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 secured to the diver or diving suit by a lower part, known as a "neck dam", "breastplate", or "corselet", depending on the construction and regional language preferences. The helmet seals the whole of 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 (and possibly other 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 typically 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 (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. 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 (usually of the free-flow type or using a series exhaust valve system) is directly sealed to a special dry suit (commonly made of a fabric with a smooth vulcanised rubber outer surface) to completely isolate and protect the diver. This equipment is the modern equivalent of the historic "Standard diving dress". Structure. Basic components and their function: History. Deane brothers. The first successful diving  helmets were produced by the brothers Charles and John Deane in the 1820s. Inspired by a fire accident he witnessed in a stable in England, he designed and patented a "Smoke Helmet" to be used by firemen in smoke-filled areas in 1823. The apparatus comprised a copper helmet with an attached flexible collar and garment. A long leather hose attached to the rear of the helmet was to be used to supply air - the original concept being that it would be pumped using a double bellows. A short pipe allowed breathed air to escape. The garment was made of  leather or airtight cloth, secured by straps. The brothers lacked money to build the equipment themselves, so they sold the patent to their employer, Edward Barnard. In 1827, the first smoke helmets were built, by German-born British engineer Augustus Siebe. In 1828 the brothers decided to find another application for their device and converted it into a diving helmet. They marketed the helmet with a loosely attached "diving suit" so that a diver could perform salvage work, but only in a fully vertical position (otherwise water entered the suit). In 1829 the Deane brothers sailed from Whitstable for trials  of their new underwater apparatus, establishing the diving industry in the town. In 1834 Charles used his diving helmet and suit in a successful attempt on the wreck of "Royal George" at Spithead, during which he recovered 28 of the ship's cannon. In 1836, John Deane recovered from the discovered "Mary Rose" shipwreck timbers, guns, longbows, and other items. By 1836 the Deane brothers had produced the world's first diving manual, "Method of Using Deane's Patent Diving Apparatus", which explained in detail the workings of the apparatus and pump, and safety precautions. The Siebe helmet. In the  1830s the Deane brothers asked Siebe to apply his skill to improve their underwater helmet design. Expanding on improvements already made by another engineer, George Edwards, Siebe produced his own design; a helmet fitted to a full length watertight canvas diving suit. The equipment included an exhaust valve in the helmet, which allowed excess air to escape without allowing water to flow in. The closed diving suit, connected to an air pump on the surface, became the first effective standard diving dress, and the prototype of hard-hat rigs still in use today. Siebe introduced various modifications on his diving  dress design to accommodate the requirements of the salvage team on the wreck of , including making the helmet detachable from the corselet; his improved design gave rise to the typical standard diving dress which revolutionised underwater civil engineering, underwater salvage, commercial diving and naval diving. Lightweight helmets. Commercial diver and inventor Joe Savoie is credited with inventing the neck dam in the 1960s, which made possible a new era of lightweight helmets, including the Kirby Morgan Superlite series (an adaption of Morgan's existing "Band Mask" into a full helmet.) Savoie did not patent his invention, which allowed widespread development  of the concept by other manufacturers. The neck dam seals the helmet around the diver's neck in the same way that a dry suit neck seal works, using similar materials. This allows the helmet to be carried on the head and not on a corselet, so the helmet can turn with the head and can therefore be a much closer fit, which considerably reduces the volume, and as the helmet must be ballasted for neutral buoyancy, the overall weight is reduced. Types. 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." A small number of copper Heliox helmets were made for the US Navy by 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. 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 negative 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 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 ballast 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. Open circuit 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 notable manufacturers including Savoie, Miller, Gorski and Swindell. 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. Oceaneering bought out the Ratcliffe helmet, often known by its nickname "Rat Hat". It can function in either free-flow or demand mode. 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. 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 "jock 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 ad affect diver hearing. The US Navy replaced the Mark V helmet 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 	Diving helmet
25936176	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 bouyancy for stability. Airflow is directed over the faceplate to 	Diving helmet
25936177	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. 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, developed by Yves Le Masson in the 1970s, has been used in television to let viewers see the face and hear 	Diving helmet
25936178	the voice of the presenter speaking underwater. Other. An alternative to the diving helmet that allows communication with the surface is the full face diving mask. These cover the diver's face and are held onto their head by adjustable straps. Diving helmet" may also refer to a rigid safety helmet like a workman's 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. During the First World War the British Army used a few diving helmets out of  water as emergency protection from mustard gas.