Detectors 160
Detectors 160 : Particle Detectors: (4.17) Infrared Open-Path Detector: (1) Advantages and Disadvantages Versus Fixed-Point Detectors: An open path detector usually costs more than a single point detector, so there is little incentive for applications that play to a point detector's strengths: where the point detector can be placed at the known location of the highest gas concentration, and a relatively slow response is acceptable. The open path detector excels in outdoor situations where, even if the likely source of the gas release is known, the evolution of the developing cloud or plume is unpredictable. Gas will almost certainly enter an extended linear beam before finding its way to any single chosen point. Also, point detectors in exposed outdoor locations require weather shields to be fitted, increasing the response time significantly. Open path detectors can also show a cost advantage in any application where a row of point detectors would be required to achieve the same coverage, for instance monitoring along a pipeline, or around the perimeter of a plant. Not only will one detector replace several, but the costs of installation, maintenance, cabling etc. are likely to be lower. An important consideration for both types is a realistic assessment of their availability in service, distinguishing carefully between revealed and unrevealed downtime. The latter, where a detector appears to be working but is insensitive to gas, is especially serious. For instance, point detectors of the catalytic type are prone to poisoning by silicones and H2S, or by clogging of the gauze or sinter by water or ice. Point detectors of the infrared type are immune from the former but not the latter mechanism. Open path detectors suffer downtime from anything that blocks the path of the beam, such as people, vehicles or thick fog. However, they approach the ideal of eliminating unrevealed downtime because the loss of signal strength is easily made to generate a 'beam block' signal, distinct from zero gas or a hardware fault. Infrared detectors of both types, unlike passive sensors, invariably incorporate a microprocessor capable of self-checking the circuitry, so unrevealed downtime due to hardware failures is largely eliminated. The possibility of (revealed) downtime of an open path detector due to fog can be minimised by limiting the beam path to moderate lengths. In fog-prone areas, such as the North Sea, paths are often limited to 15 to 20 metres for this reason, even though the detectors are capable of much greater distances in clear air
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