Automatic control of human thermal comfort by a liquid-cooled garment.

Astronauts utilize water cooling in a liquid-cooled garment to maintain thermal comfort during extravehicular activities (EVA's). In the Apollo and Skylab Programs, manual control of the cooling water was a necessary operation to ensure proper control of body heat storage for the EVA crewman. The development of an automatic thermal control system would be a valuable asset to the conduct of EVA, relieving the crewman of a task that can interfere with his EVA objectives. An analytical model of human thermoregulation was used to develop the equations governing the operation of such an automatic controller. A series of tests verified the feasibility of a controller utilizing only a measurement of the difference in coolant temperature into and out of the astronaut's liquid-cooled garment and an estimate of environmental heat loss to maintain the proper crewman thermal balance. Three test subjects performed over a wide range of metabolic rates, and the crewman heat balance was maintained well within allowable medical limits. This study demonstrates the use of a mathematical model to generate previously unknown physiological relationships between human thermal comfort and liquid cooled garment performance. In so doing, it quantitates physiological parameters that are difficult to relate directly by experiment. Index terms: body temperature regulation, space suit cooling, temperature control, human thermoregulation.