During the last decade, sport diving has undergone major changes. The restrictive dive tables that made multi-level and repetitive diving almost impossible have been superseded by dive computers. Mixed gases permit ever-longer and deeper dives. Larger-capacity cylinders and rebreathers have extended gas duration from minutes to hours.

Pivotal to making increasingly ambitious dives has been the breaking of the thermal barrier. Making long and repetitive dives all year round in chilly waters was only made possible with the advent of drysuits.

As with other diving equipment, what meets one person's needs and budget mat not satisfy another's. Fortunately, the UK market for drysuits is a large one and the smart buyer can choose from a huge range of home-grown and imported models.

The first choice a buyer has to make is whether to select a foam-neoprene drysuit or a membrane one. Foam neoprene suits provide substantial inherent insulation, while membrane models (which include urethane, trilaminate, vulcanised rubber and crushed neoprene) are designed only to keep the wearer dry: underclothing provides the warmth. This allows the diver to use underclothing appropriate for each dive and makes the suit adaptable to conditions from near-freezing to the 20-24°C typical of popular destinations such as the Red Sea. In addition, most membrane suits roll up tightly and are lightweight, making them ideal for travelling.

Drysuits depend on four features to be waterproof: the fabric used for the body; the seams; the seals typically found at the neck and wrists; and the zip.

Foam neoprene is the material you get in wetsuits. The spongy, gas-filled rubber is a poor conductor of heat, which, combined with the insulating air inside the suit, makes it ideal for cold-water diving. A nylon lining on both sides of the neoprene increases its strength and makes it easier to put on. Suits tend to be hard-wearing and quite cheap.

There are some disadvantages, though. At the surface, for example, a 5 mm neoprene suit is warm but very buoyant, so a diver must carry lots of weight. During descent, the air bubbles inside the material compress, the suit loses warmth and buoyancy, and the diver must add lots of air to stay neutrally buoyant, making it least effective at depth. In addition, foam-neoprene suits can be 7-8 mm thick, making them bulky and heavy.

Urethane suits are usually the least expensive membrane suits. Urethane makes the suit waterproof; a nylon lining protects it from abrasion. The material offers little stretch, so must be loose-fitting, which means suits are bulky and cause drag in the water.

Trilaminate suits usually consist of a thin, waterproof layer of rubber sandwiched between two linings, which tend to be nylon. The inner lining makes it easy to slip the suit on and off; the outer one protects the rubber from abrasion.

Vulcanised rubber suits dry almost instantly and are quick to repair using glue and a patch. The seams are usually vulcanised as well, so are very tough and shouldn't leak.

Vulcanised rubber is long-lasting, although thin suits are vulnerable to punctures, so choose a heavy-weight model for hard diving.

Crushed neoprene was originally patented by the US manufacturer DUI and consists of foam neoprene that's been hydrostatically compressed to eliminate the bubbles. The result is a thin, supple and rugged material, with no insulation, so undergarments must be worn. Crushed neoprene suits are fairly close-fitting and do stretch, but they're very heavy, especially when wet.

Compressed neoprene suits offer many of the pluses of both foam neoprene and membrane drysuits. Again, the materials starts off as foam neoprene, but this time it's only partially compressed. The bubbles still remain, offering some insulation, but their reduced size means the suit is lighter than normal foam neoprene and loses less buoyancy as it compresses only a small amount at depth.

New materials and designs are always being tested and introduced by manufacturers. They can't all be covered here, particularly as many are patented and their exact composition can't be published. Keep your eyes peeled: there are some good innovations around.

Quality seams are essential to the longevity and watertightness of any drysuit. Foam suits are usually glued edge to edge and then sewn with blind stitches that don't go all the way through the material, so water can't enter through the holes. Membrane suits are usually stitched and sealed along the seam with either waterproof tape or sealant.

Seams can leak due to ageing or damage. If the sealant breaks down or is dissolved by chemicals, water can wick along the stitch and then spread through the suit lining. Vulcanised suits use a heat-pressure seal and are effectively seamless.

Because the air in a drysuit compresses with depth, a diver can feel 'shrink-wrapped' very quickly on descent. To prevent suit squeeze, and to compensate for loss of buoyancy, air or another gas must be injected into the suit.

Normally, a push-button direct feed, similar to the type found on a BCD, is fitted. Some divers use argon from a small cylinder instead. Argon helps the body stay warmer for longer as it is a poor conductor of heat.

When wearing a drysuit, it's best to use the suit for buoyancy underwater and the BCD for surface flotation, as it's very easy to achieve a horizontal position with a drysuit. However, some divers choose to add just a little air to their drysuit to avoid suit squeeze, then use their BCD.

With a membrane suit, a recreational diver who's correctly weighted should need to add only a little more than enough air to eliminate squeeze before neutral buoyancy is achieved. Technical divers using multiple tanks and making deep dives, especially wearing a neoprene suit, may have to supplement buoyancy control using their BCD.

With the many combinations of how a diver may control buoyancy and the variations in valve position and activation, it's essential to brief your buddy properly as to how to surface you safely in an emergency.

If a drysuit floods, the diver loses buoyancy, which could make him or her substantially negative. Because drysuits are often used in very cold conditions, a flooded suit may also expose the diver to the risk of hypothermia.

At the surface, drysuits float a diver at neck level, making them vulnerable to wave action. For back-up buoyancy underwater and proper surface flotation, BCDs are essential for drysuit diving.

Most drysuits leak slightly most of the time. Zips aren't always fully closed, tilting your head back can unseat a neck seal, and operating a reel often creates a gap between wrist and suit. Condensation caused by sweating, or by warm air inside the suit being chilled by the water outside, is also common. Experienced drysuit divers consider warm and damp a realistic situation. A risk with drysuit diving is a 'blow-up', or runaway buoyant ascent. This results from having too much air in the suit and often occurs as air expands on ascending. Typical reasons are poor buoyancy control by the diver or mechanical failure of the suit valves. In the latter instance, disconnecting the inflator hose and venting excess air through the exhast valve should enable you to regain control. Be prepared to vent air through your wrist and neck seals as well.

Short bursts of air reduce the risk of icing in the inflator-valve mechanism, which could cause an uncontrolled free-flow of air into the suit. Handling mini high-pressure cyclinders requires even more care than usual, since opening the tap too fast can inflate a drysuit quickly, making it difficult to regain control.

It takes longer to vent drysuits than BCDs because air has to work its way past underclothing and harness straps. Working in cold conditions usually means the diver is wearing clumsy gloves or has lost dexterity because of numb fingers: this can make it hard to disconnect a free-flowing direct feed. Gloves and hoods can also make it difficult to break a wrist or neck seal to vent the suit in the event of a blow-up.

Air caught in the feet and legs requires fast action to aviod an uncontrolled and rapid feet-first buoyant ascent. As alarming as this and previous scenarios sound, a drysuit training course should give you the appropriate skills to deal with such emergencies. You should not dive in a drysuit without first completing one.

Divers are constantly taught the importance of adequate hydration to guard against DCS. But what goes in must come out...

It's not impossible to pee underwater wearing a drysuit. For men, it's probably easiest via vent valves into the water or by using a condom-like device linked to an internal storage bag. As gross as it sounds, plastic pants and incontinence pads are an option for either sex, and some divers even use nappies.