Precision and accuracy of miniature tunable diode laser absorption spectrometers

Abstract

Tunable Diode Laser Absorption Spectroscopy (TDLAS) is finding ever increasing utility for industrial process measurement and control. The technique's sensitivity and selectivity benefit continuous concentration measurement of selected analytes in complex gas mixtures. Tradeoff options among optical path length, absorption linestrength, linewidth, cross-interferences, and sampling methodology enable sensor designers to optimize detection for specific applications. This paper describes TDLAS measurement precision and accuracy limitations in emerging applications that demand increasing volumes of distributed miniaturized sensors at diminishing costs. In these situations, the TDLAS specificity is a key attribute, while high sensitivity enables novel sampling package designs with short optical pathlengths. Under these circumstances, the traditional approaches to optimizing accuracy and precision may fail if analyzer control features are sacrificed to reduce cost. We describe here a relatively simple TDLAS sensor designed to meet the needs for acceptable cost, and discuss its theory of operation along with the implications on measurement accuracy and precision.