Insulation
Still water (without currents or convection) conducts heat away from the body by pure thermal diffusion, approximately 20 to 25 times more efficiently than still air. Water has a thermal conductivity of 0.58 Wm−1K−1 while still air has a thermal conductivity of 0.024 Wm−1K−1, so an unprotected individual can succumb to hypothermia even in warmish water on a warm day. Wetsuits are made of closed-cell, foam neoprene, a synthetic rubber that contains small bubbles of nitrogen gas when made for use as wetsuit material (neoprene as a plastic may be manufactured without foaming, and is made in that fashion for many other applications where insulating qualities are not important). Nitrogen gas (like any gas) has very low thermal conductivity with respect to water or to solids, and the small and enclosed nature of the gas bubbles minimizes heat transport through the gas by gas convection currents (this is the same principle by which air containing cloth fabrics or feathers insulate). Indeed, some modern wetsuits incorporate Merino wool and titanium fibers to add an incompressible layer of gas cells in other ways, while keeping the thickness of the suit to a minimum. The end result of all these techniques is that the fabric layer of trapped gas cavities forces heat to travel slowly by a mostly diffusive process, in a direction that mostly passes through bubbles of entrapped gas, thereby greatly reducing heat transfer from the body (or from the layer of warmed water trapped between the body and the wetsuit) to the colder water surrounding the wetsuit.
Uncompressed foam neoprene has a typical thermal conductivity in the region of 0.054 Wm−1K−1, which produces about twice the heat loss of still air, or one-tenth the loss of water. However at a depth of 15 metres (50 ft) of water, the thickness of the neoprene will be halved and its conductivity will be increased by 50%, allowing heat to be lost at three times the rate at the surface.
A wetsuit must have a snug fit to work efficiently; too loose a fit at water entry and exit points will allow water to escape from between the suit and the body, taking the body's heat with it. Cold water from the outside may enter the same way. Flexible seals at the suit cuffs aid in preventing heat loss in this fashion.
Foamed neoprene is very buoyant, helping swimmers to stay afloat, and for this reason divers need to calculate the need for extra weight based on the thickness of their suit to achieve neutral buoyancy underwater. At the same time, the suit loses buoyancy and thermal protection as the bubbles in the neoprene are compressed at depth.
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