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Appeared in DIVER May 2009
Semi-dry or dry?
Everyone wears a drysuit in cold water, don't they? Well, most do, but it's not compulsory. For some, a semi-dry suit may offer advantages, including being half the price, says John Bantin.
SEMI-DRY SUITS ARE ILL-NAMED, because they are wetsuits. They have seals at wrist and ankle that reduce the effects of cold water flushing through and chilling the diver.
The diver will feel that initial chill on immersion, but the trapped water warms up, and he or she is protected from the cold of the water surrounding him by a thermally insulating layer of neoprene.
A semi-dry suit, like the wetsuit to which it is related, clings closely to your body contours. By varying the thickness of this layer, either by using varying gauges of neoprene (usually 3, 5 and 7mm) and/or wearing more than one layer, and adding a warm hood and gloves, a semi-dry suit can be appropriate for almost any water temperature. For cold water, you'll need 5 or 7mm components.
What's more, you can always choose to take just part of your semi-dry combo abroad with you, depending on conditions at your destination.
Of course, it would be foolish to use in cold water a suit meant for the tropics, and don't be misled by pricing that refers to components of a semi-dry suit that might be worn individually in warmer climes.
Even so, when it comes to total price, a typical semi-dry, as illustrated, will cost around half as much as a typical decent drysuit.
So is it viable to use in the UK? Why not? I used to club-dive in a semi-dry, though I usually chose either to do a single dive or take a second warm and dry suit into which to change. Getting into a wet semi-dry for a second dive in the British climate is unpleasant for all but the hardiest.
WATER IS RARELY COLDER THAN 0°C, so a well-fitting 7mm semi-dry, with a similar layer in the form of a jacket or shortie worn over it, can keep a diver snug. Problems arise when leaving the water and entering air that might be appreciably colder, or wind-chilled. The cooling effect of evaporation can be devastating, so it's important to get out of the wet semi-dry quickly, dry off and don warm clothes.
Some semi-dry suits have sleeves and leg-ends that close over the seals and are closed with zips. This can add to the insulation but you need to be careful when getting out of a suit with skin that has been softened by long immersion that these zips don't scrape you.
Thanks to the super-flexible neoprene now available, it's easier than ever to get a suit that fits. Some manufacturers provide an anatomical shape that allows for the differences between men and women.
When someone recently asked me which was the best semi-dry suit I have used, my reply was that it was the one that fitted me best. Fit is everything.
A DRYSUIT KEEPS THE DIVER DRY. Membrane suits are made from various watertight materials. The most popular, often called a trilaminate, is a sandwich of butyl rubber between a layer of man-made fibre, such as Cordura or nylon.
One US manufacturer employs neoprene that is crushed after construction of the suit. Others use a pre-compressed neoprene material.
A membrane suit forms a watertight barrier with latex seals at neck and wrists, and is donned via an opening that is closed with a watertight zip. These barriers keep the water out, and insulation from the cold is typically provided by what is worn beneath it.
Neoprene drysuits need a less insulating under- layer because the neoprene itself provides insulation. They are available with either latex or neoprene seals, but if the latter are chosen there is more likelihood of slight water ingress.
In fact, because you can vary the amount of insulation from 200gsm to nothing at all, one could be persuaded that a membrane drysuit is ideal in all circumstances - but it isn't.
Swimming in a semi-dry is almost as unrestricting as swimming with no suit. If you need to cover a large distance, a semi-dry comes into its own, because a drysuit has to be "driven".
Air is injected or dumped to maintain its overall volume and hence the diver's buoyancy according to depth.
This internal air space moves around within the suit and can affect trim. Loss of control of buoyancy on an ascent can have serious consequences.
The membrane drysuit is, by its nature, baggy. Even if you have had one made to measure by the cleverest drysuit tailor in the world (and I have) there will inevitably be folds of material that conspire to make you less aquadynamic.
On a square-profile wreck dive this may not be important, but you wouldn't want to have to cover any great distance horizontally under water against a current. A neoprene drysuit, tailored for a close fit that may not need to account for a very bulky undergarment, can be better in this respect.
THEN THERE IS THE QUESTION OF LATEX versus neoprene seals. Latex may be fragile and need care in use, but it does keep water at bay. Neoprene may be more comfortable and hard-wearing but, unless you're lucky, it will tend to let trickles of water past.
A cuff dump is a cheap and effective way of jettisoning expanding air on ascent by raising an arm. But a properly sited auto-dump, or more properly named "constant-volume" dump valve, employs a spring tensioner to maintain the suit volume, and hence its buoyancy during an ascent.
Latex wrist seals allow the easy installation of dry-gloves. Dry-gloves use a warm woolly liner to keep the hands warm, and share the same internal volume with the rest of the suit.
You can get them with their own latex wrist seals, but these will crush up with the pressure of depth, losing all insulation, or balloon during an ascent. Many drysuit divers prefer to use neoprene wet gloves.
Dry hoods have not proved popular, because of the risk of reversed ear during an ascent; so wet hoods are commonly employed.
A good drysuit will give you the flexibility to employ an electrically heated under-vest once the connection for the battery-pack has been fitted.
When choosing any suit, look at how the seams are constructed. On a semi-dry, check that the stitching looks well done. Some have additional sealing. Drysuits can be stitched, glued or taped, or any combination of the three. The security of the seams will dictate the longevity of the suit.
You can pay from £550 to three times that much for a drysuit with undergarment. The price range reflects quality of construction and accessories.
Whatever you choose, be sure that it fits you properly. And if you're worried about how you'll look, take up skiing!
You can read reviews of individual suits that we have tested by going to: www.divernet.com and clicking on DIVER Tests and Group Tests under Diving Gear.
WHAT SHOULD I WEAR WHERE?
The Typhoon Seamaster neoprene drysuit at £430 is one of the least expensive drysuits available. It's made from neoprene compressed to 5mm and lined with titanium, and uses 3mm material for the arms to give greater flexibility.
The Pinnacle Stealth is one of the warmest semi-dry suits we have tested. Made from 7mm neoprene lined with natural Merino wool, it has no zips. You climb in through the neck seal, pulling the hood and bib over to seal it. It came from New Zealand and has been superseded by the Extreme 8/6, with a cross-chest dryzip for easier donning. It costs an estimated £360 with boots and gloves.