Interference Filter Correlation - Emerson ROSEMOUNT Analytical NGA 2000 Instruction Manual

NGA 2000 MLT Hardware
MEASURING PRINCIPLE
IR MEASUREMENT
When the IR radiation passes through the measurement side of the analysis cell into the detector,
a part of it is absorbed depending on gas concentration. 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.
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.
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.
In addition to simply light chopper, the chopper wheel has a special structure for measuring side
and reference side, that simulates an absorption in the analysis cell. This absorption signal is be
cut out of the normal measuring signal and is be used for automatically sensitivity control. The result
is a high long-term stability of sensitivity.
2.1.2 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 MLT and the spectral
absorption of the gases CO and CO
of these gases each coincide with the passbands of one of the interference filters.
2 - 4
are shown in Fig. 2-3. It can be seen that the absorption bands
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Instruction Manual
90002929
Emerson Process Management GmbH & Co. OHG
07/2006