page_title: Surface-supplied diving
section_title: Bonnet
hierarchical_section_title: Surface-supplied diving / Breathing apparatus / Free flow helmet / Standard diving helmet (Copper hat) / Bonnet
caption_attribution_description: Standard diving dress. Hardhat diver entering water at Stoney Cove, England
context_page_description: Surface-supplied diving is diving using equipment supplied with breathing gas using a diver's umbilical from the surface, either from the shore or from a diving support vessel, sometimes indirectly via a diving bell. This is different from scuba diving, where the diver's breathing equipment is completely self-contained and there is no link to the surface. The primary advantages of conventional surface supplied diving are lower risk of drowning and considerably larger breathing gas supply than scuba, allowing longer working periods and safer decompression. Disadvantages are the absolute limitation on diver mobility imposed by the length of the umbilical, encumbrance by the umbilical, and high logistical and equipment costs compared with scuba. The disadvantages restrict use of this mode of diving to applications where the diver operates within a small area, which is common in commercial diving work.
context_section_description: The bonnet is usually a copper shell with soldered brass fittings. It covers the diver's head and provides sufficient space to turn the head to look out of the glazed faceplate and other viewports (windows). The front port can usually be opened for ventilation and communication when the diver is on deck, by being screwed out or swung to the side on a hinge. The other lights (another name for the viewports) are generally fixed. Viewports were glass on the early helmets, with some of the later helmets using acrylic, and are usually protected by brass or bronze grilles.  The helmet has fittings to connect the air line and the diver's telephone.
  Later helmets include a non-return valve where the airline is connected, which prevents potentially fatal helmet squeeze if the pressure in the hose is lost.
  The difference in pressure between the surface and the diver can be so great that if the air line is cut at the surface and there is no non-return valve, the diver would be partly squeezed into the helmet by the external pressure, and injured or possibly killed.
  Helmets also have a spring-loaded exhaust valve which allows excess air to leave the helmet. The spring force is adjustable by the diver to prevent the suit from deflating completely or over-inflating and the diver being floated uncontrollably to the surface. Some helmets have an extra manual exhaust valve known as a spit-cock. This allows the diver to vent excess air when he is in a position where the main exhaust can not function correctly.
caption: Diver in standard dress entering the water
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