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The Measurement of Water Evaporation from Limited Areas of a Normal Body Surface

U. S. Department of Agriculture^{2, 3,}

Abstract

A method is described for measuring the rate of water evaporation from selected areas of skin without altering markedly the humidity of the air to which it is exposed. Air is circulated alternately through a capsule attached to the skin and an absorber containing a saturated solution of a salt giving the desired vapor pressure. The gain in weight of the absorber indicates the evaporation from the skin. A supplementary "maintenance" circuit is used to keep up air flow through capsules when they are not subject to measurement.

Examples are given of results obtained by the use of this apparatus on cows under hot conditions. Different areas give different rates, especially under the less hot conditions. The rate of evaporation is increased with the hotter conditions. No significant differences are found between Jersey and Sindhi-Jersey crossbred cows for areas on the forechest and paunch. The rates for these areas under severely hot conditions (105° F., and 34 mmHg vapor pressure) reach 660 gm./sq. m., hr.

The same technique applied to rabbits under the same hot conditions revealed virtually no evaporation.

¹ Professor of Physiological Climatology, Johns Hopkins University, Baltimore, Md.

² These studies were conducted by the Bureau of Dairy Industry at the Agriculture Research Center, Beltsville, Md.

³ The authors wish to express their thanks to H. G. Wiseman for suggesting the idea for the re-circulation method, to Dr. J. F. Sykes for his assistance in designing the equipment, and to Dr. João S. Veiga, University of Sao Paulo, Brazil, for his assistance during the earlier stages of development.