Abstract: A gas detector-alarm comprises a hot-wire gas sensor 1 having an actuating power control 7 that supplies pulsed actuating power with a cycle T0 when the concentration of gas detected by the hot-wire gas sensor is lower than a first reference level, with a cycle T1 that is shorter than said cycle T0 at least once when the concentration of gas exceeds a second reference level, and with a cycle T0 again after an alarm has been triggered.

Abstract: A porous cellulose tape impregnated with powdered silica or other gas adsorbent carries a coloring reagent of silver perchlorate or silver para-toluenesulfonate and a light-resistance enhancer of para-toluenesulfonic acid which are dissolved in glycol. When the tape is exposed to a sample gas, the silane contained in it dissolves in the glycol. The resulting reduction of the silver perchlorate or silver para-toluenesulfonate forms a colloid of silver as a trace of their reaction. The concentration of silane, which is proportional to the colloid of silver formed, can be determined by measuring the optical concentration of the reaction trace. The absence of strong acid detrimental to cellulose assures the determination of silane concentration with a high degree of sensitivity, without impairing the mechanical strength in the cellulose tape.