Series 4 Operating Manual - Nebula
Issue 1.00
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5 The Principle of Tunable Diode Laser Spectrometry
Tunable Diode Laser Spectrometry is an infrared absorption measurement which utilizes the
interaction of infrared light with gas molecules to determine the concentration of a target gas.
The absorption of infrared light by the gas molecules causes a decrease in IR radiation
intensity on the detector/photo diode.
A standard TDLS setup consists of a laser diode, optical lens, gas sample cell and a photo
detector. The optical lens in the system collimates the laser light to ensure the light is
perpendicular as it hits the photo diode. The laser diode in the detector is tuned to the
characteristic absorption lines of a material in a sample gas. Therefore, one laser/detector will
be used to measure one gas with little to no chance of any cross-sensitivity with other species
in the sample.
5.1
Sample System
The compact 3U 19" rack unit includes a 191 °C heated sample handling module which houses
a sample particulate filter, a sample selection solenoid, and connections to the zero and span
selection valves.
The sample handling module allows the connection of a heated sample line directly to the rear
panel preventing cold spots where sample condensation may occur. This is particularly
dangerous for this application as Ammonia is such a toxic gas. The NEBULA uses an internal
heated inlet line to maintain the temperature from the sample handling module to the detector
to prevent condensation forming on the inlet pipe. It also has a heated exhaust pipe so that
moisture build up does not occur and form aqueous ammonia in the outlet line.
