Picture of two heads

Open Access research that challenges the mind...

The Strathprints institutional repository is a digital archive of University of Strathclyde research outputs. Strathprints provides access to thousands of Open Access research papers by University of Strathclyde researchers, including those from the School of Psychological Sciences & Health - but also papers by researchers based within the Faculties of Science, Engineering, Humanities & Social Sciences, and from the Strathclyde Business School.

Discover more...

An alternative approach to wavelength modulation spectroscopy for absolute measurements of gas parameters in industrial processes

Duffin, K. and McGettrick, A.D. and Johnstone, W. and Stewart, G. (2007) An alternative approach to wavelength modulation spectroscopy for absolute measurements of gas parameters in industrial processes. In: Chemical and Biological Sensors for Industrial and Environmental Monitoring III, 2006-07-12.

Full text not available in this repository. (Request a copy from the Strathclyde author)

Abstract

Wavelength modulation spectroscopy (WMS) with tunable diode lasers (TDLs) is the preferred technique for gas composition measurement in a growing number of industrial process control applications. Those systems using optical fiber cables or networks to address single or multiple sensing points are of particular interest. However, the conventional approaches suffer from a number of calibration / scaling factor issues which, although addressable, lead to added cost and accumulated errors in the final determination of gas concentration. Such issues are particularly problematic in industrial applications where the pressure may be varying and unknown. The target signal in WMS is an amplitude modulation (AM) component generated by the interaction of frequency modulation (FM) on the laser output with a rotational / vibrational gas absorption line function. However, direct laser amplitude modulation is also present and distorts the recovered target signals again leading to errors. Here we report an alternative approach in which we exploit the phase difference between the laser AM and the FM to provide direct recovery of the absolute gas absorption line function from which both the gas concentration and the pressure may be obtained from the depth and line width respectively. The method is absolute with no need for calibration thus eliminating the difficulties with the conventional approach. In our presentation, we report the basic principles of the technique and its experimental validation through a number of methane gas concentration and pressure measurements.