GB1419146A - Smoke detector - Google Patents

Abstract

1419146 Smoke detectors COMMONWEALTH SCIENTIFIC & INDUSTRIAL RESEARCH ORGANIZATION AND AUSTRALIA THE COMMONWEALTH OF 30 May 1973 [6 June 1972] 25660/73 Heading G1A A smoke detector comprises two chambers having respective light detectors and a common light source; air from an area to be monitored, e.g. inside a building, is supplied to one chamber and air from outside the area, e.g. from the atmosphere, is supplied to the other chamber. Thus the effects of atmospheric pollution on the detector inside the area can be minimized by subtracting the two detector outputs. Fig. 1 shows parallel chambers 1,1' each having a photo-multiplier detector 10,10'. A xenon flash tube 15 acts as the common light source, and has diffusers 17, 17' associated with it. Apertured baffles 2-9 and 2'-9' define the field of view of the detectors, and allow the air supplied to inlets 12, 12, to pass through the chambers to outlets 13, 13'. The baffle plate construction is described, Fig. 2 (not shown), as is an embodiment in which the parallel chambers are spaced apart and have side tubes leading to the common source, Fig. 3 (not shown). The two chambers may be supplied with air from the outlet and inlet and an air conditioning unit to monitor the air conditioned area by comparing the air leaving it with the air entering it. The output from each photo-multiplier is passed to an integrate and hold circuit, Figs. 5 and 6 (not shown), having two capacitors, one of which receives the output all the time and the other only during the period when the flash tube is not activated. By subtracting the stored signals on these capacitors noise in the output can be removed, to leave only the output due to the light scattered from the flash tube to the photo-multiplier by smoke. This latter output is recorded for both chambers, and the difference of output of the two detectors may be recorded. When the difference exceeds a given level an alarm or fire extinguisher is activated, the record of the two detectors allowing an operator to see whether the alarm is due to a fire or is just a temporary effect such as cigarette smoke. Also the fact that atmospheric pollution takes time to penetrate a building and then remains in the building when the outside air is clear can be allowed for if a record is made.