TM 10-6665-297-13&PSECTION1SYSTEMSDESCRIPTION1-1GENERALDESCRIPTIONThe CD800/830 and CD802/832 Series Combustible Gas Alarm Systems comprise several configurations ofgas detectors and control units designed to measure concentrations of combustible gases wherever they occur.When properly installed, the systems continuously monitor atmospheres and give early warnings ofcombustible gas leaks, flammable liquid spills, and other dangerous combustible gas or vapor conditions.Typical standard features include state-of-the-art design with all solid-state electronics for maximum reliability,meter display for instantaneous readout of percent Lower Explosive Limit (L.E.L.), recorder outputs, and a dualalarm level capability. Built-in relays are provided that may be used to actuate various plant protection devicessuch as ventilation fans and blowers, sprinklers, shut-down controls, or process control switches.The CD800/830 and CD802/832 systems are similar except that the CD802/832 control units have two discretesignal channels to monitor two points individually, whereas CD800/830 individual control units have only onesignal channel to monitor a single point. (If a CD800/ 830 control unit is used to monitor two points, incomingsignals are summed in the single channel as if from a single point.)1-2GASDETECTORUNITSTo detect combustible gas in air, the existing air-gas mixture surrounding a detector is subjected to flamelesscatalytic burning on the surface of a catalyst-coated sensing element. The resulting heat raises thetemperature, and thus the electrical resistance, of the active sensing element. A Wheatstone Bridge circuit isused to compare the electrical resistance of the active sensing element with that of a companion referenceelement (noncatalytic and therefore inactive) at the same ambient temperature. The difference in electricalresistance between the active and reference elements produces a signal current in the detector bridge circuitwhich is fed to the remotely located control unit, where it is in turn electrically processed to drive an indicatingmeter and associated alarm circuitry.23-96177/1/761-1
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