ABB PXAH401 Industrial PLC Controller

The measuring principle is infrared single-line absorption spectroscopy, which is

based on the fact that each gas has distinct absorption lines at specific

wavelengths. The measuring principle is illustrated in Figure 1-1. The laser

wavelength is scanned across a chosen absorption line of the gas to be

measured. The absorption line is carefully selected to avoid cross interference

from other (background) gases. The detected light intensity varies as a function of

the laser wavelength due to absorption of the targeted gas molecules in the optical

path between transmitter and receiver. In order to increase sensitivity the

wavelength modulation technique is employed: the laser wavelength is slightly

modulated while scanning the absorption line.

The detector signal is spectrally decomposed into frequency components 

at harmonics of the laser modulation frequency.

The second harmonics of the signal is used to measure the

concentration of the absorbing gas. The line amplitude and line width are both

extracted from the second harmonics line shape, which makes the measured

concentration insensitive to line shape variations (line broadening effect) caused

by background gases.

NOTE: The AO2000-LS25 measures the concentration of only the FREE

molecules of the specific gas, thus not being sensitive to the molecules bound with

some other molecules into complexes and to the molecules attached to or

dissolved in particles and droplets. Care should be taken when comparing the

measurements with the results from other measurement techniques.