Particle detector using a second radiation emitter which becomes operative only if the signal from a first emitter signal exceeds a predetermined value

摘要

Detector has both blue and infra-red radiation emitters (3 and 3A). Light is forward scattered by the particles in scatter volume (9). The emitters are pulsed at different intermittent frequencies enabling the control means (16) to produce separate signals (21, 23) corresponding to the scattered blue and infra-red lights. A comparator (25) takes the ratio of these two signals to produce a smoke-dependent warning output to the alarm (32). Smoke particles scatter more blue light than infra red. The apparatus normally operates in monitoring mode in which the infra-red emitter (3A) is pulsed intensively but at a low flashing rate. The second emitter (blue light) is maintained inoperative, until the first emitter (infra-red) radiation scattering causes the photodiode (15) exceeds a predetermined output, at which point the blue (second) emitter is rendered operative. The state of second emitter may be changed between operative and inoperative in various ways e.g. by energising/de-energising the second emitter or by increasing/decreasing its amplitude or its intermittent frequency. Alternatively, the control system may keep the second emitter inoperative until the signal form the first emitter exceeded a predetermined value and this has elapsed a specific length of time.