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22 CLOTHING TEST METHODS maximum regains of the various fibers are rather approximate, however, since the transition between absorbed and liquid water at 100$ relative humidity is con- tinuous. Approximate values for regain at 100$ RH are: cotton, 20-24$, mercerized cotton or viscose rayon 40-45$, wool 30-36$. Curves of regain against relative humidity are given in Valko, Kolloid Chemische Grundlagen der Textilveredlung, and in International Critical Tables. For particular fibers, see: cellulose fibers, Ott, Cellulose and Cellulose Derivatives; cotton, Urquhart and Williams, J. Textile Institute 15. T 559 (1924); for wool, Speakman and Cooper, J. Textile Institute 22, T 183 (1936); for rayons, Urquhart and Eckersall, J. Textile Inst. 23, T 163 (1932); for nylon, Physical and Chemical Properties of Nylon and the Processing of Nylon Textiles, Nylon Sales Division, E. I. du Pont de Nemours and Co. Body heat, lost by evaporation. Under ordinary room conditions when there is no visible sweating, evaporation takes place on the surfaces of the skin and lungs and it is assumed that the body itself furnishes all the latent heat necessary to evaporate the water. Body heat lost by evaporation = moisture evaporated x L.H.E. The L.H.E. varies with the temperature of the water film at which evaporation takes place. The following linear relationship applies to temperatures between 25 and 100°C. L.H.E. = 538.9 + 0.599 (100 - t°), cal./Kg. H20.. Usually L.H.E. is taken as 580 cal./Kg. H20 which is close enough for most practical purposes. When liquid sweat is absorbed by the clothing on exposure to heat or during muscular work, this sweat will evaporate at some distance from the skin drawing its heat of vaporization from the clothing and surrounding air. The body does not derive the full benefit of this evaporative cooling but only that part which results from increased temperature gradients between the skin and clothing surfaces on which evaporation takes place. Sweat that drips off the body is totally wasted. Body heat loss by evaporation is also considerably reduced by condensation and freezing of perspiration in the clothes on exposure to subfreezing tempera- tures. The heat of vaporization which is released by condensation and the heat of fusion is absorbed by the clothes and surrounding air. Part of this heat is lost from the outer clothing surfaces, and the remaining part is returned to the body through decreased temperature gradients. Diffusion of water vapor through clothing is greatly reduced by freezing, and the frost may accumulate over a period of hours apparently without doing any harm until it begins to thaw. There is no way of estimating accurately body heat loss by evaporation on exposure to intense heat or cold, or during muscular exertion. The weight loss from the clothed human body (corrected for the unequality of C02 eliminated and 02 consumed) always overestimates the effective evaporation loss under such condi- tions. Clothing ventilation by bellows action, or otherwise, increases the ef- fective evaporation, and is an advantage on exposure to heat but a distinct disad- vantage in the cold. Other observations that are worthwhile recording in tests of this sort are skin area exposed and total skin area, weight and thickness of fabrics, outer circumference of clothing at the chest, waist, around the sleeve and the leg, and a description of tightness of closure at the neck, coat front, waist, wrist and and ankle.
SOLAR HEAT LOAD1 Harold F. Blum INTRODUCTION This report attempts to answer the specific question whether experiments to determine the effectiveness of field uniform fabrics in combating the solar heat load can be carried out in the laboratory or must be made in the field. Under field conditions, sunlight, both direct and reflected, forms a cer- tain portion of the total heat load. This will be referred to herein as the solar heat load. If the influence of clothing on the total heat load is to be analyzed, this factor is best treated as separate from the heat load contributed Indirectly by the sun through its influence on the temperature of the ambient air, and the terrain. The evaluation of the effect of clothing on the solar heat load by direct experimental methods presents many difficulties. Sunlight cannot be closely simulated in the laboratory, and on the other hand, testing under outdoor condi- tions presents difficulties because numerous factors cannot be accurately evaluat- ed and controlled. THE SOLAR SPECTRUM In order to view the problem properly, reference must be had to the spec- trum of sunlight. Curve 0 in Fig. 1 represents the spectral distribution of sun- light outside the earth's atmosphere. The spectral distribution is altered in passage through the atmosphere due to the fact that all wave lengths are not ab- sorbed equally. The atmospheric constituents chiefly responsible for this altera- tion of the spectrum are ozone, which absorbs the short wave length ultraviolet end of the spectrum, and water vapor which absorbs the long infrared wave lengths. The latter is of particular importance with regard to the present problem. The quantities of both ozone and water vapor in the atmosphere at different times and places vary, and the spectral distribution of sunlight is altered accordingly. The other gases in the atmosphere absorb very little within the spectral range of sunlight. The spectral distribution is also altered by scattering by gas mole- cules and by dust particles. Curves 1 and 2 in Fig. 1 represent sunlight at the earth's surface when certain quantities of ozone (2.8 mm.), water vapor (20 mm.), and dust (300 particles/cm,3) are present in the atmosphere. Curve 1 represents the spectrum when these atmospheric conditions pertain and when the sun is direct- ly overhead, while curve 2 represents the spectrum under the same 'conditions when the sun is 60° from zenith, at which time the rays pass through twice as thick a layer of atmosphere. Considering all these factors it is obvious that accurate predictions cannot be made about sunlight without direct measurements, or without knowledge of the atmospheric conditions and proper consideration of latitude, season and time of day, all of which determine the angle of the sun with respect to the zenith. Fig. 1 shows that the maximum of the solar "spectrum occurs at about 0.48^.. Thermal emission having its maximum at this wave length would be given off by a black body at 6,000°K. Such a temperature is not attainable in the laboratory 1. The substance of this article has appeared as a report from the Naval Medical Research Institute. 23 m w
