LI-COR LI-7815, Instruction Manual

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Appendix A. Measurement overview
To measure all three gases, the laser frequency scans across the full spectral range of
interest in about 0.25 seconds (see
Figure A-2 below). It uses the reliably fixed grid
spacing in combination with spectroscopic data to determine the exact frequency of
each mode. The instrument uses two ring down events—one at the beginning and
one at the end of a scan—to normalize the measured signal.
Absorption is computed at each resonance mode by comparing the normalized
sample photodiode signal with the reference photodiode signal. The concentration
of each gas is then determined by using a fitting algorithm that compares the meas-
ured absorption spectrum with internally stored high resolution spectrum.
Figure A-2. Example of a scan showing the absorption features for a number of gases. The
upper line (dark blue) is the reference signal, and the lower line (green) is the sample signal
at each of cavity resonance modes.
A-2 LI-7815 CO₂/H₂O Gas Analyzer