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MEASURING PRINCIPLE
2 - 1
ETC00781(1) Series 100 e 10/2001
2. Measuring Principle
Depending on the gas to be analyzed different measuring methods will be used.
2.1 IR Measurement
The analyzers are non-dispersive infrared photometers (NDIR) using measurement of selective
absorption in a column of gas.
The measuring effect derived from absorption of infrared radiation is due to the gas being
measured. The gas specific wavelengths of the absorption bands characterize the type of gas
while the strength of the absorption gives a measure of the concentration of the component
measured. Due to a rotation chopper wheel, the radiation intensities coming from measuring and
reference side of the analysis cell produce periodically changing signals within the detector.
The detector signal amplitude thus alternates between concentration dependent and concentra-
tion independent values. The difference between the two is a reliable measure of the concentration
of the absorbing gas component.
Dependent on measuring component and measuring concentration, two different measuring
methods will be used.
2.1.1 Interference Filter Correlation (IFC Principle)
The undivided analysis cell is alternately illuminated with filtered light concentrated in one of two
spectral separated wave length ranges. One of these two spectrally separated wave length bands
is chosen to coincide with an absorption band of the sample gas, and the other is chosen such
that none of the gas constituents expected to be encountered in practice absorbs anywhere within
the band.
The spectral transmittance curves of the interference filters used in the 100 series analyzer and
the spectral absorption of the gases CO and CO
2
are shown in Fig. 2-1. It can be seen that the
absorption bands of these gases each coincide with the passbands of one of the interference
filters. The fourth interference filter, used for generating a reference signal, has its passband in a
spectral region where none of these gases absorb. Most of the other gases of interest also do not
absorb within the passband of this reference filter.
IR MEASUREMENT