Servomex SERVOTOUGH LaserExact 2950, Operator'S Manual

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SERVOTOUGH LaserExact Monitor, Operator Manual 02950001A_1
2 C
HAPTER
3
WARNING
The Safe Area monitor is only suitable for use in unclassified areas.
WARNING
The ATEX Zone 2 and CSA CL I Div 2 Hazardous area variant
cannot be used for oxygen enriched samples; that is, gases over
21% O
2
. If used with gases containing over 21% O
2
the hazardous
area certification is invalidated.
WARNING
The Safe Area, GP and ATEX Zone 2 Variants are certified for use
with a flammable samples at a pressure of 0.8 to 1.1 bar absolute. If
used with a flammable sample beyond these limits, the hazardous
area certification is invalidated.
1.3 Measuring principle
The SERVOTOUGH LaserExact Monitor is an optical instrument transmitting infrared
laser light multiple times through a measurement chamber (the LaserExact cell). Two
mirrors are mounted inside the cell and the laser beam is reflected between these
many times resulting in a measurement path of between 0.7 and approximately 11m.
The measuring technique is based on measuring the absorption of light by the gas
molecules present in the cell.
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 the diode laser and detector. In order to increase the 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.