25-11-2013
TDLAS the Most Robust Process Analyzer Available

 Tunable Diode Laser Absorption Spectroscopy or tdlas is an optical method of detecting trace concentrations of one or more selected gases mixed with other gases. The instrument is designed to detect absorptions as small as one part in 106 under the harshest of atmospheric conditions. TDLAS can also be used for high sensitivity measurements of a wide range of gases which includes methane, water vapour, carbon dioxide, hydrogen fluoride, hydrogen chloride and nitrous oxide. Over the last decade, tdlas has evolved into a valuable and reliable instrumentation for the detection of gas species. Today tdlas is so far the most common laser based absorption technique for quantitative assessments of species in gas phase.

Basic tdlas setup

A basic tdlas setup consists of tunable diode laser light source, transmitting optics, optically accessible absorbing medium, receiving optics and detector/s. Different diode lasers are used based on the application and the range over which tuning is performed. These lasers can be tuned by either adjusting their temperature or by changing injection current density into the grain medium.

Operating conditions of tdlas

·         In a laboratory

·         At ground level

·         Balloon-borne

·         On board an aircraft for the upper troposphere-lower troposphere

·         Open-path measurements

·         High repetition rate measurements for eddy flux correlation and turbulence monitoring

·         Simultaneous air species measurement

Advantages of tdlas

·         TDLAS has the advantage of using only one measurement technique and of potentially rendering uncertainties similar to those obtained when analyzing impurities.

·         TDLAS can be used as a secondary method for the certification of gas mixtures reference materials because it has the repeatability and reproducibility that other traditional facilities like a gas chromatography can achieve.

·         The advantages of TDLAS are based on the laser source characteristics that conventional radiation sources cannot offer particularly high radiant power, monochromaticity and directionality. The small linewidth of laser radiation allows for the high selectivity of gas species while monochromaticity of the laser source can maintain undisturbed the spectrum of the measured gas by instrumental linewidth contribution while the laser is tuned over a molecular absorption line. Directionality is used for the generation of large optical path lengths particularly for multi-reflection gas cells and open-path free-of-sampling gas analysis. 

·         TDLAS can be applied to real time, sample-free and non-destructive measurements. Traditional measurement techniques cannot fulfill these needs.

·         TDLAS changes the optical patch length that allows measurement from pure to trace levels of concentrations. A change in the laser source also allows a variation in the line intensity.

The practicality of tdlas in harsh environments

·         It utilizes economical, robust and portable TDL light sources and fiber optics

·         Can yield multiple properties: species, T, P, V & m in real-time over wide conditions. T to 800K, P to 50 atm, V to 15 km/sec, multiphase flows, overcoming strong emissions, scattering, vibrations and electrical interference

·         Demonstrated in harsh environments and large scale systems like aero-engine inlets, scramjets, pulse detonation engines, IC engines, arcjets, gas turbines, shock tunnels, coal-fired combustors, rocket motors, furnaces and more.

·         Potential use in the control of practical energy systems. 


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