Gas sensing signaling system

Abstract

A gas sensing signaling system is provided in which a plurality of gas sensing units are connected over cables with a central station. To permit establishment of threshold levels such that false alarms are rejected while the system still remains sensitive to dangerous conditions and to permit ready supervision and testing of said system, a central station is provided in which timing circuits are included which start the time interval upon sensing of an alarm condition above a lower, or warning threshold level. If the lower warning level signal persists for the timing interval--for example for about one-half minute, a warning signal is given. A higher level signal generates an alarm signal, either immediately or after a much shorter time delay. The circuit includes self-holding circuitry so that, once a sensing unit transmits a sensing signal which persists beyond the previous predetermined warning or alarm time duration, the central station will indicate the respective conditions until manually reset. Besides indication, corrective action, such as energization of ventilation equipment and the like can be controlled. The cabling system preferably operates with predetermined, discrete voltage levels at the respective signal lines which, upon transmission of a warning and/or alarm signal, changes under control of a voltage control circuit in the central station, to permit additionally, by connection of a termination element, to indicate malfunction within the lines or the sensing units by detecting current flow and voltage relationships in the respective lines.