natural bias (desirable in many applications) towards the total size of an unintentional gas release, rather than the concentration of the gas that has reached any particular point. Whereas the natural units of measurement for an Infrared point sensor are parts-per-million (ppm) or the percentage of the lower flammable limit (%LFL), the natural units of measurement for an open path detector are ppm. metres (ppmm) or LFL. metres (LFLm). For instance, the fire and gas safety system on an offshore platform in the North Sea typically has detectors set to a full-scale reading of 5LFLm, with low and high alarms triggered at 1LFLm and 3LFLm respectively
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