The Secrets of Gas Analysis with Tunable Diode Lasers

29 January 2010

Plant safety, improving process efficiency and preserving plant assets in refineries and chemical processing industries have become key focus areas in recent years. The ability to determine the concentration of moisture and other problem gases, like hydrogen sulphide, in process streams has become increasingly vital. Among the most significant of the technologies to advance such detection and measurement is the tunable diode laser (TDL) based absorption spectroscopy gas analyser.

A UK company can now offer a system which has seen rapid growth in the natural gas industry in the USA. The development of today's extractive TDL is the culmination of sophisticated research by NASA's Jet Propulsion Laboratory for use on Mars.
The use of TDL technology makes it possible to alleviate the problems of using older technologies.
It is an optical system that uses a laser to produce a specific wavelength of light tuned to an absorption line, the known light frequency of the target gas. This technique produces an analyser with a fast response speed, where the laser light stimulates vibrations and rotation in the molecule, resulting in energy absorption, enabling the measurement of water vapour and other gases. Gas concentration is calculated by measuring the difference in the amount of light received via the analyser's built-in detector.

IMA Limited represent SpectraSensors in the UK, and Paul Stockwell, MD of IMA Ltd, says “We have seen an improvement in user confidence since these analysers were introduced a few years ago. We believe that their fast, accurate response and low maintenance has contributed significantly to the rapid growth in market share we have seen recently”.

Essentially, the TDL analyser uses an extractive type sample system. With its condensed design, the analyser is small enough to allow convenient installation virtually anywhere in a plant. The long optical path of the cell enables highly sensitive measurements of trace gases in small volumes. As a non-contact measurement, the advantages of the extractive TDL is its ability to rapidly measure changes in trace levels of gases, and an improved resistance to contamination, significantly improving confidence in process control.

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