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U.S. Army Soldier & Biological Chemical Command
U.S. Army Soldier Systems Center-Natick
Public Affairs Office
Kansas Street
Natick, MA 01760-5012

Contact: Chief, Public Affairs Office
(508) 233-5340

Date: July 2, 2002
No: 02-30

Miniature monitor keeps heat stress at bay

Natick, Mass. -- Knowing when to stop working and start drinking has its payoffs.

Monitoring the environment to advise troops on work/rest cycles and water consumption to prevent heat casualties has been accomplished with some variation of the Wet Bulb and Globe Temperature (WBGT) device since 1956. Now a hand-sized monitor developed at the U.S. Army Research Institute of Environmental Medicine (USARIEM), an installation partner at the U.S. Army Soldier Systems Center (Natick), has transformed heat risk assessment.

"In many cases, the WBGT is fine. Its sensors respond to most of the environmental parameters, but from a biophysical view, it's quite limited," said William Matthew, a biophysicist at USARIEM and principal investigator of the Miniature Heat Stress Monitor. "The WBGT-based guidelines use one chart based on a significant number of assumptions about environment, clothing and activity, so the potential for error is quite high. You'd have to have a telephone book of tables to cover all the variables."

Heat stress creates discomfort and inefficiency. Injuries and deaths caused by heat exhaustion or heat stroke are a regular occurrence in the military and civilian population.

"You see heat casualties, and you see them when you wouldn't think they would be there intuitively," Matthew said. "Heat casualties are less of a problem if you have the right measurements and then bring that information into a rational predictive model."

WBGT devices measure air temperature, humidity with a wet "bulb" that needs to stay moistened with water to simulate evaporation of sweat, and radiant or solar heat with a six-inch-diameter black globe. They connect to a processor to calculate an integrated WBGT index reading. Army and Marine units currently look up a table to specify how many minutes troops should work and how many liters of water they should drink per hour based on three levels of physical exertion and the measured WBGT heat index category.

At the core of the miniature monitor is USARIEM's heat strain prediction model software, which is integrated into the suite of environmental sensors. The model is a compilation of data collected during the past 25 years that predicts the amount of heat strain young, healthy male soldiers will experience depending on their work rate, environment and clothing.

"It's a systematic way to look at body heat balance," Matthew said. "Heat flow through clothing, for example, is calculated from four variables of heat and vapor transfer measured in the laboratory on thermal copper manikins."

Powered by four AA alkaline batteries, the square-shaped monitor weighs 13 ounces and is housed in an injection-molded plastic case. The back opens up to rotate the sensor module into position and replace batteries. The field-replaceable sensor module measures air temperature, radiant heat with a gumball-sized black globe and humidity through a solid-state sensor. It has two extra sensors not found on the WBGT-wind speed, accurate even at low speeds, and atmospheric pressure to adjust to various altitudes.

A five-button keypad is located next to a text and graphics-capable LCD display complete with a backlight for use at night. When switched on, the monitor automatically shows the present time, date and remaining battery power.

Programmed with a heat strain model, the user selects from a menu of clothing items and enters the work/task category choosing from rest, very light, light, moderate or heavy work. Then the monitor asks if the group is acclimated to the heat. After two minutes of processing, the monitor displays hourly water drinking requirements, optimal work/rest cycle limits, maximum safe time for continuous work and environmental data used to make those determinations.

WBGT readings are retained for a point of reference, but the monitor takes away the need for pages of printed tables for clothing and acclimatization. It can be set on default so the same information won't have to be repeatedly entered. Matthew said it would take the average user about 30 minutes to learn how to operate it.

With several other operating modes, its ability doesn't stop with a straightforward conditions report.

The user can set the current date and time, and select metric or English units for the displayed parameters in the system setup. The data log setup allows the user to select a start time, log time interval and duration that can provide operational test documentation or survey data for heat stress conditions during a 24-hour period. In data log review, the user can view all logged data, including predictive model outputs.

"This is the first time you can take a device with you in a tank, helicopter cockpit, shelter or outside where you can make these measurements and document the working conditions," Matthew said.

On the bottom of the monitor's case, a threaded mount securely attaches to a tripod for unattended data collection. A port on the lower left-hand side of the case plugs into a computer to download logged data for an alternative display or to import a spreadsheet for analysis. The same port enables program updates, calibration and diagnostics in the monitor's service mode.

Southwest Research Institute in San Antonio, Texas, engineered the item and built the first prototype monitor in 1998. USARIEM's Cooperative Research and Development Agreement partner, OCC-Consult Limited in Perth, Australia, has adopted it for use at a major copper mine. Up to 50 monitors help with the occupational safety and health program by determining whether the mine's cooling should increase or even if the workers should quit for the day.

"That's the beauty of it-it's adaptable to a wide range of user-specific models. Australia is using custom software that meets their particular needs," he said.

The monitor's measurement of ambient environment could be used for a variety of human factors engineering and development projects. Other potential applications are in foundries, offshore oil operations, agriculture, archeological digs and certain sports settings. For the military, commanders want to know the maximum work/rest situation, particularly for troops wearing chemical biological protective gear, according to Matthew.

The 6th Ranger Training Battalion evaluated it with good reviews, but "there's no strong Army proponent for fielding it right now," he said.

Natick is part of the U.S. Army Soldier and Biological Chemical Command (SBCCOM). For more information about SBCCOM or the Soldier Systems Center (Natick), please visit our website at http://www.sbccom.army.mil.


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