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MEASURING PRINCIPLE
2 - 4
ETC00781(1) Series 100 e 10/2001
In principle the detector is filled with the infrared active gas to be measured and is only sensitive
to this distinct gas with its characteristic absorption spectrum. The absorption chamber is sealed
with a window which are transparent for infrared radiation [usually CaF
2
(Calcium fluoride)].
When the IR radiation passes through the reference side of the analysis cell into the detector, no
preabsorption occurs. Thus the gas inside the absorption chamber is heated, expands and some
of it passes through the flow channel into the compensation chamber.
When the IR radiation passes through the open measurement side of the analysis cell into the
detector, a part of it is absorbed depending on sample gas concentration. The gas in the absorption
chamber then is heated less than in the case of radiation coming from reference side. Absorption
chamber gas become colder, gas pressure in the absorption chamber is reduced and some gas
of compensation chamber passes through the flow channel into the absorption chamber.
The flow channel geometry is designed in such a way that it hardly impedes the gas flow by
restriction. Due to the radiation of chopper wheel, the different radiation intensities lead to
periodically repeated flow pulses within the detector.
The microflow sensor evaluates this flow and converts it into electrical voltages.
The electronics, which follow, evaluate the signals and convert them into the corresponding display
format.
IR MEASUREMENT