Modified balanced ratio metric detection system used for high sensitive vapor detection

A modified balanced ratio metric detection (BRD) system was proposed and applied in trace measurement of water vapor. Three features, including proportion of alternating current (AC) and direct current (DC) signals, environment temperature and light intensity ratio have been stated. Their influences on measured result have been analyzed. Meanwhile, techniques including adding filter, using integrated transistor and light intensity ratio adjustment and their effects for measurement improvement have been discussed. The absorption line was chosen at 1368.597 nm and the path length was 10 cm. With application of the above techniques, an accuracy of 5 ppm for water vapor concentration has been achieved.