Detector Methodologies; Non-Dispersive Infrared (Ndir) - Emerson Rosemount Analytical CAT 100 Instruction Manual

Instruction Manual
748441-D
April 2002

1-3 DETECTOR METHODOLOGIES

The CAT 100 can employ up to two of four dif-
ferent measuring methods depending on the
configuration chosen. The methods are:
NDIR, Paramagnetic O
and Thermal Conductivity.
a.

Non-Dispersive Infrared (NDIR)

The non-dispersive infrared method is
based on the principle of absorption of in-
frared radiation by the sample gas being
measured. The gas-specific wavelengths
of the absorption bands characterize the
type of gas while the strength of the ab-
sorption gives a measure of the concen-
tration of the gas component being
measured.
An optical bench is employed comprising
an infrared light source, two analysis cells
(reference and measurement), a chopper
wheel to alternate the radiation intensity
between the reference and measurement
side, and a photometer detector. The de-
tector signal thus alternates between con-
centration dependent and concentration
independent values. The difference be-
tween the two is a reliable measure of the
concentration of the absorbing gas com-
ponent.
Depending on the gas being measured
and its concentration, one of two different
measuring methods may be used as fol-
lows:
1-2 Description and Specifications
, Electrochemical O ,
2 2
Interference Filter Correlation (IFC)
With the IFC method the analysis cell is
alternately illuminated with filtered infrared
concentrated in one of two spectrally
separated wavelength ranges. One of
these two wavelength 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 ex-
pected to be encountered in practice ab-
sorbs anywhere within the band.
The spectral transmittance curves of the
interference filters used in the CAT 100
analyzer and the spectral absorption of
the gases CO and CO
Figure 1-1 (page 1-3). It can be seen that
the absorption bands of these gases each
coincide with the pass bands of one of the
interference filters. The forth interference
filter, used for generating a reference sig-
nal, has its pass band in a spectral region
where none of these gases absorb. Most
of the other gases of interest also do not
absorb within the pass band of this refer-
ence filter.
The signal generation is accomplished
with a pyroelectrical (solid-state) detector.
The detector records the incoming infra-
red radiation. This radiation is reduced by
the absorption of the gas at the corre-
sponding wavelengths. By comparing the
measurement and reference wavelength,
an alternating voltage signal is produced.
This signal results from the cooling and
heating of the pyroelectric detector mate-
rial.
Rosemount Analytical Inc.
Model CAT 100
are shown in
2
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