Tunable Diode Laser (TDL) Systems for Trace Gas Analysis

Abstract

A relatively new technique using a Tunable Diode Laser (TDL) systemto monitor water vapour, carbon dioxide and hydrogen sulphide isproviding higher on-line accuracy, lower maintenance, than traditionaltechniques and is significantly better at coping with contamination. It is the first technique to monitor the gas itself, rather than monitoringa reaction or a change on the surface of a sensor. It combines a highdegree of accuracy with an extremely good speed of response. Tunable Diode Laser Absorption Spectroscopy (TDLAS) works very wellacross the entire spectrum of gas concentration, and an area where it particularlyexcels is accurate moisture measurements in dry gases. Currentlymany hours are spent waiting for sensors using traditional techniquesto dry down: for example, when installing a new or re-calibrated watervapour sensor in a process where moisture levels are around 1 part permillion (ppmv), users normally wait 6 to 8 hours before the system settles;a TDL takes a few seconds and remains very responsive at that level. This paper will illustrate the progress made with this non-contactmethod in applications with natural gas, and gives details of why moisturemeasurement is important. The results of field trials are also presentedshowing how the technology compares with other techniques.