Control of liquid cooling garments: subjective versus technical control of thermal comfort

Liquid cooled garments (LCG) are a powerful tool for alleviating heat strain during work in hot conditions. However, the potential advantage of an LCG depends on a more or less proper control of the cooling liquid's temperature. To gain more knowledge on this subject two experimental studies concerning manual control by the wearer were carried out. In both studies the subjects had to exercise in a warm environment. In the first study the subjects (n = 5) were asked to control the temperature directly. They used different strategies, ranging from gradual changes to oscillations of the suit temperature. This was accompanied by large differences of the chosen inlet temperature and the comfort level attained, although by repetition of the experiment it was possible to increase the subjective thermal comfort. In the second study a technical 'comfort' controller changed the inlet temperature of the LCG in the appropriate direction whenever the personal assessment of thermal sensation differed from the neutral state. The subjects (n = 6) had no information about the control mode. The experiments showed that it was possible to maintain a comfortably neutral thermal sensation throughout nearly the whole experiment. This result, however, was partly in contrast to the objective thermal state of the subjects. Two of them had problems with their heat balance, expressed by the fact that their rectal temperature did not reach a steady state. The results of both studies indicate that manual or subjective control is not optimal and should therefore be replaced by an objective control of the thermal state.