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10 Detecting principal
The detection method of RI-2000R is gas filter correlation. It is a part of NDIR
(absorption of infrared radiation) based detecting, but it has better selectivity and
stability than normal NDIR methods.
There is an inlet for gas to enter the sample cell (white cell) and an outlet to let the gas
go out. IR light enters the cell and gas absorbs the beam, so the absorption rate and the
gas concentration will correlate. The IR detector detects the IR light and will put it out
as gas concentration. Light from the radiation source goes through a rotating gas filter
wheel and enters the cell, goes out through the band pass filter and then, detected by
the IR detector. The IR beam is optically folded inside the sample cell (white cell) by
mirrors to stretch the light path and obtain effective absorption length. There are two
compartments in the CFG wheel, one is filled with target gas and the other is filled with
gas that does not interfere in the target gas. Light goes through this rotating cell
alternately and crates beam to refer. The band pass filter only lets specific wavelength
that is absorbed by the target gas. The detector will not detect any light that has
different wavelength. Also, the detector will not detect gas that does not absorb IR
rays.
