Diving helmet types may be distinguished by several characteristics. They may be open to the water at the bottom, sealed to the suit, or sealed to the diver by a neck dam. They may provide a constant flow of gas, from which the diver breathes as it passes, or supply gas only on demand, they may have neutral buoyancy, or be held in place by straps or ballast weights, and they may vent all the gas after a single pass, or recirculate it for reduced consumption. In many cases they may be switched between modes during a dive. Demand helmets usually have a valve to provide free-flow, and closed or semi-closed circuit systems can often be switched to open circuit.
Standard diving helmets (Copper hats) The original standard diving equipment was a copper helmet or "bonnet" (British English) clamped onto a copper breastplate or "corselet", which transferred the weight to the diver's shoulders. This assembly was clamped to a rubber gasket on the dry suit to make a watertight seal. Breathing air and later sometimes helium based gas mixtures were pumped through a hose to a non-return inlet valve on the helmet or breastplate, and released to the surroundings through an exhaust valve. Historically, deep sea diving helmets were described by the number of bolts used to clamp them to the rubber gasket of the diving suit, and where applicable, the number of bolts used to secure the bonnet (helmet) to the corselet (breastplate). This ranged from the no bolt, two, three, and four bolt helmets; corselets with six, eight, or 12 bolts; and Two-Three, Twelve-Four, and Twelve-Six bolt helmets. For example, the US twelve-four helmets used 12 bolts to clamp the breastplate to the suit, and four bolts to seal the helmet to the breastplate. The no-bolt helmet used a spring-loaded clamp to secure the helmet to corselet over the suit gasket, and many helmets were sealed to the breastplate by a 1/8 turn interrupted screw thread. Swedish helmets were distinctive for using a neck ring instead of a corselet, a precursor of more modern diving equipment, but cumbersome and uncomfortable for the diver. A further distinction is the number of viewports, or "lights", usually one, three or four. The front light could be opened for air and communications when the diver was out of the water. This equipment is commonly referred to as Standard diving dress and "heavy gear." Occasionally, divers would lose consciousness while working at 120 feet in standard helmets. The English physiologist
J.S. Haldane found by experiment that this was partly due to a buildup of carbon dioxide in the helmet caused by insufficient ventilation and a large dead space, and established a minimum flow rate of per minute at ambient pressure.
Gas extender helmets A small number of copper Heliox helmets were made by the US Navy for the Second World War. These helmets were Mk Vs modified by the addition of a bulky brass
carbon dioxide scrubber chamber at the rear, and are easily distinguished from the standard model. The Mk V Helium weighs about complete (bonnet, scrubber canister and corselet) These helmets and similar models manufactured by Kirby Morgan,
Yokohama Diving Apparatus Company and
DESCO used the scrubber as a gas extender, a form of
semi-closed rebreather system, where breathing gas was recirculated through the scrubber by entraining the helmet gas in the flow from an
injector supplying fresh gas, a system pioneered by Dräger in 1912.
Semi-closed rebreather helmets In 1912 the German firm
Drägerwerk of Lübeck introduced a version of standard diving dress using a gas supply from an oxygen rebreather and no surface supply. The system used a copper diving helmet and standard heavy diving suit with a back-mounted set of cylinders and scrubber. The breathing gas was circulated by using an injector system in the loop powered by the added gas. This was developed further with the
Modell 1915 "Bubikopf" helmet and the DM20 oxygen rebreather system for depths up to 20 m, and the DM40 mixed gas rebreather which used an oxygen cylinder and an air cylinder for the gas supply, producing a nitrox mixture, for depths up to 40 m. The Siebe Gorman Mine Recovery Type Self-Contained Diving Apparatus (MRS) was a diving suit based on a two-light copper helmet, with the corselet glued to the top part of a diving suit which had rubber seals round the waist of the jacket and trouser sections. The system used a nitrox mixture with 45% oxygen which was partly recirculated and scrubbed of carbon dioxide, and could be used to depths of up to 40 m. Maximum endurance wast 60 to 90 minutes depending on depth. The front viewport was oval with a hinge at the bottom. The exhaust port on the right side was unusual in appearance, with a circular perforated plate cover. The gas extender unit with scrubber was back mounted, with three small cylinders, and circulated gas to the helmet through corrugated hoses attached behind the ears.
Shallow-water helmets The shallow water helmet is a very simple concept: a helmet with viewports which is fitted by lowering over the diver's head to rest on the shoulders. It must be slightly negatively buoyant when filled with air so that it does not float off the diver in use. Air is supplied through a low pressure hose and escapes at the bottom of the helmet, which is not sealed to the suit, and can be lifted off by the diver in an emergency. The helmet will flood if the diver leans over or falls over. The shallow water helmet generally has a handle on top to help the tender lift it onto and off the diver when out of the water. The structure is variable, and ranges from relatively heavy metal castings to lighter sheet metal shells with additional ballast. The concept has been used for recreational diving as a breathing system for use by untrained tourists in the direct care of a dive leader in a benign diving environment, marketed as the
Sea Trek diving system.
Lightweight demand helmets The lightweight diving helmet is a type which is fitted more closely to the diver's head, reducing the interior volume, and thereby reducing the displaced volume of the helmet, so less mass is required to make the helmet's buoyancy neutral. The consequence is a reduced overall mass for the equipment carried by the diver, who must not be buoyant in the water. This reduction in volume and mass allows the diver to more safely support the helmet on the head and neck when out of the water, so when it is immersed and neutrally buoyant, it is comfortable to move around with the head, allowing the diver to use neck movement to change the direction of view, which in turn increases the diver's total field of vision while working. Since the lightweight helmet can be supported by the head and neck, it can be sealed to the neck, using a neck dam, independent of the diving suit, making operations equally convenient with dry suits and wetsuits, including hot water suits. Some models can be sealed directly to a dry suit for maximum isolation from the environment. The foam neoprene or latex neck dam of many of the popular Kirby-Morgan helmets is fitted to an oval metal neck ring which hooks onto the bottom of the helmet in front. A folding locking collar at the back of the helmet swings forward and up to push the back of the neck ring up into the base of the helmet, and also prevents the helmet from lifting off the head by partly occluding the neck ring opening at the back. The locking collar is secured in the locked position by two spring loaded pull-pin latches. The helmet seals over the neck ring with a barrel seal O-ring. Other arrangements may be used with similar effect on other models, such as the KMSL 17B, where the seal is made on the outside of the helmet to an O-ring seated in a groove in the fibreglass rim. A lever operated clamp with a yoke is mounted on the neck dam and seals to the helmet rim, or a moulded rubber seal bonded to a dry suit is clamped to the helmet using a similar clamp system.
Open circuit demand helmets Notable modern commercial helmets include the
Kirby Morgan Superlite-17 from 1975 and developments from that model. These helmets are of the demand type, usually built on a fiberglass shell with chrome-plated brass fittings, and are considered the standard in modern commercial diving for most operations. Kirby Morgan dominates the new helmet market, but there have been other manufacturers including
Savoie, Miller,
Gorski,
Composite-Beat Engel,
Divex, and Advanced Diving Equipment Company. Many of these are still in use; a new helmet represents an investment of several thousand dollars, and most divers purchase their own or rent one from their employer.
Reclaim helmets Reclaim helmets use a surface supply system to provide breathing gas to the diver in the same way as in the open circuit helmets, but also have a return system to reclaim and recycle the exhaled gas to save the expensive helium diluent, which would be discharged to the surrounding water and lost in an open circuit system. The reclaimed gas is discharged from the helmet through a back-pressure regulator and returned to the surface through a hose in the umbilical which is provided for this purpose, passed through a scrubber to remove carbon dioxide, blended with oxygen to the required mix and repressurised for immediate re-use or stored for later use. In order to allow the exhaust gas to be discharged from the helmet safely, it must pass through an exhaust back-pressure regulator, which works on the same principle to a
built-in breathing system exhaust valve, activated by the pressure difference between the interior of the helmet and the ambient pressure. The reclaim exhaust valve may be a two-stage valve for lower resistance, and will generally have a manual bypass valve which allows exhaust to the ambient water. The helmet will have an emergency flood valve to prevent possible exhaust regulator failure from causing a helmet squeeze before the diver can bypass it manually.
Free-flow helmets In a free-flow or constant flow helmet, gas is delivered at an approximately constant rate, set by the panel operator, and usually adjustable by the diver, independent of the diver's breathing, and flows out through an exhaust valve against a slight adjustable over-pressure. 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 (such as heliox) 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 due to simple construction and fewer components. The
DESCO "air hat" is a metal free-flow helmet, designed in 1968 and still in production. Although it has been updated several times, the basic design has remained constant and all upgrades can be retrofitted to older helmets. Its robust and simple design (it can be completely disassembled in the field with only a screwdriver and wrench) makes it popular for shallow-water operations and hazardous materials diving. The helmet is secured to the diving suit by a neck ring, and held in place on the diver against buoyancy by means of a "jocking strap" which runs between the legs. Buoyancy can be fine-tuned by adjusting intake and exhaust valves to control the internal pressure, which will control the volume of gas in the attached dry suit. Concept and operation are very similar to the standard diving helmet. Noise level can be high and can interfere with communications and affect diver hearing. The US Navy replaced the Mark V helmet in 1980 with the
Morse Engineering Mark 12 deep water helmet which has a fibreglass shell with a distinctive large rectangular front faceplate for a better field of vision for work. It also has side and top viewports for peripheral vision. This helmet can also be used for mixed gas either for open circuit or as part of a modular semi-closed circuit system, which uses a back mounted recirculating scrubber unit connected to the lower back of the helmet by flexible breathing hoses. The helmet uses a neck dam or can be connected directly to a dry suit, and uses a jocking harness to keep the helmet in position, but is ballasted to provide neutral buoyancy and a centre of gravity at the centre of buoyancy for stability. Airflow is directed over the faceplate to prevent fogging. Both the Mk V and the Mk 12 were in use in 1981. The noise level in the Mk 12 in open circuit mode can have adverse effects on diver hearing. Sound intensity levels have been measured at 97.3 dB(A) at 30.5 msw depth. The Mk 12 was phased out in 1993. Light-weight transparent dome type helmets have also been used. For example, the Sea Trek surface supplied system, developed in 1998 by Sub Sea Systems, is used for recreational diving. Also the Lama, a near spherical acrylic dome helmet developed by Yves Le Masson in the 1970s, has been used in
television to let viewers see the face and hear the voice of the presenter speaking underwater, and was also used with the Lockout Breathing System, a back mounted umbilical supplied rebreather system rated to 300msw, and used in the North Sea. File:AH3 free flow helmet front viewDSC01101.jpg|Front view of an AH3 free flow diving helmet File:AH3 free flowhelmet side view DSC01102.jpg|Side view of an AH3 free flow diving helmet File:Dräger DM 220.jpg|Dräger DM 220
Push-pull helmets These are helmets which use a flow of supply gas which is recovered and recycled in a closed circuit system, such as from the atmosphere of a saturation system like a closed bell, underwater habitat, or lock-out submersible. The gas is pumped to the diver through the umbilical, and pumped back to the life-support system for carbon dioxide scrubbing and oxygen replenishment. Pressure in the helmet is maintained at ambient pressure, and the work of breathing is relatively low. A high flow rate must be maintained in a continuous flow system to compensate for potential dead space in the helmet, but as the gas is recycled, very little is lost. Lateral excursions are limited by the umbilical reach, but vertical excursions are restricted by the ability of the control valves to manage pressure variations between gas source and the helmet while providing acceptable work of breathing. The
Divex Arawak system is an example of a successful push-pull system used in the
SEALAB projects The Arawak V system, patented in 1968, is rated to be used at depths between . The breathing circuit for the Arawak has no carbon dioxide scrubber, gas mixing facilities, or gas monitoring system, as it makes use of the breathing gas of the subsurface base unit, which is already continuously controlled and monitored. This makes the preparation and after-dive maintenance very simple in comparison with a conventional rebreather. Limitations include lateral range restriction according to the length of the umbilical, and excursion depth restrictions due to work of breathing changes caused by the regulator valves when the hydrostatic pressure variations are too large. The helmet is rated for upward excursions of and downward excursions of . ==Safety==