Because of water's great thermal conductivity (25 times that of air) all but the warmest diving conditions require some form of insulating suit. Several different types of diving suit exist including Wetsuit, Semi-dry suit and Drysuit. The most appropriate suit for any given set of conditions is usually governed by the water temperature.
Probably the most widely used form of thermal protection,the wetsuit, is made from synthetic rubber (usually neoprene) that has been foamed or expanded. The gas bubbles which are trapped in the rubber are separated from each other so the material does not absorb water. This bubble layer provides effective insultation combined with elasticity.
Because the suit material stretches, a wetsuit can be reasonably close fitting. Although water is not prevented from entering between the suit and the body, the close fit restricts the amount to a minimum. This water soon heats to body temperature and is isolated from the cold surrounding water by the layer of insulating bubbles in the rubber. Suit thicknesses can be as little as 2mm for warmer water and up to 8mm for cold water.
This is similar in most respects to the full-length wetsuit and is usually made from 8mm neoprene. The major difference is the addition of wrist and ankle seals. These seals help to reduce the flushing of water through the suit and thereby increase the insulating properties. A semi-dry suit usually has a built in hood. The main disadvantage of the semi-dry suit and the wetsuit is that their insulation and buoyancy vary with depth. This means that in deep water the diver is both colder and more negatively buoyant. The greater the thickness of the material, the greater the change in thermal insulation and buoyancy.
The drysuit, as the name suggests, keeps the diver dry. The insulation afforded by a drysuit comes mainly from a layer of air trapped within the suit. Two types of drysuit are commonly available: these are the neoprene and the membrane suits.
The neoprene suit is made from the same foam used for wetsuits but has waterproof seals at the wrist and neck and built in boots. Some of the insulation in a neoprene drysuit comes from the material itself, the rest being provided by undergarments. The membrane suit is constructed from a thin flexible waterproof material. Unlike neoprene, the material itself has no insulating properties and all the insulation is provided by the clothes worn inside the suit.
The air inflated drysuit is widely used by sports divers and is entered by means of a waterproof zip. Air can be fed into the suit by means of a valve which is connected via a direct feed to the regulator. This addition of air helps to compensate for the loss of insulation/buoyancy as the diver descends. The suits must also be fitted with an exhaust valve to allow excess air to be vented on ascent.
