page_title: Surface-supplied diving
section_title: Bailout gas supply
hierarchical_section_title: Surface-supplied diving / Breathing gas supply / Bailout gas supply
caption_attribution_description: English: Detail of the RH side of a Kirby Morgan 18 band mask showing the bailout block and associated fittings
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: Bailout gas is usually carried by the diver in a scuba cylinder, mounted on the back of the harness in the same position as is used with recreational scuba. The size of the cylinder will depend on operational variables. There should be sufficient gas to enable the diver to reach a place of safety on the bailout gas in an emergency. For surface oriented dives, this may require gas for decompression, and bailout sets generally start at about 7 litres internal capacity and can be larger.
  Bell diving bailout options: For bell dives there is no requirement for decompression gas, as the bell itself carries bailout gas. However at extreme depths the diver will use gas fast, and there have been cases where twin 10 litre 300 bar sets were required to supply sufficient gas. Another option which has been used for extreme depth is a rebreather bailout set. A limitation for this service is that the diver must be able to get in and out of the bell while wearing the bailout equipment.
  Mounting options: The bailout cylinder may be mounted with the valve at the top or at the bottom, depending on local codes of practice. A generally used arrangement is to mount the cylinder with the valve up, as this is better protected while kitting up, and the cylinder valve is left fully open while the diver is in the water. This means that the regulator and supply hose to the bailout block will be pressurised during the dive, and ready for immediate use by opening the bailout valve on the harness or helmet.
  The bailout block is a small manifold fitted either to the harness where it is in a convenient but protected position, commonly on the right side on the waist strap, or on the helmet, also usually on the right side of the temple, with the valve knob to the side to distinguish it from the free-flow or defogging valve which is commonly to the front. The bailout block has a connection for the main gas supply from the umbilical through a non-return valve. This route can not be closed and supplies the helmet demand valve and free flow valve under normal circumstances. The bailout gas from the back mounted cylinder passes through a conventional scuba first stage at the cylinder valve, to the bailout block, where it is normally isolated by the bailout valve. When the diver needs to switch over to bailout gas he simply opens the bailout valve and the gas is supplied to the helmet or mask. As the valve is normally closed, a leak in the first stage regulator seat will cause the interstage pressure to rise, and unless an overpressure relief valve is fitted to the first stage the hose may burst. Aftermarket overpressure valves are available which can be fitted into a standard low-pressure port of most first stages.
  Bailout supply pressure options: If the interstage pressure for the bailout regulator is lower than the main supply pressure, the main supply will override the bailout gas, and continue to flow. This can be a problem if the diver switches to bailout because main supply is contaminated. If on the other hand, bailout pressure is higher than main supply pressure, the bailout gas will override the main gas supply if the valve is opened. This will result in the bailout gas being used up if the valve leaks. The diver should periodically check that bailout pressure is still sufficient for the rest of the dive, and abort the dive if it is not. For this reason the bailout regulator must be fitted with a submersible pressure gauge to which the diver can refer to check the pressure. This is usually clipped off or tucked into the harness on the left side, where it can be easily reached to read, but is unlikely to snag on anything.
caption: The bailout block on a KM18 band mask, showing the bailout valve (upper left), the non-return valve for main air supply (lower left), and the free-flow valve (right)
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