Research On High Sensitivity Aspirating Smoke And Gas Combined Fire Detector

Photoelectrical smoke fire detectors work on the principle of light scattering. Thescattering behavior can be described through Mie Theory. The intensity of scattering lightin a given direction is proportional to the smoke particle concentration.One of the most effective methods to find a fire in very early stage is aspiratingsmoke detection where a technique of actively drawing in, instead of passively waitingfor, air samples is employed. The sensitivity of an aspirating system can be hundredstimes high as compared with that of general smoke detectors. The aspirating firedetectors mainly produced by foreign manufacturers usually use laser LED as the lightsource. The prices are too high to our market. Another problem is that the focusing lensesused in these detectors likely trigger false alarms in the conditions that the atmospherictemperature changes rapidly or the humidity is high. The aspirating fire detectordiscussed in this paper uses an infrared LED as the light source and does not use any lens.CAD is introduced into the dark chamber design and result in a simple structure. Theemitted IR light beam is attenuated by multiple reflections from the inside surfaces of thespecially designed dark chamber before reaching the photodiode, which reduces theintensity of background signal, improves the signal to noise ratio and increases thesensitivity for fire. CO sensing combined with smoke detecting intrinsically help to bringabout false alarm immunity in most cases provided that materials burned in a firecontaining carbon element.The aspirating fire detector studied in this paper has similar specifications that theforeign products have, but is low cost, only about one third of the foreign similar. At thesame time the false alarm rate is much lower. The study result will be meaningful forreference in product design and engineering application.