Hohenstein says the overall comfort of combat uniforms for cold environments is not only due to the thermal insulation characteristics of the garments but also influenced strongly by the water vapour resistance (“breathability”). Both insulation and breathability is of great importance for the soldier and it is hard to say if one is more important than the other. Breathability affects the amount of humidity in the microclimate of the garment and thus the perceived wear comfort which is very closely related to the mental and physical performance of the soldier.
We now understand that low water vapour resistance (i.e. good breathability) extends the range of utility of combat gear to warmer temperatures. This is essential if the clothing system is to have high utility and robust use under cold and warmer conditions and is even more important during period of high physical activity. Hohenstein says the overall comfort of combat uniforms for cold environments is not only due to the thermal insulation characteristics of the garments but also influenced strongly by the water vapour #
Especially for cold protective clothing a low water vapour resistance is of high importance because it leads to reduced sweat accumulation in the clothing system. This is not only the case for “normal” temperatures of 20°C / 68°F but also at freezing point and far below at -20°C / -4°F.
In the following the authors show how to improve cold protective gear producing enhanced human thermoregulation in cold environments as well as thermal and moisture management characterisation methods for fabrics and full clothing systems.
The human body generates heat energy at a steady state “metabolic rate”. It varies from 80 Watts while sleeping up to 800 Watts in very high physical activity. To maintain the body core temperature constant at about 37°C / 98.6°F within a limit of only ± 2°C / 3.6°F at varying metabolic rates, the human body has its own thermoregulatory mechanism. Excess energy has to be dissipated by sweat evaporation and an energy loss in cold environments has to be compensated by cold shivering.
Both excessive sweating and shivering result in losses in human performance effficiency, so it is much desired to control these consequences through better protective clothing with appropriate thermal insulation.
The challenge for clothing designers is to achieve an even energy balance in the clothing system though understanding and balancing breathability and insulation. The thermal insulation of combat uniform in cold environments depends on the ambient temperature and the metabolic rate of the soldier. A good example for this is the European standard EN 342, which measures thermal insulation for cold protective clothing. In this standard, the thermal Insulation of the garment is measured for a maximum wearing time at a certain metabolic rate under known ambient temperature.
This information produces the criteria required for optimal performance. Thermal insulation (i.e. thermal resistance) may also be determined on a guarded sweating hotplate, i.e. the Hohenstein skin model acc. to EN 31092 / ISO 11092. This can be used for fabric measurement and design. When testing whole garments and/or whole clothing systems, thermal manikins acc. to ISO/DIS 15831 are used. Therefore there are good existing systems to aid in the design of cold weather gear that can begin with fabric performance through the entire garment design process.
Thermal insulation itself mainly depends on the enclosed air volume in the garment, as air has a very low thermal conductivity and is a good insulator. However, when a garment gets saturate and wet the insulating air is replaced by humidity first as water vapour and then as water, which in contrast is a very good conductor of heat and a much poorer insulator. Therefore, body heat is lost rapidly under wet conditions.
To keep a garment dry from inside while sweat is evaporated by the wearer due to increased physical activity, a low water vapour resistance is essential. To evaluate a fabric in regard of the water vapour resistance also the skin model acc. to EN 31092 / ISO 11092 is used. The lower the water vapour resistance the higher is the breathability.
As a final test sequence, evaluating all performance aspects of a combat uniform for cold environment, are subject wearing trials in a climatic chamber under realistic temperatures and physical activity / metabolic rates.
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