The Effect of Thermal Factors on the Measurement of Wet Bulb Globe Temperature

Transcription

1 The Effect of Thermal Factors on the Measurement of Wet Bulb Globe Temperature Research Articles The Effect of Thermal Factors on the Measurement of Wet Bulb Globe Temperature Yow-Jer Juang 1,2, Yi-Chang Lin 1,3 1 Institute of Environmental Health, College of Public Health, National Taiwan University 2 Department of Occupational Safety and Health, College of Human Science and Technology, Chung Hwa University of Medical Technology 3 Department of Environmental and Occupational Health, Toko University Abstract The wet bulb globe temperature (WBGT) can be evaluated by measuring the globe temperature (Tg), the natural wet-bulb temperature (Tnwb), and the air temperature (Ta, only with solar radiation outdoors) to provide a fast and useful index of heat stress in hot environments. For the past few decades, the WBGT has been used as the standard for protecting workers in hot environments in Taiwan. The aim of this paper is to investigate measurement errors in Tnwb and Tg. All the tests were conducted in a climate chamber where the environmental factors (Ta, RH, Va) were controlled in order to study the effect of globe size and radiator temperature (Tp) on Tg, Tnwb, and WBGT. A greater globe size yielded higher Tg measurements for different Va and Tp with significant radiation. The response time of Tg increased with globe size and decreased with air velocity for all test conditions with radiant heat. The measured values of Tnwb under all experimental conditions was greater than the psychrometric wet bulb temperature (Tpwb) estimated by the ambient vapor pressure and air temperature in different combined conditions. Air velocity (Va) is critical for the globe temperature and natural wet-bulb temperature measurement, which can cause errors in heat stress evaluation when the air temperature is greater than skin temperature with significant radiant heat. Key words: Heat stress evaluation, Globe temperature, Natural wet bulb temperature, WBGT index Accepted 25 July, 2007 *Correspondence to: Yi-Chang, Lin, Department of Environmental and Occupational Health, Toko University. No. 51, Sec. 2 University Road, Pu-Tzu City, Chia-Yi County, 613, Taiwan R.O.C. Tel: Ext. 389, Fax: , yichang@mail.toko.edu.tw 191

2 Journal of Occupational Safety and Health 15: (2007) Introduction A detailed analysis of the influence of the environment on heat stress requires the following four basic parameters: air temperature (Ta), mean radiant temperature (Tmrt), air velocity (Va), and humidity [1]. The most common method involves the wet bulb globe temperature (WBGT) index or a direct-reading meter, which is commercially available. The WBGT was originally proposed by Yaglou and Minard [2] to serve as a substitute for corrected effective temperature (CET). The success of the WBGT is due in part to its simplicity and the fact that it is not necessary to measure air velocity [3]. Although WBGT is not a complete calculation for the many environmental and physical factors inf luencing heat strain, it provides useful guidelines for protecting people who work or exercise in hot environments. The WBGT index combines the measurement of two derived parameters, the natural wet-bulb temperature (Tnwb) and the globe temperature (Tg), with and air temperature outdoors under solar radiation (Ta). In the presence of radiation, Tg integrates the effects of air velocity, air temperature, and radiant heat. The natural wet bulb value is the same as that of the psychrometric wet bulb at high air velocities but at low velocities may be up to 5 C higher, depending on the air temperature, vapor pressure, and radiant heat [4]. The WBGT values were calculated using one of the following equations: 1). Indoors or outdoors without solar load: Table 1 Recommendations for thermometry of WBGT index in different organizations Measurement/ Characteristic ACGIH OSHA NIOSH ISO Dry bulb temperature Sensor Any sensor protected from radiation without impeding air flow Range -5 to 50 C -5 to 50 C to 60 C Accuracy ±0.5 C ±0.5 C -- ±1 C Natural wet bulb temperature Sensor Cylindrical; 30(±5) 6(±1) mm Range -5 to 50 C -5 to 50 C -1.1 to 48.9 C (30~120 F) 5 to 40 C Accuracy ±0.5 C ±0.5 C ±0.5 F ±0.5 C Wick material Highly absorbent (e.g., cotton) Sensor cover The wick should extend over the bulb of the thermometer, covering the stem about one additional bulb length Wick wetting Directly; 30 min before reading Stabilizing period 25 min minimum ½ hour 20 min -- Globe temperature Sensor Range -5 to 100 C -5 to 100 C -1.1 to C (30~220 F) 20 to 120 C Accuracy ±0.5 C ±0.5 C ±1 F ±0.5 C (20-50 C) ±1 C ( C) Globe diameter 15 cm 15 cm 6-inch (15 cm) 150 mm Globe thickness Thin as possible Emission coefficient Globe material Copper painted matt black Copper painted matt black Copper painted matt black Matt black Stabilizing period 25 min minimum 25 min minimum 20 min -- --: not available 192

3 The Effect of Thermal Factors on the Measurement of Wet Bulb Globe Temperature WBGT = 0.7Tnwb + 0.3Tg Equation (1) 2). Outdoors with solar load: WBGT = 0.7Tnwb + 0.2Tg + 0.1Ta Equation (2) The WBGT index has been used as the standard for protecting workers in hot environments in Taiwan for the past few decades [5]. In recent times, international and technical agencies, such as ISO[1], NIOSH[6], ACGIH[7], and OSHA[8], have introduced several modifications to the WBGT when evaluating heat stress. The NIOSH adopts a ceiling level, the ISO recommends measuring five different levels of workload, and the ACGIH proposes limits for the work/rest regimen in the same place and in cooler locations. OSHA developed these WBGT standards or reference values and recommended evaluating heat stress for three different workload levels at two different air velocity ranges. Table 1 contains the recommendations for WBGT thermometry made by these organizations. WBGT instruments are commercially available from a number of manufacturers, but some misrepresent the WBGT, using a psychrometric rather than the natural wet bulb thermometer; others use smaller globes or unconventional radiant heat sensors, with little validation that the adjustment (if any) made in calculating their WBGT is acceptable. To measure the W BGT index of an environment, a dry-bulb, natural wet-bulb, and globe thermometer are required. Air velocity readings are not required as the effects of air movement are ref lected in the natural wet bulb temperature. All of these instruments are described as follows: 1. Dry-bulb thermometer The air temperature is measured by an ordinary alcohol-in-glass or mercury-inglass thermometer whose bulb is kept dry and shielded from radiation. A variety of electronic sensors can be used in place of conventional thermometers; if properly constructed some of these (e.g., thermocouples and thermistors) may require comparatively little shielding from radiant heat transfer. 2. Natural wet-bulb thermometer The natural wet-bulb thermometer, which is covered by a wetted cotton wick and exposed only to naturally prevailing air movement, is used to obtain the equilibrium temperature (Tnwb). When used to calculate the WBGT, it is generally not shielded from radiation [8-14]. NIOSH [6], however, recommends shielding natural wet bulb thermometers when measuring the WBGT. Based on the results of Lee [15], radiant heat and air velocity affect measurements of Tnwb, and the relationship between shielded and unshielded Tnwb is a function of air velocity and radiant heat. Tnwb readings have been the source of some inaccuracy in the past due to misuse of instruments, but the most common errors (drying of the wick or positioning of the thermometer stem in the water reservoir) lead to falsely high readings and are therefore not 193

4 Journal of Occupational Safety and Health 15: (2007) dangerous [2,16]. 3. Globe thermometer The thermometer consists of a black globe in which is placed a temperature sensor such as the bulb of a mercury thermometer, a thermocouple, or a resistance probe. Globe thermometers with liquid-in-glass thermometers, for diameters as large as 15 cm, require about 20 minutes to reach equilibrium at different air velocities. Smaller globes, from 2-10 cm outside diameter, have been developed to shorten the equilibrium times [17-19]. The difference between the standard and smaller globes is small in indoor measurements related to thermal comfort without significant radiation [17]. The globe temperature is equal to air temperature without radiant heat and does not change with humidity. The difference between small and standard globes becomes considerable at high air velocities and there are greater differences between the dry bulb and globe temperatures when a radiant heat load is applied [20]. The mean radiant temperature (Tmrt) can be calculated according to measurements of the air temperature, air velocity, and globe temperature at different diameters using the following equations [21]: Equation (3) In the case of the standard globe, d = 0.15 m, ε= 0.95 (matt black paint) and Equation (3) becomes Equation (4) Much of the literature, however, uses the simplified formula in Equation (5) to predict Tmrt with a K value in the range of [6, 9, 12-13,15,22-26]: Equation (5) The WBGT index has been applied to heat stress evaluation for almost 50 years. There is a strong correlation between the WBGT and the physiological strain on the worker in a hot workplace. The present study investigates the effect of globe sizes, radiator temperature (Tp), Va, and response time on Tg and Tnwb as well as WBGT in a climate chamber, where the environmental factors are well controlled. Materials and Methods All the mercury-in-glass thermometers and thermocouples in this study were calibrated against an NML (National Measurement Laboratory, Hsinchu, Taiwan) primary standard thermometer (Amarell, Germany; measuring range -10~50 C) using the water bath method with various changes in water temperature. Air velocity was measured by a hot bulb probe (TESTO, Germany) 20 cm from the thermometers or instruments. The covering and wick of Tnwb was boiled for approximately 15 minutes in an aqueous solution of approximately 5% (m/m) sodium carbonate dehydrate and then 194

5 The Effect of Thermal Factors on the Measurement of Wet Bulb Globe Temperature thoroughly rinsed in pure water before being boiled in pure water for at least 15 minutes [27]. The Tnwb was measured according to ISO 7243 [1]. The covering of the Tnwb thermometer was made from hydrophilic undressed white cotton thread of linear density between 10 and 25 tex [27]. The covering extended about 30 mm onto the stem to avoid error in the observed Tnwb due to heat conduction along the stem [28]. Conducted in the climate chamber where the environmental factors (Ta, Va, RH and Tp) were well controlled, this study investigates the effect of globe size, radiator temperature (Tp), Va, and response time on Tg and Tnwb as well as WBGT. Tp was simulated by a radiator (50cm 50cm 7cm) with a temperature thermostat controlled to ± 2 C from room temperature to 700 C. This study consisted of three trials. The first trial was to study the response time of two commercial globes (QUEST, USA) with outside diameters of 15 cm and 5 cm placed 40 cm from the radiator for a specified condition (Ta = 40 C; RH = 40%; Tp= 600 C) with four levels of air velocity (Va=0.499, 1.046, 1.389, m/s) and equilibrium temperatures of Tg and Tg(5), respectively. The globe was kept in an air-conditioned room with the room temperature at 25 C for about 30 minutes before being moved into the climate chamber. The second trial studied the equilibrium temperatures of Tg and Tg(5) at different Tp ( 300~600 C ) for a specified condition (Ta = 40 C; RH= 50 %; Va= m/s). The third trial compared measurements of Tg, Tnwb in 81 experimental conditions with different air temperatures (Ta= 27.5 C, 32.5 C, 37.5 C), air velocities (Va= 0.65 m/s, 1.30 m/s, 2.60 m/s), radiator temperatures (no radiant heat, Tp=300 C, 500 C) and relative humidities (RH= 30%, 50%, 70%, 85%). Visual observations were made on the instruments after reaching a stable condition (about 10 min) at regular intervals (every two minutes, n=15) and an automated data collection system was also used to obtain data from thermocouples attached to the instruments 45 cm away from the radiator. First Tnwb was read, then Ta and Tnwb again for confirmation [29]. All the Tnwb readings were measured by a primary standard thermometer, which was unshielded from radiation. Results First Trial: Effects of globe size and air velocity The temperature change and response time of different globe sizes at four air velocities for a specified condition (Ta=40 C, Tp=600 C, and RH=40%) are shown in Figure 1. The equilibrium temperature readings of Tg at different air velocities (0.499~2.124 m/s) were measured with Ta, RH, Tp, and distance from radiator (40 cm) kept constant. The response time of Tg depends on Va and globe size. The response time of Tg increased with globe size for all velocities. The minimum time lag in reaching equilibrium with its environment should be 195

6 Journal of Occupational Safety and Health 15: (2007) less than 15 min for the standard globe. It was found that a greater globe size yielded higher Tg measurements for different Va and Tp with significant radiation (Tp=600 C). The results show that the Va is critical for smaller globes. The temperature of small globes decreases at higher air velocities. Second Trial: Effect of radiant heat load Figure 2 shows the effect of radiator Figure 1 The temperature change and response time of different globe sizes at four various air velocities for a specified condition (Ta= 40 C, Tp= 600 C, and RH= 40%). Figure 2 The effect of radiator temperature and globe size on globe temperature for a specified condition (Ta=40 C; RH=50%;Va=0.423 m/s) 196

7 The Effect of Thermal Factors on the Measurement of Wet Bulb Globe Temperature temperature and globe size on globe temperature with significant radiant heat at low air velocity. Larger Tp and globe size yield higher Tg. All measurements of Tg at different globe sizes increased with greater Tp for the specified experimental condition (Ta = 40 C; RH= 50%; Va= m/s). The difference between Tg and Tg(5) at Tp=600 C will be above 2 C. Third Trial: Tnwb, Tg, and WBGT under 81 experimental conditions The result of Tnwb measurements in different experimental conditions was shown in Table 2. It indicted that Tp, RH, Va, and Ta have significant effects on Tnwb measurements. Tnwb increased as higher RH and Tp and Ta increasing and lower Va have a higher Tnwb in the same Ta and Tp. All Tnwb measurements in this study are greater than Tpwb which calculated by psychrometric calculator program (HCON, General Eastern's Humidity Conversion program for Windows) for air velocity less than 2.60 m/s. The result of Tg measurements was Table 2 Evaluation of natural wet bulb temperature (Tnwb a ) under different experimental conditions Ta Va RH Tpwb b Radiator Temperature ( o C) (m/s) (%) ( o C) No Radiant Heat Tp=300 o C Tp=500 o C (0.30) 20.80(0.11) (0.20) 21.17(0.29) 24.82(0.18) (0.54) 25.40(0.43) 26.53(0.28) (0.22) (0.14) 19.00(0.20) (0.09) 20.61(0.52) 22.82(0.08) (0.21) 24.04(0.32) 25.32(0.12) (0.19) (0.21) 17.37(0.40) (0.30) 20.23(0.36) 21.45(0.14) (0.33) 23.92(0.18) 24.59(0.18) (0.21) (0.35) (0.34) 25.71(0.57) 27.03(0.28) (0.19) 29.60(0.26) 31.89(0.32) (0.04) 32.05(0.07) (0.36) (0.40) 24.59(0.20) 26.22(0.51) (0.21) 28.77(0.31) 30.34(0.29) (0.06) 31.31(0.18) (0.31) (0.72) 24.33(0.39) 25.91(0.13) (0.22) 28.51(0.23) 29.01(0.29) (0.50) 30.72(0.29) (0.63) (0.59) 30.04(0.60) 31.70(0.21) (0.80) 33.49(0.27) 35.63(0.37) (0.45) 36.57(0.12) (0.56) (0.40) 29.30(0.29) 30.94(0.15) (0.11) 32.90(0.19) 34.42(0.11) (0.09) 35.57(0.23) (0.25) (0.14) 28.56(0.40) 29.57(0.12) (0.97) 32.60(0.30) 33.14(0.37) (0.21) 35.12(0.43) -- a: All values expressed as mean ± SD (n=15). b: Psychrometric wet bulb temperature (Tpwb) calculated by HCON Humidity Conversion Program (General Eastern). --: not available 197

8 Journal of Occupational Safety and Health 15: (2007) shown in Table 3. Tg was almost equal to Ta for experiments with no radiant heat. The Tg values at Tp = 500 C was always greater than at Tp = 300 C or with no radiant heat, holding Ta, Va, and RH constant. The difference in the Tg measurement decreased with increasing Va for the same Tp, Ta, and RH. The WBGT measurements calculated based on Equation (1) from the mean values of Tnwb and Tg in 81 experimental conditions are shown in Table 4. The WBGT values in this study represent moderately to extremely hot environments (WBGT: C) in Taiwan. Discussion This paper compares Tnwb and Tg to evaluate the WBGT index. Normally the instrumentation described in the measurement procedure include a standard globe with a 15 cm diameter, but today many prefer to use electronic monitors that respond more rapidly. Globe thermometers have a longer response time than dry bulb thermometers, so they Table 3 Evaluation of globe temperature (Tg a ) under different experimental conditions Ta Va RH Radiator Temperature ( o C) (m/s) (%) No Radiant Heat Tp=300 o C Tp=500 o C (0.13) 41.27(0.36) (0.09) 31.06(0.25) 40.47(0.22) (0.23) 32.79(0.47) 40.34(0.19) (0.22) (0.25) 34.96(0.22) (0.10) 29.62(0.26) 35.13(0.11) (0.21) 31.42(0.41) 35.17(0.35) (0.11) (0.13) 32.09(0.19) (0.49) 30.12(0.16) 32.28(0.20) (0.54) 30.34(0.16) 32.57(0.24) (0.36) (0.31) (0.17) 35.81(0.15) 43.62(0.38) (0.14) 37.69(0.17) 45.26(0.84) (0.13) 37.81(0.57) (0.11) (0.43) 34.57(0.10) 40.17(0.21) (0.19) 36.06(0.23) 40.69(0.33) (0.07) 36.00(0.41) (0.15) (0.33) 33.78(0.18) 37.48(0.18) (0.58) 35.32(0.22) 37.41(0.11) (0.77) 35.40(0.19) (0.35) (0.23) 42.37(0.58) 48.04(0.20) (0.17) 42.03(0.50) 50.83(0.92) (0.16) 42.35(0.34) (0.21) (0.22) 40.91(0.24) 45.55(0.33) (0.05) 40.87(0.28) 45.38(0.20) (0.00) 40.93(0.25) (0.10) (0.36) 40.64(0.24) 42.35(0.33) (0.31) 40.42(0.27) 42.42(0.20) (0.33) 40.00(0.25) -- a: All values expressed as mean ± SD (n=15). --: not available 198

9 The Effect of Thermal Factors on the Measurement of Wet Bulb Globe Temperature cannot assess the heat stress of non-steady state environments. Smaller globes respond faster than standard ones because there is less air volume within the hollow copper globe [30]. Smaller globes have shorter response times but also lead to lower readings at high radiation. The regression analysis of Tmrt calculated from ISO 7726 (Tmrt_ISO) and other studies (Tmrt_predicted) based on experimental data from this study is shown in Table 5. The best fit K value in simplified equation ( regression slope = 1; r = 0.996) is equal to 1.86 for Tmrt_ISO in the range of C (Tmrt_predicted=Tg+1.86 (Tg Ta) Va 0.5 ). The difference between smaller and standard globes becomes considerable and corrections are needed at high Va and when there are large differences between Ta and Tg (e.g. outdoor work in the sun and in some metal industries). When using a smaller globe temperature to predict Tmrt by Equation (3), measurements of Ta and Va are required. Smaller globes are more appropriate when fast response times are needed. It should be noted that the smaller the diameter of the globe, the Table 4 Evaluation of WBGT a index under different environmental conditions Ta Va RH Radiator Temperature ( o C) (m/s) (%) No Radiant Heat Tp=300 o C Tp=500 o C a: All WBGT values were calculated from Table 2 and 3 according to the following equation: WBGT=0.7Tnwb+0.3Tg. --: non-available 199

10 Journal of Occupational Safety and Health 15: (2007) greater the effect of Ta and Va, reducing the accuracy of the Tmrt measurement. For values of Tmrt predicted by smaller globes, Equation (6) can be applied to find the equivalent Tg of a standard globe. Equation (6) Tnwb is related to the rate of evaporation of the sensor through convection and radiation. The difference between Tnwb and Tpwb at lower air velocities increases with Tp, but this trend is reversed with increasing RH. For conditions at high air velocity (Va 2.60 m/s), high humidity (RH = 85%), and without radiant heat, Tnwb Table 5 Regression analysis of Tmrt calculated from ISO 7726 (Tmrt_ISO) and other studies (Tmrt_predicted) based on experimental data in this study Tmrt_predicted Equation Linear Regression Equation Correlation coefficient Author(s) Tg (Tg - Ta) Va 0.5 y = 1.153x r = Bedford & Warner, 1934 Tg (Tg - Ta) Va 0.5 y = 0.511x r = Givoni, 1969 Tg (Tg - Ta) Va 0.5 y = 4.779x r = WHO, 1969 Tg (Tg - Ta) Va 0.5 y = 0.990x r = AIHA, 1975 Tg (Tg - Ta) Va 0.5 y = 1.174x r = McIntyre, 1980 Tg (Tg - Ta) Va 0.5 y = 0.981x r = NIOSH, 1986 Tg (Tg - Ta) Va 0.5 y = 1.123x r = Lee, 1986 Tg (Tg - Ta) Va 0.5 y = 1.192x r = Brotherhood, 1987 Tg (Tg - Ta) Va 0.5 y = 1.102x r = Allan, 1989 Tg (Tg - Ta) Va 0.5 y = 0.505x r = Epstein & Moran, 2006 Tg (Tg - Ta) Va 0.5 y = x r = Proposed in this study * Tmrt_ ISO= (Tg + 273) Va 0.6 (Tg - Ta) ( ISO7726, 1998-in forced convection; d = 0.15 m ) is almost equal to Tpwb. Measurements of Tnwb should be treated carefully in heat stress evaluation with radiant heat loads such as a steel plant or outdoor workers. High readings of Tnwb will result due to drying of the wick in these extremely hot dry environments (RH<30%). For air temperatures (Ta 37.5 C) greater than skin temperature, with or without radiation, increasing Va causes more heat gain for the exposed worker. It is necessary to interpret carefully the heat transfer between the human body and its surrounding environments. Because air velocity is critical for the measurement of Tg and Tnwb, there may be errors in the heat stress evaluation of extremely hot environments when Ta is greater than skin temperature with significant radiant heat. In the case of high Va (> 2.60 m/s) and low RH (<30%), it will cause the overestimation of Tnwb and WBGT. References [1] ISO 7243: Hot environments- estimation of the heat stress on working man, based on the WBGT- index (Wet Bulb Globe Temperature). Geneva: ISO; [2] Yaglou CP, Minard CD. Control of heat casualties at military training centers. AMA Arch Ind Health 1957; 16: [3] Kerslake D. The stress of hot environments. 200

11 The Effect of Thermal Factors on the Measurement of Wet Bulb Globe Temperature Cambridge, Cambridge University Press; [4] Minard D, Goldsmith R, Far rier PH, Lambiotte BJ. Physological evaluation of industrial heat stress. Am Ind Hyg Assoc J 1971; 32: [5] Counsel of Labor Affair (CLA). Standard of work-rest regimen for worker in hot workplace. Taipei: CLA; [6] Nat ional I nst it ute for Occupat ional Safety and Health (NIOSH). Criteria for a recommended standard: occupational exposure to hot environments. Revised criteria USDHHS (NIOSH) , Washington, DC; [7] American Conference of Governmental Industrial Hygienists (ACGIH). Threshold Limit Values for chemical substances and physical agents. Cincinnati: ACGIH; [8] O c c u p a t i o n a l S a f e t y a n d H e a l t h Administration (OSHA). Heat stress. In: OSHA Technical Manual, 4th ed. Government Institutes Inc; [9] American Industrial Hygiene Association (AIHA). Heating and cooling for man in industry. 2nd ed. AIHA; [10] Ramsey JD. Heat stress standard: OSHA s advisory committee recommendations. National Safety News, 1975; 6: [11] Astrand IAO, Erikson, U, Olander L. Heat stress in occupational work. AMBIO 1975; 4: [12] Brotherhood JR. The practical assessment of heat stress. In: Hales JRS, Richards DAB, editors. Heat stress: physical exertion and environment. Elsevier Science Publishers; p [13] Allan JR. Thermal stresses in occupations. In: Waldron HA, editor. Occupational health practice. 3rd ed. London: Butterworth- Heinemann Ltd;1989. [14] Davis WJ. Typical WBGT indices in various industrial environments. ASHRAE Transactions 1976; 82: [15] Lee CH. Effects of wick contamination and thermal component variation on thermal indices [dissertation]. Texas: Texas Tech Univ.; [16] Buonanno G, Frattolillo A, Vanoli L. Direct and indirect measurement of WBGT index in transversal flow. Measurement 2001; 29: [17] Humphreys MA. The optimum diameter for a globe thermometer for use indoors. Ann Occup Hyg 1977; 20: [18] Graves KW. Globe thermometer evaluation. Am Ind Hyg Assoc J 1974; 35: [19] Hey EN. Small globe thermometers. J Sci Instru 1968; 1: [20] Hatch TF. Design requirements and limitations of a single-reading heat stress meter. Am Ind Hyg Assoc J 1973; 34: [21] ISO 7726: Ergonomics of the thermal environment- instruments for measuring physical quantities. Geneva: ISO, [22] Epstein Y, Moran D. Thermal comfort and 201

12 Journal of Occupational Safety and Health 15: (2007) the heat stress indices. Ind Health 2006; 44: [23] Givoni B. Man, climate and architecture. 2nd ed. New York: Van Nostrand Reinhold Company Ltd; [24] B e d fo r d T, Wa r n e r B. T h e g l o b e thermometer in studies of heating and ventilation. J Hyg 1934; 34: [25] McIntyre DA. Indoor climate. London: Applied Science Publishers Ltd;1980. [26] World Health Organization (WHO). Health factors involved in working under conditions of heat stress. Technical Report Series No. 412, Geneva;1969. [27] ISO 4677/1: Atmospheres for conditioning and testing- Determination of relative humidity- Part 1: Aspirated psychrometer method. Geneva: ISO, [28] Malchaire JB. Evaluation of natural wet bulb and wet globe thermometers. Ann Occup Hyg 1976; 19: [29] Japanese Industrial Standard, JIS Z 8806: Humidity- Measurement methods. Tokyo, JIS, [30] Ramsey J D. Practical evaluation of hot working areas. Professional Safety 1987; 32: Appendix List of Symbols and Abbreviations Symbol Term Unit/Value ACGIH AIHA NIOSH OSHA CLA ISO American Conference of Governmental Industrial Hygienists American Industrial Hygiene Association National Institute for Occupational Safety and Health Occupational Safety and Health Administration Counsel of Labor Affairs (in Taiwan) International Organization for Standardization Tg standard globe temperature with 15 cm outer diameter globe ºC Tg(5) globe temperature with 5 cm outer diameter globe ºC Tnwb natural wet bulb temperature ºC Ta air temperature or dry bulb temperature ºC Tp radiator temperature or hot plate temperature ºC Tpwb psychrometric wet bulb temperature ºC Tmrt mean radiant temperature ºC Tmrt_ISO mean radiant temperature calculated according to ISO 7726 ºC Tmrt_k mean radiant temperature predicted based on simplified equation ºC RH relative humidity % Va air velocity m/s WBGT wet bulb globe temperature index ºC CET corrected effective temperature ºC ε emissivity of a black globe Dimensionless ( 0.95) σ Stefan-Boltzmann constant Wm -2 K -4 d diameter of globe m k coefficient constant based on globe size dimensionless convection heat transfer coefficient Wm -2 K

13 綜合溫度熱指數測定評估影響因素之探討 綜合溫度熱指數測定評估影響因素之探討 1,2 莊侑哲 1,3 林宜長 國立台灣大學公共衛生學院環境衛生研究所中華醫事科技大學民生與科技學院職業安全衛生系稻江科技暨管理學院環境暨職業衛生學系 摘 要 綜合溫度熱指數 (WBGT) 是由自然濕球溫度 黑球溫度與乾球溫度 ( 只有在戶外有日曬時需採用 ) 之測值計算求得, 可提供熱環境快速且有用的熱壓力評估指標, 並已被我國採用當作高溫作業判定之基準 本研究的目的係利用可以控制環境參數之熱暴露艙並搭配架設可控制表面溫度之加熱板來模擬輻射熱源, 藉以探討影響自然濕球溫度與黑球溫度測定誤差的因素 首先評估不同的黑球尺寸 輻射溫度與環境參數 ( 氣溫 風速與相對濕度 ) 對黑球溫度測值之影響及評估其回應時間, 其次則是評估在不同氣溫 風速 相對濕度與輻射溫度條件下之自然濕球溫度測值變化情形 研究結果發現, 當有輻射熱源存在時, 黑球的尺寸與輻射溫度愈大其黑球溫度測值也愈大, 並且隨著黑球尺寸的加大, 其回應時間也愈長 ; 而風速愈大則將使其測值變小, 如僅有相對濕度改變時則對黑球溫度測值無影響 在風速小於 2.60 m/s 之所有測試條件下, 自然濕球溫度均較通風濕球溫度測值大 ; 而且自然濕球溫度測值隨氣溫 相對濕度與輻射溫度增加而增加, 但隨風速增加而降低 當熱環境的氣溫大於皮膚溫度並伴隨有明顯的輻射熱源存在時, 則風速愈大會造成自然濕球溫度 黑球溫度測值與綜合溫度熱指數愈低而導致熱壓力評估的判斷錯誤 因此, 建議在採用綜合溫度熱指數當作熱壓力評估指標時, 氣溫大於皮膚溫度時需將風速測定列入考慮 關鍵字 : 熱壓力評估 黑球溫度 自然濕球溫度 綜合溫度熱指數 民國 95 年 5 月 25 日收稿,96 年 6 月 21 日修稿,96 年 7 月 25 日接受 通訊作者 : 林宜長, 稻江科技暨管理學院環境暨職業衛生學系,613 嘉義縣朴子市大學路二段 51 號, yichang@mail.toko.edu.tw 203