AO2000-LS25
LASER ANALYZERS | OI/AO2000-LS25-EN REV. D
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The main board incorporates all electronics required for device
operation such as diode laser current and temperature control
and analogue-to-digital signal conversion.
An LCD continuously shows the concentration of the gas, the
laser beam transmission and the device status.
The RS-232 interface can be used for direct serial communication
with a PC.
The Ethernet module provides TCP/IP communication over LAN
(Local Area Network), which can be used instead of serial
communication.
All plug connectors are Phoenix VARIOSUB type and waterproof.
Measuring principle
The analyzer is an optical instrument for continuous in-situ gas
monitoring in stack, pipes, process chambers or similar and is
based on tunable diode laser absorption spectroscopy (TDLAS).
The analyzer utilizes a transmitter / receiver configuration
(mounted diametrically opposite each other) to measure the
average gas concentration along the line-of-sight path.
1
Measuring medium (Gas)
2
Transmitter unit
3
Optical path
4
Receiver unit
Figure 10: Measuring arrangement
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
1
Transmission
2
Absorption line
Figure 11: Absorption line of AO2000-LS25
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 analyzer 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.
