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.

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.

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 dry suit made of a fabric with a smooth vulcanised rubber outer coating to completely isolate and protect the diver.

These cover most of the diver's face, specifically including eyes, nose and mouth, and are held onto their head by adjustable straps.

[6][7] Basic components and their functions: The first successful diving helmets were produced by the brothers Charles and John Deane in the 1820s.

[10] Inspired by a fire accident he witnessed in a stable in England,[11] he designed and patented a "Smoke Helmet" to be used by firemen in smoke-filled areas in 1823.

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.

The brothers lacked money to build the equipment themselves, so they sold the patent to their employer, Edward Barnard.

[10] 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 cannons.

In 1836, John Deane recovered from the discovered Mary Rose shipwreck timbers, guns, longbows, and other items.

[12] 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.

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.

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 1.5 cubic feet (42 L) per minute at ambient pressure.

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.

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.

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.

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.

[25] Kirby Morgan dominates the new helmet market, but there have been other manufacturers including Savoie, Miller, Gorski, Composite-Beat Engel,[26][27]Divex, and Advanced Diving Equipment Company.

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.

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.

[35][22] 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.

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[39][40] Use of a sealed helmet for diving is generally safer than a full-face or half mask, as the airway is relatively well protected, and the diver can survive a loss of consciousness until rescued in most circumstances, provided the breathing gas supply is not interrupted.

If sealed to a dry suit, and fitted with a suitable exhaust system, it is also effective against contaminated ambient water.

One of the more obvious hazards is the potential for flooding, but as long as an adequate breathing gas supply is available, the helmet can be purged of water that gets into it.

The neck dam is more vulnerable, but even a major tear can be managed by keeping the head upright to prevent flooding up against the gas inside.

[30] The Anthony and Yvonne Pardoe Collection of Diving Helmets and Equipment – illustrated catalogue (PDF).