SECTION 1 SYSTEMS DESCRIPTION

TM 10-6665-297-13&P SECTION 1 SYSTEMS DESCRIPTION 1-1 GENERAL DESCRIPTION The CD800/830 and CD802/832 Series Combustible Gas Alarm Systems comprise several configurations of gas 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 of combustible 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 dual alarm level capability. Built-in relays are provided that may be used to actuate various plant protection devices such 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 discrete signal channels to monitor two points individually, whereas CD800/830 individual control units have only one signal channel to monitor a single point. (If a CD800/ 830 control unit is used to monitor two points, incoming signals are summed in the single channel as if from a single point.) 1-2 GAS DETECTOR UNITS To detect combustible gas in air, the existing air-gas mixture surrounding a detector is subjected to flameless catalytic burning on the surface of a catalyst-coated sensing element. The resulting heat raises the temperature, and thus the electrical resistance, of the active sensing element. A Wheatstone Bridge circuit is used to compare the electrical resistance of the active sensing element with that of a companion reference element (noncatalytic and therefore inactive) at the same ambient temperature. The difference in electrical resistance between the active and reference elements produces a signal current in the detector bridge circuit which is fed to the remotely located control unit, where it is in turn electrically processed to drive an indicating meter and associated alarm circuitry. 23-9617 7/1/76 1-1