1.6 Operating Principles
1.6.1 Infrared Absorption
With the exception of Oxygen, the instrument measures gas concentrations using the effect of
infrared absorption.
All heteroatomic gases absorb (or transmit) light energy in the infrared region at specific
wavelengths that are dependent upon the chemical composition of each type of gas.
Furthermore, the level of absorption is proportional to the mass of the gas present. This
characteristic is utilised in infrared gas analysis (IRGA) benches.
The benches incorporate a source of infrared energy (usually in the form of a heated filament),
which is then optically filtered to reduce the radiation spectrum to that of the absorption band
of the gas being measured.
A pyro-electric detector is used to measure the difference in energy between a reference zero
(no absorption) condition and that after some absorption in the gas has occurred. By
calibrating the bench at the zero condition and a known span concentration, a reliable measure
of concentration over the span range is obtained.
To improve the stability and to assist in the detection process, the infrared source is modulated
mechanically at 3-18hz depending on the type of bench being used.
The relationship between absorption and concentration is not linear. For this reason the non-
linear output from the detector is modified electronically to give a linear output to the chart
recorder and display outputs.
The performance of an IRGA bench is compromised by the presence of 'interference' gases in
the sample. Such gases, of which the most common is water vapour, have an overlapping
infrared response to that of the measured gas and their effects are minimised by careful
filtering and the selection of the absorption band.
The level of absorption measured is relative and therefore an IRGA provides a comparative
measurement, not an absolute one, against 'zero' and 'span' reference gases.
A 'zero' gas has no detectable concentration of the gas being measured. Depending on the
gas, fresh air, chemically stripped air or nitrogen may be used. A 'span' gas has a known
concentration of the measured gas and is used to set the gain (or span) of the analysis in the
mid-range region. Under normal circumstances, the concentration is equivalent to about 80%
of the displayed full-scale reading.
1.6.2 The Optical Bench In Use
In order to obtain and maintain the optimum performance from the instrument, a number of
guidelines are offered associated with the use of the instrument, and, in particular, those
elements under the control of the User.
The IRGA bench is essentially an optical system and can suffer some deterioration in the
performance if it is internally contaminated by the condition of the incoming gases.
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