The Secrets of Gas Analysis with Tunable Diode Lasers

The ability to determine the moisture concentration and other problem gases in process streams has become increasingly vital to plant safety, improving process efficiency and to preserve plant assets in refineries and chemical processing industries. Among the most significant of the technologies to advance such detection and measurement is the tunable diode laser (TDL) based absorption spectroscopy gas analysers.

A company can now offer the UK market a system which has seen a 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 greatly 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, thereby enabling the sensing of water vapour and other gases. Gas concentration is calculated by measuring the difference in the amount of light received via the tunable diode laser (TDL) analyser’s built-in detector.

IMA Limited (UK) represent SpectraSensors (USA) in the UK and Paul Stockwell, MD of IMA Ltd, says “We have seen an improvement in user confidence since these analysers were introduced by SpectraSensors 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 in recent years”.

Essentially, the TDL analyser system design uses an extractive type sample system. With its condensed design, the extractive TDL analyser is small enough to permit convenient installation virtually anywhere in a plant. The long optical path of the cell enables highly sensitive measurements of trace gases in small volumes. The advantage of the extractive TDL is its ability to measure trace levels for process control where fast response enables control not previously possible.