Feeling the Heat

By Kevin S. Honeyager

Contact: Keith Bartels, Ph.D.

Senior Research Analyst Kevin Honeyager of SwRI's Bioengineering Department in the Automation and Data Systems Division is manager of the HSM project. At the Institute since 1984, Honeyager specializes in designing instruments to carry out sensor acquisition and processing with microcontroller-based embedded systems.

It's not the heat -- it's the humidity. This familiar Sun Belt complaint may be oversimplified, but it's not far from the mark. There is more to summertime discomfort than a mere temperature reading. But for some outdoor activities such as military training exercises, and in some especially hot workplaces such as mines and foundries, the combination of heat, humidity, and strenuous work goes beyond discomfort. Here, heat stress can be deadly.

Beating extreme heat requires monitoring the full range of stress-inducing parameters. Not just the obvious ones, such as temperature and humidity, but also such contributing factors as air flow, protective clothing, and physical exertion. Historically, predicting heat stress has required two actions: first, to place the multiple sensing instruments and needed to assess heat stress; then, to integrate the combined data and arrive at a work/stop-work decision. This approach is both cumbersome and time-consuming, preventing health personnel from rapidly assessing the safety of the environment.

Southwest Research Institute (SwRI) biomedical engineers have solved the problem by transforming a veritable "weather station" into a miniature handheld package, rugged enough to be used in the field and simple enough to be operated without specialized training. Their prototype heat stress monitor, recently delivered to the U.S. Army, will provide both the measurements and the computed guidelines to ensure safety and maximize productivity.

Accurate monitoring of human heat stress in mines, foundries, and oil rigs, as well as in military exercises, can reduce costs and contribute to improved health and safety.

The SwRI Bioengineering Department has a history of expertise in developing medical devices with embedded systems that carry out sensor acquisition and processing. Using the latest surface-mount technology and low-voltage electronics, the department has developed a variety of successful instruments including a life detector, a cardiac output computer, a continuous arterial blood gas monitor, and a series of blood pressure monitors.

The SwRI-developed miniature heat stress monitor (HSM) has potential applications in military training exercises and actual missions, and in mining, foundries, agriculture, offshore oil operations, and also at archeological digs and in specialized sports. All of these are activities where accurate and rapid prediction of heat stress can be critical in preventing immediate distress or long-term damage to the human body.

The HSM allows viewing of environmental data in real-time for monitoring purposes. Data are collected, averaged, and updated once per second. There is also a menu for unattended logging of data. The user interface is sufficiently flexible to be adapted for alternative applications, and sensor modules can easily be developed for measuring additional environmental parameters.

Heat stress causes discomfort and reduced productivity and can lead to more serious health effects such as accidents, illness, and even death. Prolonged exposure to high temperatures alone can lead to excessive fluid loss, shock, or heat stroke. High humidity reinforces the effects of temperature by reducing the cooling effects of sweating. Extended and strenuous exercise, or labor that increases the heat produced by muscles, also contributes to the risk of illness in the form of cramps, exhaustion, or heat stroke. Internal body temperatures, normally 37°C, are considered hazardous between 40-41°C. Death is likely to occur at 42°C and above.

Much research into heat stress has been sponsored by the mining industries in tropical areas of countries such as South Africa and Australia. In some of the deeper mines in the Rand of South Africa and at Mount Isa in Queensland, Australia, where working rock face temperatures can exceed 60°C and humidity may be close to 100 percent, more than half of mining production costs are spent for ventilation and cooling. One South African study found that in an average entering labor pool, 52 percent of workers had no problems with heat stress, but 46 percent needed careful acclimatization to the work over several months. Two percent were completely unable to tolerate even the artificially cooled and ventilated working temperatures that vary between 30-40°C at the mine face. In work situations such as these, the HSM could prove an invaluable tool for ensuring the health and safety of employees and helping management to plan appropriate work schedules.

The environmental parameters typically measured to identify heat stress are the dry bulb, wet bulb, and black globe temperatures. From these three readings the wet bulb globe temperature (WBGT) index is calculated. The WBGT is used as an industry standard metric for assessing the level of heat stress within a given environment. In addition to these parameters, the HSM also measures wind speed, which significantly affects evaporative cooling, and barometric pressure. The data are combined with user inputs of clothing type and work level. The degree of acclimatization status in a specific group under surveillance can also be factored. All data are then combined using algorithms derived from a heat strain model developed at the U.S. Army Research Institute of Environmental Medicine (USARIEM) in Natick, Massachusetts. The output of the model provides specific guidance on selected features such as optimal work/rest cycles, the maximum safe length of a work shift, and hourly water requirements.

The USARIEM physiological heat strain model was developed empirically using data collected from soldiers in maneuvers under various environmental, clothing, and work conditions. Before development of a unified model, work patterns in many hot environments depended on the judgment of a supervisor or commander, or a single climatic indicator such as a wet bulb thermometer reading. Mining activities in the Federal German Republic, for example, are restricted after the temperature exceeds 38°C. Such simple methods to determine work patterns can substantially increase operating costs. A more sophisticated model such as USARIEM's can allow for more flexible work schedules.

In 1991, the U.S. Army contracted SwRI to design and build a portable, integrated heat stress monitor prototype which would incorporate the USARIEM model. They requested a simple device that required no assembly and could measure air temperature, relative humidity, black globe temperature, and windspeed. The device was also required to incorporate data on a selection of work force characteristics such as clothing, acclimatization, and work level, that could then be integrated with specific environmental readings to provide the appropriate information to supervisors. As much as possible, the instrument was to be built from off-the-shelf components to expedite future manufacturing plans and to reduce cost.

The first HSM prototypes, completed in 1993, demonstrated the technical feasibility of the concept but were eventually determined to be too large and heavy. A follow-on project, initiated in 1997 by USARIEM, added more precise and demanding specifications. Those included a 40 percent reduction in size and weight; increased reliability; a modular, removable environmental data sensor system that could be automatically deployed and replaced without difficulty; a liquid crystal display (LCD) with low power consumption and good contrast in sunlight as well as indoor lighting; real-time data logging with the capability of downloading to a personal computer; and finally, a flash memory for field-upgradeable software.

SwRI engineers met all these goals with the recently completed prototype and produced a smaller, lighter instrument of less than 21 cubic inches. A volume reduction of 40 percent has been achieved and the specified weight reduction of 40 percent has been exceeded. The liquid crystal display has been improved, and the unit has more user interface flexibility, greater ease of sensor deployment, and higher reliability.

The most challenging design problems were meeting the size and weight requirements, designing the modular sensor system, selecting a suitable LCD, and fabricating the black globe thermometer. The weight reduction was accomplished primarily by fabricating the enclosure package from nylon using a rapid prototyping process called selective laser sintering. Also, the use of low-voltage electronics and a highly efficient design allowed the power supply to be changed from two nonstandard, C-sized lithium batteries to four standard AA-sized, 1.5V, alkaline batteries.

Because of size constraints, it was not possible to have the sensors permanently deployed. As a result they were located in a module that extends from the body of the instrument when measurements are being taken, and rotates and stows away into the rear of the device when inactive, for storage and compact packaging. This design allows the module to be deployed without danger of contamination and reduces the risk of breakage. The entire module can be removed for replacement, repair, or calibration.

The HSM is equipped with thermistor-based sensors to measure air temperature, wind speed, and solar radiation, a capacitive polymer-based sensor to measure humidity, and a piezoresistive absolute pressure sensor for measuring barometric pressure and estimated altitude. All calibration data and signal conditioning electronics are contained in the sensor module. A digital serial interface is used to perform data acquisition and control.

The choice of the LCD also was critical because of the need to combine small dimensions with sufficient resolution to display all the required environmental parameters without excessive paging. After much searching, a commercially available 119 by 73 pixel graphics LCD with built-in electroluminescent backlight was found with dimensions of only 59 by 40 by 7 millimeters.

A black globe is used to measure radiant heat. The fabrication process for the globe used a copper electroforming method. This was expensive, but the process allowed a uniformly thin (0.006 inches thick) globe to be created as one piece. The globe was then chemically treated to turn it dull black. The globe's thin walls and single-piece construction give a fast response time, allowing accurate measurement within two minutes.

All environmental data to calculate a single heat strain measurement can be displayed in real time. The instrument also performs unattended data logging with time stamping using an onboard clock with an alarm that wakes up the system to perform a measurement. Data logging results can be viewed onscreen or downloaded to a desktop computer for analysis.

Future work on the heat stress monitor includes the construction and field testing of a number of preproduction prototypes and the development of a PC interface to perform calibration, software upgrades, and data retrieval.

Acknowledgments

The author would like to thank Terrie McDaniel, Larry Canady, and Steve Solis for their contributions to this project.

Published in the Summer 1998 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.

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