Teledyne 7300A, Operating Instructions Manual

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Infrared Gas Analyzer Start-up and Operation
3-3
Teledyne Analytical Instruments
are not used during automatic sampling.
3.3 Theory of operation
3.3.1 General
T he non-dispersive infrared (NDIR) analyzer is one of the major components
of the system. It employs the basic principles of spectroscopic analysis to
measure a specific concentration of one gas in a multicomponent gas system.
The concentration of a gas is determined by exposing a chamber filled with a
gas mixture to infrared radiant energy and measuring how much of the
specific (non-dispersive) infrared wavelength is absorbed by the gas being
measured.
As an example, the NDIR analyzer is used most in flue gas applications
where the amount of carbon monoxide in a flue gas mixture is measured.
The specific infrared wavelength at which the carbon monoxide molecule
absorbs infrared energy is at 4.65 microns. The more carbon monoxide
present in the measurement cell, the more energy its molecules absorb.
The NDIR analyzer needs four basic components to measure the spectral
absorbance.
a) A source of emitted infrared radiation to be absorbed by the gas of
interest.
b) A chamber opened to accept a flowing sample gas.
c) A detector specifically tuned to measure only the wavelength of infrared
energy that will be absorbed by the gas being measured. For example,
carbon monoxide requires a detector turned to 4.65 microns, while carbon
dioxide needs one tuned to 4.27 microns.
d) An electronics system to process the changes in the electronic resistance
of the detector, and to convert these changes into specific electronics signals
that deliver a voltage at current linearly proportional to the concentration of
the gas measured. The Teledyne NDIR analyzer employs all of the above
features.
3.3.2 Analyzer
The emitted IR energy is generated by one specially configured miniature
lamps in parabola’s focused and operating at low power of only 0.5 watts.