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Abstract

A reactive gas detection system provides early warning of gas emissions that often occur in developing fire conditions in environments such as telephone system central offices where halogenated substances, for example polyvinyl chloride wire insulation and brominated fire retardant materials, are prevalent. Multiple microbalance detectors (122) comprising quartz crystal oscillators coated with a layer of zinc or zinc compound are distributed about a premises and the rate of change of crystal oscillation frequency is cyclically monitored by a frequency counter (130) under control of a data processor (112). The detectors are specifically reactive to the halogen acid gases emitted during pyrolysis of the noted substances and exhibit a significant differential frequency change in the presence one or more of these gases well in advance of the outbreak of flame, smoke, or other normally detectable combustion products. In order to maintain a high rate of system monitoring, the cycle period of each frequency measurement is greatly reduced during normal conditions, with resulting low measurement resolution. Upon the occurrence of a significant threshold frequency change in any of the detectors, subsequent frequency measurements are preferentially taken at the suspect detector over an extended cycle period with high resolution to confirm that the threshold is being exceeded. Continued excessive excursion of measured frequency change beyond a preset limit initiates the generation of an alarm signal.

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