the years, various detectors have been developed for use with infrared
gas analyzers. While most modern instruments employ a solid state,
photo-conductive detector, Luft-type and microflow detectors are commonly
all detectors translate the difference in infrared energy into a sine
wave that has a frequency derermined by the chopping rate and a magnitude
proportional to the energy difference.
solid state detector is constructed of a material that changes its
electrical conductance in response to IR light energy falling on it.
Changes in current flowing through the detection circuit are proportional
to the difference in transmitted IR energy. The solid state detector
has the advantage of being insensitive to external mechanical vibration.
In addition, careful selection of detector material allow for simplified
optical filter design. The Signal Model 7000FM GFC/IR
Luft type detector is the oldest type found in common use. It is found
in dual beam analyzers and uses a pair of chambers that are filled
with the adsorbing gas interest. One chamber recieves light through
the inert gas-filled reference cell, the other recieves light through
the sample gas-filled cell. The chambers are seperated by a capacitive
diaphragm which distends in response to the difference in adsorption-induced
gas expansion between the two detector chambers. This detector is
extremely sensitive and selective, but is expensive to produce. The
Signal Model 418 uses a variation of the luft detector.
Microflow detector is similar to the Luft detector, but employs a
small orifice between the two detection chambers. Instead of differential
pressure, the resulting flow is measured. A thermocouple arrangement
in the orifice is used for this purpose. This detector is relatively
immune to mechanical vibration, but is not as simple as the solid
species of gas have overlapping infrared absorption spectra which
can cause cross-interference in the basic technique. Example are water
vapor, CO, CO2, NO, NO2, NH3 and
SO2. Selectivity and interference rejection in NDIR analyzers
is achieved either through the addition of optical band-pass filters,
or gas-filled cells. More advanced examples of this technique, employ
multiple filters in combination on rotating wheels, which align to
allow the detection of several different gases within the same instrument.