A Water Vapor Monitor Using Differential Infrared Absorption

摘要

A water vapor monitor has been developed with adequate sensitivity and versatility for a variety of applications. Two applications for which the instrument has been designed are the continuous monitoring of water in ambient air and the measuring of the mass of water desorbed from aerosol filters. The sample gas may be held static, or it may flow continuously through the 56 cc sample cell, which is temperature controlled at 45 degrees C. Infrared energy from a tungsten-iodide bulb passes through a rotating filter wheel and the sample cell to a PbS detector. The infrared beam passes through the sample gas twice to produce a total optical path of 40 cm. As the filter wheel rotates at 1800 rpm, the infrared beam passes alternately through two semicircular narrow bandpass filters; one is centered in a spectral region of strong absorption and the other is centered nearby in a region of weak absorption. Absorption by the water vapor in the sample produces a 30-Hz modulation of the detector signal that is proportional to the water concentration. The zero-setting of the monitor is maintained quite stable by controlling the temperatures of the detector and the filters. The r.m.s. noise level corresponds to approximately 3 ppm of water. The maximum concentration that can be measured accurately is approximately 5%; higher concentrations could be measured by shortening the sample cell.