AN APPARATUS AND METHOD FOR THE CONTINUOUS MEASUREMENT OF EVAPORATIVE WATER LOSS FROM HUMAN SUBJECTS

The evaporation of water from the skin and lung surfaces is an important factor in the thermal balance of man. Methods formerly employed for the determination of evaporative rate gave only average values for relatively long intervals of time and, hence, could not be used to detect rapid changes in rate. The measurement of high sweat rates by these methods was also very difficult. It was desirable, therefore, to develop an apparatus and method which would record continuously the rate of evaporation of water from the body under a wide range of experimental conditions. An infrared gas analysis apparatus was developed which gave a continuous record of evaporative rate. The subject was placed in a chamber through which a steady flow of air at several closely controlled temperatures and humidities was maintained by an air conditioning unit. The inlet and outlet air streams of the chamber were analysed for water vapor simultaneously by a special modification of an N. D. R. C. Selective Gas Analyser, Model IV. The optical system of the analyser was altered so that one beam of infra-red radiation passed through a sample of inlet air and a parallel beam through a sample of outlet air. The difference in absorption of radiation caused an electrical imbalance in the receiving thermopile, and this was amplified and recorded as a continuous tracing.