Surface-supplied diving equipment
Surface-supplied diving equipment is the equipment required for surface-supplied diving. The essential aspect of surface-supplied diving is that breathing gas is supplied from the surface, either from a specialised diving compressor, high-pressure gas storage cylinders, or both. In commercial and military surface-supplied diving, a backup source of surface-supplied breathing gas should always be present in case the primary supply fails. The diver may also wear a bailout cylinder which can provide self-contained breathing gas in an emergency. Thus, the surface-supplied diver is less likely to have an "out-of-air" emergency than a scuba diver using a single gas supply, as there are normally two alternative breathing gas sources available. Surface-supplied diving equipment usually includes communication capability with the surface, which improves the safety and efficiency of the working diver.
The equipment needed for surface supplied diving can be broadly grouped as diving and support equipment, but the distinction is not always clear. Diving support equipment is equipment used to facilitate a diving operation. It is either not taken into the water during the dive, such as the gas panel and compressor, or is not integral to the actual diving, being there to make the dive easier or safer, such as a surface decompression chamber. Some equipment, like a diving stage, is not easily categorised as diving or support equipment, and may be considered as either. Equipment required only to do the planned underwater work is not usually considered diving or support equipment.
Surface-supplied diving equipment is required for a large proportion of the commercial diving operations conducted in many countries, either by direct legislation, or by authorised codes of practice, as in the case of IMCA operations. Surface-supplied equipment is also required under the US Navy operational guidance for diving in harsh contaminated environments which was drawn up by the Navy Experimental Diving Unit.
Breathing apparatus
The definitive equipment for surface-supplied diving is the breathing apparatus which is supplied with primary breathing gas from the surface via a hose, which is usually part of a diver's umbilical connecting the surface supply systems with the diver, sometimes directly, otherwise via a bell umbilical and bell panel.Lightweight demand helmets
Lightweight demand helmets are rigid structures which fully enclose the head of the diver and supply breathing gas "on demand". The flow of gas from the supply line is activated by inhalation reducing the pressure in the helmet to slightly below ambient, and a diaphragm in the demand valve senses this pressure difference and moves a lever to open the valve to allow breathing gas to flow into the helmet. This flow continues until the pressure inside the helmet again balances the ambient pressure and the lever returns to the shut position. This is exactly the same principle as used for scuba demand valves, and in some cases the same internal components are used. Sensitivity of the lever can often be adjusted by the diver by turning a knob on the side of the demand valve. Lightweight demand helmets are available in open circuit systems and closed circuit systems.The helmet may be of metal or reinforced plastic composite, and is either connected to a neck dam or clamped directly to a dry suit. The neck dam is the lower part of the helmet, which seals against the neck of the diver in the same way that the neck seal of a dry suit works. Neck dams may have neoprene or latex seals, depending on diver preference. Attachment to the neck dam is critical to diver safety and a reliable locking mechanism is needed to ensure that it is not inadvertently released during a dive. When using a dry suit, the neck dam may be permanently omitted and the lower part of the helmet assembly attached directly to the suit.
The term "Lightweight" is relative; the helmets are only light in comparison with the old copper hats. They are supported only by the head and neck of the diver, and are uncomfortably heavy out of the water, as they must be ballasted for neutral buoyancy during the dive, so they don't tend to lift the diver's head with excess buoyancy. There is little difference in weight between the metal shell and GRP shell helmets because of this ballasting, and the weight is directly proportional to the total volume - smaller helmets are lighter. To avoid fatigue, divers avoid donning the helmet until just prior to entering the water. Having the helmet supported by the head has the advantage that the diver can turn the helmet to face the job without having to turn the entire upper torso. This is particularly an advantage when looking upwards. This allows the helmet to have a relatively small faceplate, which reduces overall volume and hence the weight.
Demand breathing systems reduce the amount of gas required to adequately ventilate the diver, as it needs only to be supplied when the diver inhales, but the slightly increased work of breathing caused by this system is a disadvantage at extreme levels of exertion, where free-flow systems may be better. The demand system is also quieter than free-flow, particularly during the non-inhalation phase of breathing. This can make voice communication more effective. The breathing of the diver is also audible to the surface team over the communications system, and this helps to monitor the condition of the diver and is a valuable safety feature.
Open circuit demand helmets
The open circuit demand system exhausts gas to the environment at ambient pressure. As a result, all exhaled gas is lost to the surroundings. For most surface orientated commercial diving where air is the breathing gas in use, this is no problem, as air is cheap and freely available. Even with nitrox it is generally more cost effective to use open circuit, as oxygen is an easily available and relatively inexpensive gas, and blending nitrox is technologically simple, both to mix and to analyse.Reclaim helmets
In the case of compressed air, or nitrox mixtures, the exhaled gas is not valuable enough to justify the expense of recycling, but helium-based mixtures are considerably more expensive, and as the depth increases, the amount of gas used increases in direct proportion to the ambient pressure. As a result, gas cost is a significant factor in deep open circuit diving with helium-based mixtures for long periods. By using a return line for the exhaled gas, it can be recompressed and used again, almost indefinitely. It is necessary to remove carbon dioxide from the reclaimed gas, but this is relatively cheap and uncomplicated. It is generally removed by a scrubber, which is a filter packed with a chemical which reacts with and removes the carbon dioxide from the gas. The reclaimed gas is also filtered to remove odour and microorganisms, and oxygen is added to the required concentration. The gas is compressed for storage between uses.Recovery of the exhaled gas requires special equipment. Simply venting it to a return hose through a non-return valve will not work, as the hose must be maintained at exactly the ambient pressure at the depth of the helmet, otherwise the gas from the helmet will either free-flow out under pressure, or not flow out at all because of back pressure. This obstacle is overcome by using a back-pressure regulator exhaust valve, which opens the exhaust valve by using the leverage of a diaphragm sensing the pressure difference between the helmet interior pressure and the ambient pressure, This only requires the pressure in the reclaim hose to be lower than ambient at the diver to function. The same principle is used in a diving chamber's built-in breathing system.
Free flow helmets
A free flow helmet supplies a continuous flow of air to the diver, and he breathes this as it flows past. Work of breathing is minimal, but flow rate must be high if the diver works hard, and this is noisy, affecting communications and requiring hearing protection to avoid damage to the ears. This type of helmet is popular where divers have to work hard in relatively shallow water for long periods. It is also useful when diving in contaminated environments, where the helmet is sealed onto a dry suit, and the entire system is kept at a slight positive pressure by adjusting the back-pressure of the exhaust valve, to ensure that there is no leakage into the helmet. This type of helmet is often large in volume, and as it is attached to the suit, it does not move with the head. The diver must move his body to face anything he wants to see. For this reason the faceplate is large and there is often an upper window or side windows to improve the field of vision.Standard diving helmet (Copper hat)
The helmet is usually made of two main parts: the bonnet, which covers the diver's head, and the corselet which supports the weight of the helmet on the diver's shoulders, and is clamped to the suit to create a watertight seal. The bonnet is attached and sealed to the corselet at the neck, either by bolts or an interrupted screw-thread, with some form of locking mechanism.The helmet may be described by the number of bolts which hold it to the suit or to the corselet, and the number of vision ports, known as lights. For example, a helmet with four vision ports, and twelve studs securing the suit to the corselet, would be known as a "four light, twelve bolt helmet", and a three-bolt helmet used three bolts to secure the bonnet to the corselet, clamping the flange of the neck seal between the two parts of the helmet.
When the telephone was invented, it was applied to the standard diving dress for greatly improved communication with the diver.
Image:Standard diving dress 1.jpg|thumb|left|Diver in standard dress entering the water
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. 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 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.
All except the earliest 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 or near 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 valve known as a spit-cock, which can be used to vent excess air when the diver is in a position where the main exhaust can not function correctly.
The corselet, also known as a breastplate or gorget, is an oval or rectangular collar-piece resting on the shoulders, chest and back, to support the helmet and seal it to the suit, usually made from copper and brass, but occasionally steel. The helmet is usually connected to the suit by placing the holes around the rubberised collar of the suit over bolts along the rim of the corselet, and then clamping the brass straps known as brailes against the collar with wing nuts to press the rubber against the metal of the corselet rim to make a water-tight seal. An alternative method was to bolt the bonnet to the corselet over a rubber collar bonded to the top of the suit.
Most six and twelve bolt bonnets are joined to the corselet by 1/8th turn interrupted thread. The helmet neck thread is placed onto the neck of the corselet facing the divers left front, where the threads do not engage, and then rotated forward, engaging the thread and seating on a leather gasket to make a watertight seal. The helmet usually has a safety lock which prevents the bonnet from rotating back and separating underwater. Other styles of connection are also used, with the joint secured by clamps or bolts.