MAINTAINABILITY DESIGN TECHNIQUES METRIC - AcqNotes.com

Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)10-3.1.1.2 Wet-Bulb Global TemperatureFor military personnel who must work outside in hotclimates in ranges beyond the comfort and discomfortzone of heat stress—the Wet-Bulb Global Temperature(WBGT) Index is more applicable than the ET Index. TheWBGT Index takes into consideration dry-bulb temperature;relative humidity calculated with ambient airmovement, rather than at a standardized rate; and thesolar, or radiant, heat load. This index and its use aredescribed in Ref. 11. The WBGT is calculated as follows(Ref. 7):Generally, the activities of unacclimatized individuals arerestricted when the WBGI exceeds 25°C (77°F) (Ref. 7).See Refs. 7 and 12 relative to the WBGT Index and limitsof heat exposure.Figure 10-1. Error Increase Due to Rise inEffective Temperature10-3.1.1.1 Effective TemperatureThe effective temperature (ET) of an environment is anempirical thermal index that considers how combinationsof dry-bulb air temperature, humidity, air movement, andclothing affect people (Ref. 7), Numerically, the ET isequal to the temperature of still saturated air, whichwould give the same sensation. The effective temperaturemay be read from Fig. 10-2 (Ref. 7).The optimum temperatures for personnel vary accordingto the nature of the tasks performed, the conditionsunder which the tasks are performed, and the clothing thepersonnel are wearing. For maximum physical comfortwhile normally dressed appropriate to the season orclimate, the optimum range of effective temperature foraccomplishing light work is (Ref. 7)1. 21° to 27°C (70° to 81°F) in a warm climate orduring summer2. 18° to 24°C (64° to 75°F) in a colder climate orduring winter.Fig. 10-3 (Ref. 7) indicates summer and winter comfortzones and thermal tolerances in hours for inhabited areas.10-3.1.1.3 Windchill IndexNo general index, such as effective temperature, isavailable for expressing all the factors involved in coldexposure, but the Windchill Index commonly is used toexpress the severity of cold environments. Althoughwindchill is not based on physical cooling and is probablynot very accurate as an expression of human cooling, ithas come into use as a single-value, practical guide to theseverity of temperature-wind combinations. Fig. 10-4(Ref. 7) portrays a windchill chart. Note, for example.-40°C (-40° F) with air movement of 0.1 m/s (0.3 ft/s) (seeLine A) has the same windchill value and, therefore, thessme sensation on exposed skin—as-24°C (-13°F) with a0.5-m/s (1.5-ft/s) wind (see Line B). The Windchill Indexdoes not account for physiological adaptations or adjustments,however, and should not be used rigorously. Aqualitative description of human reaction to windchillvalues on exposed skin is shown in Table 10-5 (Ref. 7).10-3.1.1.4 Guidelines for MaintenanceOperations in Extreme ClimatesGuidelines for operations in a hot climate follow:1. Where possible, provide air conditioning if temperaturesexceed 32°C (90°F). Proper ventilation shouldbe provided in equipment trailers or other locationswhere personnel are monitoring, servicing, or performingother maintenance tasks.10-6