TM 10-6665-297-13&P1-2GASDETECTORUNITS(continued)With the sensing element surrounded by a double flame arrestor, the detector is suitable for location in Class I,Divisions 1 and 2, Groups A, B, C, and D hazardous locations as defined by National Electrical Codes.NOTE: Standard detectorsshouldnot be used in oxygen-enriched atmospheres. Special detectorsshould be chosen for such atmospheres in consultation with the factory. Oxygen-deficient and inertatmospheres may be monitored by standard detectors if sampled gases are diluted by air tosupport an oxidation reaction on the catalytic surface of the sensing element.1-2.1DETECTORSENSINGDEVICE. (Figure 1-1) The sensing element of the combustible gas detector isa platinum-covered bead protected by a porous metal, cylindrical cap that freely passes gas and air whileexcluding airborne, nongaseous materials from the area surrounding the element. Individual elements for usein CD800/830 Series detectors normally sense the presence of combustible gases of any kind. Elements arealso available with special sensivities to particular gases such as methane, hydrogen, or ethylene oxide.The sensitivity of all catalytic materials may be inhibited by certain compounds, usually called “poisons.”Tetraethyl lead and silicone compounds are the most critical. J-W sensing elements utilize a unique designand are resistant to catalytic poisoning. But aerosol sprays, polishes, waxes, and lubricants containing siliconesshould not be used in the vicinity of sensing elements. Where poisons cannot be eliminated, frequent gastesting is essential to ensure safe and accurate operation of the gas detection system.Figure 1-1. Detector Sensing Element1-2.2 DETECTORELECTRICALCIRCUITRY. (Figure 1-2) Regulated 6-volt (nominal) DC power from thecontrol module is applied to balanced pairs of resistances in the detector. In normal atmospheres free ofcombustible gases, the paired resistances are in electrical balance, so that no signal current is sensed by themeasuring circuit of the control unit. Oxidation of combustible gas on the catalytic element acts to increase itsresistance,23-96177/1/761-2
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