Dry suit
A dry suit or drysuit provides the wearer with environmental protection by way of thermal insulation and exclusion of water, and is worn by divers, boaters, water sports enthusiasts, and others who work or play in or near cold or contaminated water. A dry suit normally protects the whole body except the head, hands, and possibly the feet. In hazmat configurations, however, all of these are covered as well.
The main difference between dry suits and wetsuits is that dry suits are designed to prevent water from entering. This generally allows better insulation, making them more suitable for use in cold water. Dry suits can be uncomfortably hot in warm or hot air, and are typically more expensive and more complex to don. For divers, they add some degree of operational complexity and hazard as the suit must be inflated and deflated with changes in depth in order to minimize "squeeze" on descent or uncontrolled rapid ascent due to excessive buoyancy, which requires additional skills for safe use. Dry suits provide passive thermal protection: Undergarments are worn for thermal insulation against heat transfer to the environment and are chosen to suit expected conditions. When this is insufficient, active warming or cooling may be provided by chemical or electrically powered heating accessories.
The essential components are the waterproof shell, the seals, and the watertight entry closure. A number of accessories are commonly fitted, particularly to dry suits used for diving, for safety, comfort and convenience of use. Gas inflation and exhaust equipment are generally used for diving applications, primarily for maintaining the thermal insulation of the undergarments, but also for buoyancy control and to prevent [|squeeze].
Function
The dry suit is a form of exposure suit, a garment worn to protect the user from adverse environmental conditions. The two most common purposes are to insulate the wearer against excessive heat loss, and to isolate the wearer from direct contact with a liquid environment during immersion or repeated multi-directional contact with bulk liquids or spray. Most often the liquid is water, usually without significant contaminants, but dry suits also have applications in isolation from hazardous materials and biological contaminants.Most of the insulation function is provided by passive thermal protection in the form of garments worn under the dry suit, The suit itself has the primary function of keeping the insulating garments dry, and allowing them to be maintained at sufficient loft to provide adequate insulation by adding dry gas to the interior of the suit and releasing excess gas. Active heating systems may also be used but are less popular.
Isolation of the wearer from contact with the environment for purposes other than thermal insulation usually requires the entire surface of the skin to be kept dry and uncontaminated by the ambient environment. This requires that the seal between the breathing apparatus and the suit is also reliably watertight, which is most effectively provided by sealing the suit to a helmet with redundant series exhaust valves, or a return of exhaled gas to the surface by hose, similar to a gas reclaim system, though there are applications where a lesser level of isolation is acceptable.
Dry suits should not leak, but once the suit is sealed, interior humidity rises to 100% and condensation will occur on cold surfaces such as the inside of the suit. A certain amount of dampness is inevitable and common on the inside of the suit after a dive, and is acceptable provided the diver remains warm. Flexing the wrists and large movements of the head may allow water to enter along raised or sunken tendons. This is normal, and to some extent can be avoided or reduced with practice. It can be prevented by attaching the gloves directly to the suit and by sealing the suit to the helmet.
Heat loss
There are two physiological aspects of heat loss of particular relevance to the diver: Cold shock response and hypothermia.Cold shock response is the physiological response of organisms to sudden cold, especially cold water, and is a common cause of death from immersion in very cold water, such as by falling through thin ice. The immediate shock of the cold causes involuntary inhalation, which if underwater can result in drowning. The cold water can also cause heart attack due to vasoconstriction; the heart has to work harder to pump the same volume of blood throughout the body, and for people with heart disease, this additional workload can cause the heart to go into arrest. This effect is prevented or mitigated by almost any dry suit, as the cold water is kept from direct contact with most of the body and the immediate heat loss is reduced considerably. A person who survives the initial minute of trauma after falling into icy water can survive for at least thirty minutes before succumbing to hypothermia provided they don't drown. However, the ability to perform useful work like staying afloat declines substantially after ten minutes as the body protectively cuts off blood flow to "non-essential" muscles.
Hypothermia is reduced body temperature that happens when a body dissipates more heat than it absorbs and produces, and is a major limitation to swimming or diving in cold water. The reduction in finger dexterity due to pain or numbness decreases general safety and work capacity, which consequently increases the risk of other injuries. Body heat is lost much more quickly in water than in air, so water temperatures that would be quite reasonable as outdoor air temperatures can lead to hypothermia in inadequately protected divers, although it is not often the direct clinical cause of death. The effectiveness of a dry suit in preventing or delaying hypothermia depends on its insulating value.
There are two major routes for heat loss. Respiratory and through the skin. The mechanisms of respiratory heat loss are heating the inspired gas and humidifying the inspired gas by latent heat of evaporation. While they are major factors in diver compfort and safety, these are not influenced by the use of a dry suit. The loss of heat through the skin by radiation, conduction, and convection is the aspect which can be controlled by an exposure suit, and the one for which dry suits are effective and appropriate.
Skin will heat up gas and clothing inside a dry suit by radiation and conduction. Convection will transport heated gas within the suit, and may take it to places where it may be transferred through the suit shell more rapidly. Heat transfer by radiation occurs through a medium that is transparent to infrared radiation of the relevant wavelengths. This is mostly the gas, and the radiation paths are short, multiple, and with small temperature differences, so the effects are relatively small. Heat conduction is kinetic energy transfer by molecular or atomic collision. It has the more important role in heat transfer through a diving suit. Conduction occurs between the diver's skin and the gas and suit materials in contact with it, and through these materials to the shell, through the shell to the surrounding water, where it is rapidly removed by convection. Conduction heat loss is strongly influenced by thermal conductivity of the gas in the suit.
Convective heat transfer is the consequence of movement of heat carried by a gas or liquid from one place to another, where conduction can occur. It can considerably speed up heat transfer, so thermal protection of the undersuit is improved when it limits convection of the gas within the suit. Convective heat transfer in the suit is strongly influenced by the freedom of the gas in the suit to move around, which is increased when there are large gas spaces, and reduced when the gas is constrained by the loft of the fabric. There is also heat transfer within the suit by evaporation of moisture in contact with the skin, and condensation on the inner surface of the shell. This is reduced by wicking it away from the skin before it evaporates, and preventing condensate on the inside of the shell from wetting the inner layer of the undersuit.
Essential components
The essential components include a shell of watertight material, sufficiently flexible to allow the wearer to function adequately, seals where parts of the body pass through the suit while in use, and a method of sealing the access opening while the suit is worn. An inflation valve with gas supply and dump valve are generally provided on dry suits used for diving, but were not standard on early models, and are not needed when the suit is sealed to the helmet and the helmet space is open to the interior of the suit, or for suits only used for surface activities.Shell
The main part of the dry suit is a waterproof shell made from a membrane type material, closed cell foamed neoprene or a hybrid of both. Insulation may be provided in part by the suit shell, but is usually mainly provided by thermal insulation clothing worn under the suit, which relies to a large extent on trapped air for its insulating properties.Membrane
Membrane dry suits are made from thin materials which have little thermal insulation. They are commonly made of stockinette fabric coated with vulcanized rubber, laminated layers of nylon and butyl rubber known as, or Cordura proofed with an inner layer of polyurethane. With the exception of the rubber-coated stockinette, membrane dry suits typically do not stretch, so they need to be made slightly oversized and baggy to allow flexibility at the joints through the wearer's range of motion and to allow the hands and feet to pass through without difficulty. This makes membrane dry suits easy to put on and take off, provides a good range of motion for the wearer when correctly sized and sufficiently inflated, and makes them relatively comfortable to wear for long periods out of the water compared to a wetsuit or close-fitting neoprene dry suit, as the wearer does not have to pull against rubber elasticity to move or keep joints flexed.To stay warm in a membrane suit, the user must wear a thermally insulating undersuit, typically made from synthetic fiber, which is considered preferable to natural materials, since synthetic materials have better insulating properties when damp or wet from sweat, seepage, or a leak. A low capacity for water absorption, retention of loft under mild compression, and quick drying after use are also desirable characteristics.
Reasonable care must be taken not to puncture or tear membrane dry suits, because buoyancy and insulation depend entirely on the air layer held in the undersuit,. The dry suit material offers essentially no buoyancy or insulation itself, so if the dry suit leaks or is torn, water can soak the undersuit, with a corresponding loss of buoyancy and insulation.
Membrane dry suits for surface use may also be made of a waterproof but breathable material like Gore-Tex to enable comfortable wear without excessive humidity and buildup of condensation. This function does not work underwater. Sailors and boaters who intend to stay out of the water may prefer this type of suit, but the fabric is less tolerant of rough usage, and may develop leaks more easily.
Membrane suits rely entirely on thermal undergarments for thermal insulation. The thermal undergarments rely on large volumes of trapped air for insulation, and any excess air trapped within the suit is not well constrained from migrating to the high points of the suit when diving. The loose fit necessary to allow reasonable freedom of movement and to make it possible to get in and out of the suit creates baggy air pockets where trapped air accumulates if it is not vented immediately, and some of these air pockets form in the parts of the suit where they are least easily vented by a diver trimmed for efficient horizontal swimming. This combination makes it necessary for the diver to be more vigilant and increases task loading in buoyancy control, and thereby increases risk of overinflation incidents and uncontrolled ascents. These risks are reduced by use of a suit which has the minimum excess volume, which in most cases requires precise custom fitting. The large and baggy standard diving suits had the option of lacing up the back of the legs to reduce suit volume in the place where it was most hazardous, but this feature is not available on more recent suits, and the nearest functional substitute is gaiters over the lower legs. Bagginess in the torso and arms is less problematic as excess gas in these areas is much easier to vent, and will usually do so automatically if the dump valve is set correctly.