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The Deep sea dive helmet ranges from the no bolt to two bolt to four bolt helmets; helmets with six, eight, or 12 bolts; and Two-Three, Twelve-Four, and Twelve-Six bolt helmets. Bolts being the technique of fastening the helmet to the diving suit.
The helmet could also be secured to the breastplate (corselet) by bolts as in the example of US twelve-four helmets (12 bolts to the suit, four bolts seal helmet to corselet). The no bolt helmet applied a spring-loaded clamp to secure the helmet to corselet over the suit. Swedish helmets were distinctive for applying a neck ring rather than a corselet, a pioneer of modern diving equipment but tremendously clumsy and awkward for the underwater diver. This equipment is generally referred to as "heavy gear."
These helmets are worn chiefly by professional divers engaged in surface supplied diving although a lot of models can be modified for use with scuba equipment.
The helmet seals the whole of the underwater divers head from the water, permits the diver to see clearly underwater, supplies the diver with breathing gas , protects the diver's head when doing hard or dangerous work, and commonly supplies voice communications with the surface (and possibly additional divers).
If a helmeted diver becomes unconscious but is still breathing, the helmet will remain in place and continue to deliver breathing gas until the diver can be rescued .
In contrast, the scuba regulator commonly used by amateur divers must be held in the mouth, otherwise it could come out of an unconscious diver's mouth and lead to drowning this doesn't apply to a full face mask which also continues to serve air if the diver is unconscious).
Prior to the invention of the demand regulator, all helmets used a free-flow design. Gas was delivered at a constant rate, irrespective of the diver's respiration, and flowed out through an exhaust valve. Nearly all modern helmets incorporate a demand valve then the helmet only delivers breathing gas when the diver breathes in.
The free-flow dive helmet use very much larger quantities of gas than demand helmets, which can cause logistical difficulties and are very highly priced when special breathing gases such as heliox are used. They also develop a constant noise inside the helmet, which may cause communication troubles. Free-flow helmets are still favored for risky materials diving, because their positive-pressure nature can prevent the ingress of hazardous material in case the integrity of the suit or dive helmet is compromised.
They also remain relatively more common in shallow-water air diving, where gas consumption is of little concern, and in nuclear diving because they must be discarded of after some period of use due to irradiation; free-flow helmets are significantly less expensive to buy and maintain than demand types.
Most modern helmet designs are sealed at the neck using a neoprene "neck dam" which is independent of the suit, permitting the diver his choice of suits depending upon the dive conditions. When a neck dam is installed into a dry-suit, however, the full body is isolated from the surrounding liquid, giving an extra degree of warmth and protective cover.
When divers must work in hazardous surrounds such as sewage or life-threatening chemicals, a dive helmet (normally of the free-flow type or using a series exhaust valve system) is sealed to a special dry-suit (normally made of a fabric with a smooth vulcanized rubber outer surface) to entirely isolate and protect the diver. This equipment is the modern equivalent of the historic Mark V "Standard Diving Dress .
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