Even soldiers have to sleep - Thermophysiological properties of sleeping bags

Nowadays people mostly sleep at home - safely protected by the house's walls and with a bed system providing sound sleep. Camping, however, has become an attractive way to spend the holidays, especially after the financial crisis. For a relaxing sleep in outdoor and camping surroundings tents, thermal mats and sleeping bags are usually used. Particularly important with respect to sleep comfort and optimal regeneration is the sleeping bag, because of its direct contact with the sleeper's body. This is even more important for soldiers who have to sleep outdoors more frequently than other people. Therefore, military forces should not fail to provide optimized sleeping bags for their troops, because physical recovery of the soldiers may turn out to be a crucial factor in field operations.

Thermophysiological properties of sleeping bags

To provide the necessary thermal comfort and hence a sound sleep sleeping bags should optimally support the thermoregulation of the soldier. On the one hand, the soldier's body produces heat – even during sleep about 80 Watt. On the other, this heat is lost via convection, conduction, sweat evaporation, radiation and respiration.

Depending on its specific thermal and water vapour resistance a certain sleeping bag is basically suitable for different climatic conditions. The knowledge about this range of utility is especially important for explorers or mountaineers who use sleeping bags in extreme climates. For this reason, different test methods to define a certain range of utility were developed. At the beginning of the 19th century, sleeping bags appeared for the first time. They were especially made for explorers and mountaineers, who did not know their range of utility in advance, but had to find it out by trial and error. Approximately since 1900, sleeping bags are manufactured in mass production and are tested by the manufacturers by means of test persons. But such tests with persons are time and cost intensive. What is more, they can only be done within small temperature ranges. That is why laboratory based test methods were developed to measure the thermal insulation of sleeping bags.

Much research on it was done, different measurement tools and analyzing methods were developed - at first only for fabric layers, e.g. Togmeter or hot plates. Later on, thermal manikins with the shape and weight of a male human being, and consisting of several, heated segments were developed. Such modern thermal manikins have made it possible now to test ready-made sleeping bags under realistic conditions.

In the mid-1990s, different research labs started to create a European standard for the requirements of sleeping bags. The resulting standard EN 13537 was implemented in 2002. According to this norm, thermal insulation of a sleeping bag is measured by a thermal manikin lying on a thermal mat, a wooden board and a campbed in a climatic chamber.

In addition to this test set-up, a physiological model allows to predict different temperature limits using the measured thermal insulation. Those temperatures define a certain range of utility including a comfort, transitional and risk zone.

Does the standard EN 13537 on the requirements of sleeping bags meet military demands?

On the one hand, the EN 13537 is applicable for military sleeping bags, too. The thermal insulation is measured with an accuracy of less than 5 % (standard deviation of 3 measurements)." will remain the same even if minor adaptations have to be made to simulate realistic, military situations. Furthermore, sleeping bags can be clearly distinguished by their thermal resistance and thus can be chosen with regard to the expected temperatures. From the data, the risk of hypothermia and frost bites can be calculated. In the case of sleeping bag S1, for example, the risk of hypothermia and frost bites already starts at a temperature of +3.1°C, whereas in the case of sleeping bag S9 the limit is -46.2°C.

On the other hand, EN 15537 itself states that it does not apply to sleeping bags intended for military use, because of the differences of physical condition and in use between soldiers and civilians with regard to their physical condition and the intended use of the sleeping bags. Simultaneously, it is beyond controversy that the indication of a range of utility can be of great importance for soldiers, too. Even soldiers have to sleep in order to relax and to prevent exhaustion. Besides such "comfort" reasons, a shivering soldier is not able to successfully aim at a target and is endangered by hypothermia as well. But how is it possible to adapt the standard to military sleeping bags?

The basic test design simulates camping in a tent. That is why the manikin lies on a campbed with a thermal mat, and a simulated air flow of only 0.3m/s. To get a more realistic, military scenario the manikin should be bedded on a military thermal mat on the ground or even on earth. And since soldiers do not necessarily sleep in a tent, higher air flows could be simulated by means of wind machines.

In addition, the physiological model used for the calculation of the temperature limits has to be adapted. The physiological model takes into account heat production of the sleeper and heat loss through the sleeping bag. EN 15537 assumes an untrained, middle aged and inexperienced camper or hiker as a basis. Soldiers, on the other hand, can be assumed to be younger and far better trained than average campers. Accordingly, they have a higher heat production. At the same time, soldiers are used to sleep outdoors. This makes only small changes to the sensation of cold and the body's reaction to it, but the behaviour is optimized. Heat loss, for example, can be lowered by lying in a foetus position. Compared to the assumptions of the standard, the physiological model for military use could work with higher heat productions and lower limits for hypothermia. Of course one has to keep in mind that in war situations fatigue and inappropriate nutrition may also lead to a lowered heat production - the consequence of which would be a lowered resistance of cold and higher temperature limits.

Conclusion
The known temperature range of utility is important for sleeping bags. Therefore, sleeping bags for civilian use have to meet the requirements of the European Standard EN 13537. But the test design and physiological model do not simulate realistic military situations. On the other hand, the standard can be adapted with minor modifications. The test conditions could be changed for example by using other thermal mats or applying more wind to simulate military use more realistically. Furthermore, the physiological model used for the analysis has to be changed to live up to the better physical condition of soldiers.

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