ABB GLA131 Series Microportable Analyzers
User Manual
ABB PROPRIETARY INFORMATION
83
Appendix A Theory of Operation
For gas measurements based on conventional laser-absorption spectroscopy, a
laser beam is directed through a sample, and the mixing ratio (or mole fraction) of
gas is determined from the measured absorption using Beer’s Law, which may be
expressed with Equation 1.
Equation 1
𝑰
𝒗
𝑰
𝒐
=
𝒆
−𝑺𝑳
𝒙
𝑷𝝓
𝒗
where:
Iv
= transmitted intensity through the sample at frequency v
Io
= (reference) laser intensity prior to entering the cell
S
= absorption line strength of the probed transition
L
= optical path length of the laser beam through the sample
X
= mole fraction
P
= gas pressure
Φ
v
= line-shape function of the transition at frequency v
In this case:
Equation 2
∫
𝝓
(
𝒗
)
𝒅𝒗
=
𝟏
If the laser line width is much narrower than the width of the absorption feature,
high-resolution absorption spectra may be recorded by tuning the laser
wavelength over the probed feature.
Integration of the measured spectra with the measured values of:
Gas temperature
Gas pressure
Path length
Line strength of the probed transition
determines the mole fraction directly from the relation:
Equation 3
𝒙
=
−𝟏
𝑺𝑳𝑷
∫ 𝒍𝒏
𝒗
𝑰
𝒗
𝑰
𝒐
𝒅𝒗
This equation is used to determine gas concentrations, even in hostile
environments, without using calibration gases or reference standards.
The values measured are:
Mixtures containing several species
Flows at elevated temperatures and pressures
Calibrated gases would normally be used to verify measurement accuracy, as a
monitor to a fix process and for troubleshooting.
