Abstract :
Reliability of the transmission-type optical smoke detectors is limited by the high sensitivity of optical components to parasitic effects, especially to contamination and temperature. Influences of the high rate temperature variations cannot be eliminated by filtering, as their spectral range partially overlaps with the spectral range of a smoke absorption index. Hence, sensitivity to these effects has to be eliminated through the mutual cancellation of the corresponding components of the output signal. First determining the temperature dependence of the light source and the receiver in the generalized block diagram of the transmission-type optical smoke detector, transfer functions are derived, specifying the detector output signal as a function of the temperature variations of the blocks considered. Using the second-order thermal model for the thermal system of the optoelectronic diodes, an expression is given for the steady-state solution of the difference of the source and the receiver diode junction temperatures, when the ambient temperature is a ramp function of time. On the basis of the thermal system analysis, a block diagram is shown to be equivalent to RC low-pass (LP) filters that model the pn junction temperature dependence on ambient temperature. For the completed block diagram, with the help of the expressions derived, an analysis is performed to establish how to eliminate the detector threshold sensitivity to ambient temperature variations, or to adjust it to realize a multicriteria fire detector.
