If you need a real thermal comfort when diving, if you want to stay warm the entire dive, not only non-hypothermic, but doggone-it, downright toasty, buy a dry suit.
They are made from a mixture of waterproof materials such as vulcanized rubber, waterproofed nylons and compressed neoprene. The majority of suits is designed to seal around the wrists and neck, and are equipped with waterproof zippers. A low-pressure port in the regulator's first stage in the dry suits is used to inflate the interior of the suit, thus preventing suit squeeze at depth and providing buoyancy. Dry suits have exhaust valves, often located on the sleeves, allowing air to escape on ascent.
The function of the dry suit is to give a waterproof, protective "outer skin" under which the diver is able to wear warm underclothing. Also, the air within the dry suit gradually warms up through body heat and this also helps to insulate the diver against the surrounding cold water. You should also bear in mind that dry-suit underwear does not compress considerably under pressure since, if it does, it will not work efficiently at depth.
The seams, the wrist and neck seals and the zip are the main items contributing to the dryness of the suit. There should be no thread that passes from the outside of the suit through to the inside, as this would provide a path for water to follow. A good solution is when any exterior stitching is hidden by tape, particularly if the tape is vulcanized to the suit. Seals at the wrist and neck are usually made of thin neoprene or latex. Latex seals do not offer any insulation and can be fragile, though they are generally drier. Though it will be more difficult to get a good fit with a neoprene seal as the surface has a tendency to develop wrinkles with time, it is tough, easy to repair, and provides insulation. A perfect solution is top-of-the-range suit fit a latex seal with a neoprene protective collar.
Usually the same manufacturers make most zippers, but it is the way in which they are attached to a suit that can cause problems. If the zip is longer that usually makes the suit easier to put on, but it also pushes up the price. Usually the "across the shoulder" zip is the shortest and cheapest, followed by the front "shoulder to hip" and then the "around-the-neck" versions.
If you are hesitating with which hood, attached or loose to buy, you should choose it accordingly your type of diving. In case you will spend long periods in your suit out of the water, it is better to take with a separate hood so that you can don it just before entering the water. And if you may go into the water soon after dressing in your suit, an attached hood lets less water in.
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