10.4.1 Smoke and chemical measuring

10.4.1 Smoke and chemical measuring devices
Infrared sensors detect changes in the signal received from a light emitting
diode due to the presence of smoke in the light path or the presence of an object
in the light path.
Ionization chambers are devices that detect the leakage current between two
plates that have a voltage between them. The leakage occurs when carbon particles
from smoke are present and provide a conductive path between the plates.
Taguchi-type sensors are used for the detection of hydrocarbon gases, such as
carbon monoxide and dioxide, methane, and propane. The Taguchi sensor has
an element coated with an oxide of tin that combines with hydrocarbon to give
a change in electrical resistance which can be detected. Periodically the element
is heated and the chemical reaction is reversed, reducing the coating back to tin
oxide. Likewise, the sensing process can be repeated. The tin oxide can be made
sensitive to different hydrocarbons by using different oxides of tin, different deposition
techniques, and so on.