Recording Dielectric Hygrometer for Expired Air

Abstract

A method for rapid continuous measurement of water vapor in expired air was needed for studies of subjects under cold ambient conditions. The method chosen senses water vapor content by measuring the air dielectric constant. Air passes between the plates of a small capacitor so that changes in the air dielectric constant, which depend on water vapor concentration, cause capacitance changes. These small changes cause resonant frequency shifts in a 2‐Mc oscillator. Design features which insure stability in the element and oscillator are presented. The frequency changes are detected by the beat frequency method, converted to proportional linear dc voltage, and used to drive the pen of a magnetic ink‐writing oscillograph. The system detects changes in absolute humidity of 1.0‐mgm water vapor per liter, but the sensitivity could be increased or decreased for different applications. Precautions regarding the need for simultaneous temperature measurement are pointed out, a sample tracing is shown, and the calibration procedure is mentioned. There is a brief discussion of previous developments along similar lines, followed by a listing of some of the advantages of the method.