Document 8VaY70kdkDV8VL2K7Oxp94XnZ

108 Grade (MR IS? Col per (M'j'(hr)* CHAPTER 7 1965 -Guide' And Data* Boole tore, and the given dew point, and then computing ET. ft is believed that, by this modification, the error will be small jj the air movement is low, but it may be considerable in high, wind velocities. Wet-Bulb Globe-Temperature Index of Yaglou (WBGT) ' The.Effective Temperature Index previously described re quires the integration ofthree values: dry-bulb temperature humidity,'and air movement. Proper measurement of air movement is a most^difficult procedure, requiring specialised apparatus and skilled-personnel. It'has been suggested, for : spaces in which radiant heat is present, that an index (WBGT) ' can be obtained more simply by the use of wet-bulb and black globe, temperatures. With this procedure, it would cot be necessary to obtain air movement measurements, since tbs globe temperature will automatically reflect air movement, 7 provided that radiantheat is present. In this method, the wetbulb temperature is multiplied by a factor (approximately 0.7), the globe temperature is multiplied by a factor (ap proximately 0.3), and the two are added to give a new index.' This index is currently being evaluated in the field and looks promising, particularly in outdoor applications where the environment ordinarily has a radiant heat load. Results to date indicate values of the same order of significance as those obtained by more tedious methods. Index of Physiological Effect (Ep)*8 In the construction of this index, heat stress was evaluated from increases of heart rate, skin temperature, rectal tem' perature, and sweat rate of four subjects working or resting in tore, humidity, and movement of air upon the human body. In repeated, experiments, the subjective response by. groups, of individuals to variations of temperature, humidity, and air, movement was studied. Combinations of temperatures, humidities, and velocities, which produced the same feeling of warmth were assigned the same effective temperature value. If the temperature,'humidity, and air movement are known, the effectivetemperatore can be read from Fig. 7. The choice of chart to be-used 'will depend upon whether'the mien ore normally clothed or stripped to the waist. In the presence of radiant heat, as .when, the globe temperature exceeds the ambient temperature by more than 2 F deg, the globetom- peratore should be-used instead of the dry-bulb temperature in computing the ET from Fig. 7. According to this scale, an' ET,of 78' F represents the'threshold of sweating, while' an ET of 90 F marks the upper.limit for continuous exposure of heat-acclimatized men engaged in light, activities. The' upper permissible limit for moderately hard work is an ET, of 85 F, and for,`heavy work, 80 F. In hot spaces of Naval ships (underway), an EX. of 91 F is well tolerated during the usual 4-hr watches. Limitations of Effective Temperature , ` The ET index h most applicable to .warm atmospheres when ridiation effects are not significant. This scale makes too much allowance for humidity at low temperatures and not enough allowance at high temperatures. In the. presence- of radiant heat, significant errors are reported if the ET index isused. Bedford*suggested that'this can be corrected by using an unbftfttflri globe thermometer, instead of the air tempera- Fig. 9 i ... Physiological .Effects of Environment on . Two Qoihed; Men -Walking at 2.8 mph on the Level (MR 125 Cal/(M*) (hr) . theWboratcwy at various temperatures and humidities, with a constant air movement of-180 fpm. Contour curves, representLogiKnes of equal- philological strain, were plotted on charts (figs.8,9, and 10). The chart to be used will depend upon the mptaWdic rate invoived.' If the dry-bulb and wet-bulb tem peratures and the approximate metabolic rate are-known, and provided the subjects are dressed in hot weather clothing, it's possible to estimate-#? values in the range of 50 to 400. Thermal1 equilibrium is possible when the Ep value is under 200; heat begins to accumulate in the body when Ep exceeds 250/and conditions become intolerable with Ep values of over 400.-Fp'has 'certain distinct'advantages and disadvantages, the Predicted Four-Hour Sweat Rate (P4SR)S7 . 7 The predicted four-hour sweat rate uses only the rate of creating as a criterion of heat stres in environments that are hot gnpugK to cause sweating. On the basis of British' experi mented'work, empirical nomograms have been developed for predicting'the probable amount of sweat in liters that would be secreted over a 4-hour' period by fit, acclimatized men. These nomograms incorporated environmental factors, meta bolic rate','add the amount of clothing worn. A P4SR of 4.5 titers was provisionally adopted as the upper limit of tolerance for physically fit' men. On the basis of this value, leas revere conditions can be evaluated. This method, of determining thermal stress is difficult to apply in the field and is' primarily a research tool The Beldmg-HatdvHeat Stress Index (HSI)*8 ...The Belding-Hatch index expresses the heat load in terms of the amount of sweat which must be evaporated in order.to 109 Table 3 .. Estimates ofEnergy Metabolism (M) of Various Types of Activity (Voim* apply ^ e 1S4 b Dan, and do not Mdi rod paamij ' XU of Work AdMf M tin/hr Sleeping. . .'........... .................................... Sitting quietly............................................. 250 400 Light ' Work' '' ; *' T '*** ` Sitting, moderate arm and trunk movements.............................................. ...... (e.g., desk work, typing) Sitting, moderate arm ' and leg movements. .......................... ............ ' (e.g., playing organ, driving car in traffic) Standing, light work at machine or bench, mostly arms................................ 459-550 550-650 559-650 Moderate" Sitting, heavy arm and leg movements. 659-800 Work Standing, light work at machine or bench, some walking about............. 650-750 Standing, moderate work at machine or bench, some walking about............ 759-1000 ` Walking about, with moderate li/tingor pushing........................................ .............. 1000-1400 Heavy Work : . -s Intermittent heavy lifting, pushing or pulling........................................................ 1500-2000 (e.g., pick and shovel work)'- Hardest sustained work.:........................ 2000-2400 maintain' heat balance at ah arbitrarily assumed skin tem perature of 95 F. The evaporation rate required is estimated from the'metabolic rate and body heat loss by radiation and convectibn,: using empirical "equations dr nomographs. By comparing this rate with'the maximum evaporative capacity of the atmosphere on. a man whose skin is at a temperature of 95 F; and is completely wet with perforation, it is posable to evaluate the heat stress and resulting strain by the difference between' the rates or by their ratio. Although this index may have future possibilities with new facta coming to light, it, too, is primarily a research tool at present.. Tables 3 and 4, and Fig.. 11, are included for informational purposes. It should be noted that, several factors- incorporated- in this, index are currently, under, study, and may .need modification (convec tion factor, evaporation factor).......... , esTVIRGNMENTAL CONDITIONS AND COMFORT, As h&S'been previously indicated, changes in body-tem perature' or pulse rate provide useful scales for evaluating the effects of thermal stress on the human body. However, there isho1precise physiological observation by which'comfort can be evaluated. Within the comfort sone, or the zone of thermal neutrality (Table 5), the body is able to maintain a balance between'heat production and: heat loss without significant changes in any of the readily-measurable indices which have been;tried:'Mean skin'temperature was at one time thought to offer"some promise, but this, too, has proven to be an unreliable index. '