Infrared gas analysis has been in use as an industrial process for decades.
Normally the gas is sampled from the stack and conveyed to an analyser
system. As this requires the gas to be extracted, filtered and cooled to
remove condensates, other methods have been developed. These include
across-stack gas analysis where an infrared transmitter is placed on one
side of the stack and a receiver on the other side. As there is now no
need to remove gas from the stack, there are advantages of simplicity and
avoidance of loss of soluble gases with the condensate.
However, it is not possible to do span and zero tests with the across-stack
gas analyser as the measuring path is open.
A method which combines the advantages of across-stack and extractive gas
analysis is the in-situ gas analyser. In this instrument, a probe is in
the stack which fills with stack gas because it is made out of a porous
material. This probe has an infrared beam of suitable wavelengths inside.
The high quality rigid path compensates for the fact that it is a shorter
path than the across-stack gas analyser.
The overriding advantage is that zero and span gases can be introduced as
they can flush out the porous sampling tube.
However, all the advantages of non-sampling are maintained and this method
allows the accurate analysis of soluble gases such as NO2, SO2 and NH3.