A convenient handheld WBGT meter tells you thermal conditions around you and “protects” you from heat stroke while you are in sporting or working activities.
The WBGT index is specified by ISO-7243, which defines the following formula based on Ta (natural dry-bulb Temperature), Tnw (natural wet-bulb temperature ) and Tg (Global Temperature):
Outdoor under sunlight: WBGT = 0.7Tnw+0.2Tg+0.1Ta
Indoor without sunlight: WBGT = 0.7Tnw+0.3Tg
The dry bulb temperature is measured by the built in thermistor and the wet-bulb temperature is obtained by calculating RH and Ta. The Tg is converted value from 24mm-diameter black hemisphere to the standard 150 mm-black bulb temperature.
For more information on WBGT meters, see “Wet Bulb Globe Temperature Meter (WBGT8758)” (http://inspectusa.com/bulb-globe-temperature-meter-wbgt8758-40mm-black-globe-p-713.html) . (Edited in 2011 to reflect new source of device)
NOTE ADDED IN EDITING OF THIS SUBMISSION:
A few years back I was in US Army training at Ft. Sam Houston Texas during the summer. The big item on the assembly/exercise quad was a WBGT device with a 24″ diameter black hemisphere that was a forerunner of this device mentioned here.
One morning during strenuous calisthenics a medical officer came out and forced the training cadre to cease all physical exercise because the WBGT index was excessive. Needless to say, it may have saved more than a few of us from heat stroke that day.
The natural question is: What do the Index values mean or how does one use them?
Here’s what an official US Army publications says about the index values and their meaning:
“3. Use of the WBGT Index in Control of Physical Activity.
“The proponents of the WBGT Index have proposed the following as a standard for application of the Index. IT MUST BE EMPHASIZED that the measurements must be taken in a location which is the same as, or closely approximates, the environment to which personnel are exposed.
“a. When the WBGT index reaches 78 ??F (26 ??C), extremely intense physical exertion may precipitate heat exhaustion or heat stroke; therefore, caution should be taken.
“b. When the WBGT index reaches 82 ??F (28 ??C), discretion should be used in planning heavy exercise for unseasoned personnel.
“c. When the WBGT index reaches 85 ??F(29 ??C), strenuous exercise such as marching at standard cadence should be suspended in unseasoned personnel during their first three weeks of training. At this temperature training activities may be continued on a reduced scale after the second week of training.
“d. Outdoor classes in the sun should be avoided when the WBGT exceeds 85 ??F (29 ??C).
“e. When the WBGT reaches 88 ??F (31 ??C), strenuous exercise should be curtailed for all recruits and other trainees with less than 12 weeks training in hot weather. Hardened personnel, after having been acclimatized each season, can carry on limited activity at WBGT of 88 ??F to 90 ??F (31 ??C-32 ??C) for periods not exceeding six hours a day.
“f. When the WBGT index is 90 ??F (32 ??C) and above, physical training and strenuous exercise should be suspended for all personnel (excluding essential operational commitments not for training purposes, where the risk of heat casualties may be warranted).”
There is more detail at this OSHA Page: http://www.osha.gov/dts/osta/otm/otm_iii/otm_iii_4.html