ABB GLA232 Series User Manual page 50

ABB LGR-ICOS™ GLA232 Series Methane/Ethane Analyzer User Manual
Because the path length depends only on optical losses in the cavity and not on a unique beam
trajectory (like conventional multi-pass cells or cavity-ringdown systems), the optical alignment
is very robust allowing for reliable operation in the field. The effective optical path length is
determined routinely by simply switching the laser off and measuring the necessary time for
light to leave the cavity (typically tens of microseconds).
As with conventional tunable-laser absorption-spectroscopy methods:

The wavelength of the laser is tuned over a selected absorption feature of the target
species.

The measured absorption spectra are recorded and used to determine a quantitative
measurement of mixing ratio directly and without external calibration when combined
with the recorded:
o Measured gas temperature and pressure in the cell
o Effective path length
o Known line strength
ABB PROPRIETARY INFORMATION 50