Diving helmet

A diving helmet is a rigid head enclosure with a breathing gas supply used in underwater diving. They are worn mainly by professional divers engaged in surface-supplied diving, though some models can be used with scuba equipment. The upper part of the helmet, known colloquially as the hat or bonnet, may be sealed directly to the diver using a neck dam, connected to a diving suit by a lower part, known as a breastplate, or corselet, depending on regional language preferences, or simply rest on the diver's shoulders, with an open bottom, for shallow water use.
The helmet isolates the diver's head from the water, allows the diver to see clearly underwater, provides the diver with breathing gas, protects the diver's head when doing heavy or dangerous work, and usually provides voice communications with the surface. If a helmeted diver becomes unconscious but is still breathing, most helmets will remain in place and continue to deliver breathing gas until the diver can be rescued. In contrast, the scuba regulator used by recreational divers must be held in the mouth by bite grips, and it can fall out of an unconscious diver's mouth and result in drowning.
Before the invention of the demand regulator, all diving helmets used a free-flow design. Gas was delivered at an approximately constant rate, independent of the diver's breathing, and flowed out through an exhaust valve against a slight over-pressure. Most modern helmets incorporate a demand valve so the helmet only delivers breathing gas when the diver inhales. Free-flow helmets use much larger quantities of gas than demand helmets, which can cause logistical difficulties and is very expensive when special breathing gases are used. They also produce a constant noise inside the helmet, which can cause communication difficulties. Free-flow helmets are still preferred for some applications of hazardous materials diving, because their positive-pressure nature can prevent the ingress of hazardous material in case the integrity of the suit or helmet is compromised. They also remain relatively common in shallow-water air diving, where gas consumption is of little concern, and in nuclear diving because they must be disposed of after some period of use due to irradiation; free-flow helmets are significantly less expensive to purchase and maintain than demand types.
Most modern helmet designs are sealed to the diver's skin at the neck using a neoprene or latex "neck dam" which is independent of the suit, allowing the diver a choice of suits depending on the dive conditions. When divers must work in contaminated environments such as sewage or dangerous chemicals, the helmet is directly sealed to a dry suit made of a fabric with a smooth vulcanised rubber outer coating to completely isolate and protect the diver. This equipment is the modern equivalent of the historic "standard diving dress".

Function and structure

The usual meaning of diving helmet is a piece of diving equipment that encases the user's head and delivers breathing gas to the diver, but the term "diving helmet", or "cave diving helmet" may also refer to a safety helmet like a climbing helmet or caving helmet that covers the top and back of the head, but is not sealed. These may be worn with a full-face mask or half mask to provide impact protection when diving under an overhead, and may also be used to mount lights and video cameras.
An alternative to the diving helmet that allows communication with the surface is the full-face diving mask. These cover most of the diver's face, specifically including eyes, nose and mouth, and are held onto their head by adjustable straps. Like the diving helmet, the full-face mask is part of the breathing apparatus.
Another style of helmet construction, seldom used, is the, which uses a front section with a hinged back section, clamped closed, and sealed along the joint. These were seldom satisfactory due to problems with the seal. Prototypes of this type were made by Kirby Morgan and Joe Savoie.

Components

Basic components and their functions:

Helmet casing or shell – A rigid watertight structure that encloses the diver's head and supports most of the other components. The part of a standard helmet that encloses the head may also be called the "bonnet", and the whole unit may be called the "hat" by professional divers. Traditionally made from spun copper, more recently from glass fibre reinforced resin or stainless steel.
Lower seal – Most deep-water helmets have a means of excluding water from the helmet regardless of the posture of the diver. Shallow water helmets rely on the diver keeping the helmet approximately upright, and the flow of breathing air keeps the water level lower than the diver's nose and mouth, and excess air escapes at the bottom of the helmet.
*Direct to the dry suit – The helmet may be sealed directly to the neck opening of the dry suit, making the helmet and suit a single watertight unit. The weight of the helmet may be carried by the head and neck, so it must be nearly neutrally buoyant, or can be supported by a breastplate or corselet, in which case it may be negatively buoyant, or positively buoyant and held down by jocking straps.
*Via a neck dam – The more recent development is for the helmet to clamp to a neck dam, supported by a rigid ring, or clamped to the lower edge of the helmet. The neck dam seals against the skin of the divers neck in the same way that the neck seal of a dry suit works, making the helmet a sealed unit independent of the suit, which may be a dry suit, wet suit or hot-water suit, or even just a pair of overalls in warm water. The weight of the helmet is carried by the head and neck, so it must be nearly neutrally buoyant, and is generally slightly heavy in the water so that it rests on the head and does not have a tendency to float off. When the helmet is buoyant, it is held down by a jocking strap.
*Breastplate/corselet – An alternative system is for the helmet to seal to a corselet or breastplate, which in turn is sealed to the dry suit. The helmet and suit become a single sealed unit, somewhat more complex than with a direct seal, but easier to put on and take off. The weight of the helmet is carried on the shoulders via the breastplate, so it does not have to be neutrally buoyant, and can be directly weighted or held down by a jocking strap.
' – The diver's window to the world. A transparent window in the front of the helmet or full-face mask. If the helmet is light and carried directly on the head and neck, and can move with the head, it is usually relatively small, and the helmet too is compact and relatively light. If the helmet is supported by the shoulders it cannot turn with the head, and must have a larger volume with a larger viewport or more than one viewport to give an adequate field of vision. A four-light helmet was a common design, with a front port which could be opened when out of the water, two side-lights, to the left and right, and an upper light above the front light to give an upward view.
Gas supply – The breathing gas supply is connected to the helmet. This is usually a low pressure surface supply hose attached through a non-return valve to a gas block with a bailout gas supply connected to the bailout valve on the gas block, but other systems have been used.
*Inlet valve or demand valve – The primary gas supply may be free-flow or demand controlled. If it is demand controlled there is usually a free-flow bypass, which may also serve as a defogging system, by blowing air over the inner face of the front viewport.
* An internal oro-nasal mask is used in demand supplied helmets to minimise dead space. The oro-nasal mask seals around the nose and mouth, forming a small volume gas space through which the breathing gas normally flows from demand valve to nose or mouth, and from nose or mouth to the exhaust valves. There are also one-way valves from the main helmet space into the oro-nasal mask, to allow flow into it while breathing from the free-flow supply, or from a pneumo-hose for emergency supply.
Gas exhaust system – Exhaled gas is exhausted from the helmet through non-return valves, either directly to the surrounding water, or via a reclaim regulator system through a hose to the surface. There may be two or more sets of non-return valves in series to reduce the risk of back-flow of contaminated water.
Voice communications microphone and headphone speakers connected to the surface via copper conductors in the umbilical cable.
Nose blocker, also known as a valsalva device – A device is provided which the diver can use to block the nose for ear equalising maneuvers.
Other accessories may be present, such as a lifting handle,, light and video camera brackets, a welding visor, spitcock and internal padding.
* A lifting handle allows the attendant to lift the helmet with one hand, without it swinging or tilting excessively.
* Ballast weights provide neutral or slightly negative buoyancy, and place the centre of gravity where it will not cause off-centre loads on the diver's neck.
* Brackets allow easy mounting of lights and video cameras so that the diver can see in dark conditions, and the supervisor can see what the diver is doing.
* A welding visor is fitted on a hinge to protect the diver from bright light and ultraviolet light produced by welding arcs or cutting flames. It is flipped down over the faceplate for use when needed. Most welding visors are heavily tinted glass, and must be manually flipped down for use, but electronically controlled auto-darkening visors are also available.
* A ' was provided on some standard helmets to allow the diver to suck in a mouthful of water which would then be spat onto the inner surface of a viewport to wash off condensation fogging. The water would then usually be trapped between the outside of the suit and the inside of the breastplate. The spitcock could also be opened as an auxiliary exhaust port when the diver was working in some non-upright positions.
* Internal padding is fitted to lightweight helmets to cushion the diver's head against shock loads, to support the helmet more comfortably and so that it will closely follow the head movement. A chin strap can be used to help with these functions. A close-fitting insulated cap may be provided for warmth.
* A ' or jocking harness can be used to transfer excess buoyancy forces of the helmet to the diver's weighting system, or part of the weighting system can be directly attached to the helmet.
* Part of the water used to heat a hot water suit can be routed through a water jacket around part of the breathing gas supply tubing on the helmet, typically the metal tube between the bailout valve block and the demand valve inlet to heat the gas just before delivery through the demand valve. As a large part of body heat loss is in heating the inspired air to body temperature on every breath, this can reduce heat loss significantly on deep dives in cold water.
A ', or is a component that folds under the bottom opening of a lightweight helmet making the opening smaller than the head, and when locked in place, prevents the helmet from lifting off the diver's head.