Teledyne IR7000 Operating Instructions Manual page 17

NDIR Gas Analyzer
The detector consists of 2 chambers filled with the gas of interest in
optical series with a sensitive mass flow sensor. The sensor measures a
fluctuating mass flow between the 2 chambers due to a differential in
infrared absorption between the chambers.
The 2 chambers of the detector are of unequal volume, the first
chamber, called the primary chamber, is much smaller than the trailing
chamber, or secondary chamber. A small passageway connects the 2
chambers and contains the mass flow sensor. During assembly at the
factory, both chambers are filled with the gas of interest and due to the
unequal volume, a vastly different optical path length exists between the
chambers.
Initially, with only nitrogen (zero gas) passing through the sample
cell, pulsed IR radiation from the source passes through the cell. Since this
is the zero gas, no differential absorption takes place. At the rear of the
sample cell an IR transparent window (typically sapphire but may be some
other material depending on the application) allows the radiation to pass
into the primary detector chamber. Due to the heteroatomic nature of the
gas contained within the chambers (identical to the gas to be monitored),
IR absorption takes place at a few characteristic wavelengths
corresponding to the most strongly absorbed lines for that particular gas in
the IR spectrum. The remaining radiation passes through to the secondary
chamber.
The secondary chamber has a much greater path length and therefore
additional absorption takes place but at different energies. Due to the
longer residence time of the optical beam in this chamber, absorption
occurs at weaker absorption bands in the IR and accounts for the less
intense absorption relative to the primary chamber. The remaining
unabsorbed energy is eventually dissipated.
Essentially, the front chamber absorbs IR differentially at specific
wavelengths characteristic of the gas of interest within the detector
Teledyne Analytical Instruments., - Rev. 3
Figure 2-1: Optical Bench Components
Operational Theory 2
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