Drysuits are used typically when diving in water temperatures between 0 and 15 C (32 to 60 F).
Seals at the wrists and neck prevent water entering the suit. Even so, the diver will be damp after a dive in a drysuit due to sweat and condensation. The seals are either made from latex rubber or neoprene. Latex seals survive for a maximum of two years but are supple. Neoprene seals last longer but let more water enter because, being stiffer, they do not make effective seals in the contours of the wrist and neck.
A modern drysuit has an air inflation valve, which lets the diver control the buoyancy of the suit by injecting gas from the diving regulator to avoid squeeze during descent. Some old-type frogman's drysuits had a small "jack cylinder" to be inflated from, or the frogman (who was using an oxygen rebreather and so limited to about 30 feet (10 m) depth) had to put up with the suit squeeze.
A typical drysuit has an air vent valve , which lets the diver vent off higher pressure gas from the suit during the ascent. Vent valves can be automatic, operating as pressure relief valves, or manual, where the diver must raise the valve to vent. Automatic vents are generally located at the shoulder and manual vents are located at the wrist. Some drysuits have no vents, but the diver must pull one of the wrist or neck seals open to vent the drysuit.
Most drysuits have built-in boots, but some have ankle seals instead.
Modern drysuits have a zipper, for entry and exit, across the back of the shoulders, or diagonally across the front of the torso, or straight down the middle of the front. At least one make of old-type British frogman's drysuit was one-piece with a wide neck hole for entry; the bottom of the hood and the edge of the suit's neck hole were clamped together by a large circular steel clamp around his neck; there was a watertight seal in the bottom of the hood.
There are two types of drysuit:
- Membrane dry suits are made from materials with low thermal insulation such as vulcanised rubber or a trilaminate of nylon, butyl rubber and nylon. So the diver must wear an insulating undersuit. Membrane drysuits are comfortable to put on, get off and wear. They can be unreliable because the suit's buoyancy and insulation depends on the air trapped in the under suit: if the suits is punctured the buoyancy and insulation is lost. Some divers in warm water wear a membrane drysuit without an undersuit. Membrane drysuits may also be constructed with a waterproof and breathable membrane to enable comfortable wear for periods out of water.
- Neoprene dry suits are constructed from neoprene, a buoyant and thermally insulating material. This built-in buoyancy and thermal protection makes them safer to wear than membrane dry suits when punctured because they keep some of those properties when flooded. Being made of a fairly rigid heavy material, they are difficult to get on and off, and their buoyancy and thermal protection decreases with depth as the neoprene is compressed. Neoprene also tends to shrink over the years. An alternative is crushed neoprene, which is less susceptible to volume changes when under pressure and shrinks less.
Semi-dry suits are used typically when diving in water temperatures between 10 and 20 C (50 to 70 F). They are effectively a thick wetsuit with better-than-usual seals at wrist, neck and ankles.
The seals limit the volume of water entering and leaving the suit. The diver gets wet in a semi-dry suit but the water that enters is soon warmed up and does not leave the suit readily, so the diver remains warm. The trapped layer of water does not add to the suit's insulating ability. Any residual water circulation past the seals still causes heat loss. But semi-dry suits are cheap and simple compared to dry suits. They are made from thick neoprene, which provides good thermal protection. They lose buoyancy and thermal protection as the trapped gas bubbles in the neoprene compress at depth. Semi-dry suits can come in various configurations including a single piece or two pieces, made of 'long johns' and a separate 'jacket'. Semi dry suits do not usually include boots, so a separate pair of insulating boots are worn.
Dive skins and jeans
Skins are used typically when diving in water temperatures above 25 C (77 F). They are made from Lycra and provide little thermal protection but simply protect the skin from stings and abrasion. Lycra became popular about 20 years ago, and is styled after women's nylon stockings. Australian lifeguards wore nylons to protect against jellyfish stings when on rescues. The down side of lycra is that it can shred when touching an abrasive surface, and that it can be costly.
The original dive skins were a neoprene jacket and tight jeans and known as a 'Top and Levis'. Jeans have a negative buoyancy of about 8 ounces force (2 N). Some divers believe that they are excellent diving skins. Levis, when broken-in properly and shrunk-to-fit, are practical for outdoor activities, most athletics, and aquatics. The famous 'top and levis' -- a neoprene jacket and shrink-to-fit Levis remains one of the best skindiving and scuba suits or diveskins, offering protection from overexposure to the sun, stings, and abrasion. Levis 501, 505 and 512 Red Tab are the best styles from Levis Strauss for swimming and diving, and provide the same thermal protection as 1 mm neoprene. When worn with a 2-3 mm top, you can swim and dive comfortably in water 5 - 10 degress f. cooler than you normally would. Levis skins are effective in water above 68 degrees F. In water below 68 degrees F., depending on the water athlete's tolerance to cold, a neoprene dive suit or skin should be worn to prevent hypothermia.
In addition, Levis (and Wranglers, etc.) are approximatley 1.5 pounds negatively buoyant, enabling a diver -- either on scuba or freediving, to descend underwater more easily than with an all-neoprene diveskin (which is positively buoyant, and requires lead weights to achieve the same result). This is especially important in open water and in the ocean - where salt water makes the diver more buoyant than in fresh water. The very same jeans that might feel 'heavy' to a beginner swimmer when they go into a pool, are a second skin to the experienced intrepid swimmer/diver in open water. There is no comparison in fit between street jeans and water jeans. Some swimmers and divers reserve specific pairs of jeans and diveskin jeans for the water, just as they do with their neoprene wetsuits and lycra skins. The preferred approach is to be able to go from dryland to water and back to dryland wearing the same jeans or skins always -- when on, in, or underwater. The truly aquatic see no distinction between the two, and shrink-to-fit jeans and diveskins offer the freedom to transition without extra time and preparation. In cooler weather, a "Warm Wind" - style coat can be worn over the diver's skins to wick away moisture and prevent chill.
Levis 'skins' provide swimmers and divers with additional convenience because in warm weather and climates where a complete diveskin is needed underwater, but not at the warmer surface (which might cause over-heating), the neoprene jacket can be removed and you can swim with just your jeans or with a lycra top. Most swimmers and diver combine their 'skins' with dive fins and a mask/snorkle for a very practical and functional set of swim/dive gear. The serge in the denim fabric acts as shark dendrils in that it funnels the water over the swimmer/diver's body allowing them to become hydrodynamic and swim faster underwater.
They offer minimal thermal protection but good protection against sun, abrasion, and stings. They can be purchased slim fit or tapered and will shrink to fit. Jeans can also be tucked into dive boots or cuffed to prevent drag underwater. With street clothes and a t-shirt or dive top one has the basics for a safe and fun dive. However, jeans provide poor thermal characteristics out of the water on cold days. So modern dive skins may be preferable in all but the warmest conditions. In very warm water a boilersuit is good protection against cuts and scrapes, but is heavy when wet after the dive and more difficult to take off than when it is dry after work on land.
Diving suit combinations
- Some divers wear a wetsuit under a membrane drysuit.
- Some divers wear a thin "shorty" wetsuit under a full wetsuit.
- Some divers wear a "skins" under a wetsuit. That started with divers (of both sexes) wearing women's body tights under a wetsuit to get a bit of extra warmth.