Abstract: A system for sensing and annunciating the presence at any one of a plurality of locations of accumulations of potentially hazardous gaseous and vapor mixtures prior to their reaching a lower explosive limit thereof. The system has particular applicability to aircraft, and has a number of advantageous features such as adjustable annunciator set points which are settable between relatively wide lower explosive level limits, compensation in the sensor circuitry for temperature variations, built in diagnostic and test features, and a circuit design wherein multiple sensors at multiple locations are operable independently of each other.