Ascending and descending (diving)
In underwater diving, ascending and descending is done using strict protocols to avoid problems caused by the changes in ambient pressure and the hazards of obstacles near the surface or collision with vessels. Diver certification and accreditation organisations place importance on these protocols early in their diver training programmes.
The procedures vary depending on whether the diver is using scuba or surface supplied equipment. Scuba divers control their own descent and ascent rate, while surface supplied divers may control their own ascents and descents, or be lowered and lifted by the surface team, either by their umbilical, or on a diving stage or in a diving bell.
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- The divers swim towards the point of descent and meet up with the companion diver ("buddy"). The OK-signal is given.
- The snorkel is removed and the second stage of the regulator is placed in the mouth (if not already done).
- The divers orient themselves towards the diving target, keeping their buddy in view.
- The divers give the signal to descend (thumb down).
- The inflater hose is held out of the water and the deflation button pressed. The BCD deflates, which will start the sinking process.
- Every few metres, the divers will "clear their ears" to equalise pressure in the middle ear.
- At a pre-arranged point in the descent, the divers halt and check their buddy's equipment for any air leaks. After this "bubble check", the descent is continued.
A commonly used procedure for ascent in open water when not ascending along a shot line or anchor cable:
- The accompanying divers are informed of the intention to ascend, using the thumb up hand signal.
- The signal buoy may be inflated and released to notify any vessel in the vicinity of the ascent. This can be done before starting the ascent, or at any time during the ascent. It is usually easiest to deploy at the bottom, but some divers prefer to deploy at the first decompression stop or safety stop.
- The buoyancy compensator device (BCD) is set to neutral or slightly negative, the inflator hose is held ready to release excess gas, and the divers slowly swim upwards, releasing excess air from the BCD as it expands due to the reduced water pressure. Increased buoyancy of the BCD could cause a runaway ascent so expanded air is vented as the divers ascend to retain approximately neutral buoyancy. The divers keep looking upwards to avoid any obstacles.
- Ascent speed is limited to the requirement of the decompression schedule in use - (commonly kept below 10 meters per minute). A dive computer may be used to help judge this speed.
- The divers suspend ascent at the depths of any required decompression stops for the appropriate stop time, remaining as close as practicable to the specified depth for the duration of the stop. Buddy pairs will usually decompress to the schedule of the diver needing the longest decompression.
- A safety stop of 1–3 minutes may be made at 3–6 metres from the water surface. This is an optional stop, but it is predicted by some decompression models to further reduce the risk of decompression sickness.
- After the stop, the last part of the ascent is done, often very slowly (at no more than 5 metres per minute). Before surfacing the divers check for approaching vessels.
- When the divers are at the surface, the BCDs are inflated to establish positive buoyancy, and the surface team or boat are notified that the divers are well by using hand signals.
In emergencies when a diver runs out of air in the cylinder in current use, and when there is no buddy around to donate air, the use of a redundant air supply (such as independent twins or a pony bottle), allows a diver to perform an ascent in a controlled manner, breathing as normal.
When no redundant air supply is available, the diver can make a controlled emergency swimming ascent. The diver starts to swim up exhaling steadily along the ascent unless trying to inhale. The mouthpiece is kept in as the cylinder still contains some air and it will become available as the ambient pressure decreases. It is important not to hold the breath, to avoid over-expansion of the air in the lungs due to pressure decrease as the depth decreases, which could cause the lung tissues to tear. The speed of ascent has to be a compromise between too slow (and running out of oxygen before reaching the surface) and too fast (risking decompression sickness). Lung barotrauma is unlikely in a heathy diver who allows the air to escape freely from the lungs.
Surface supplied diving
Surface oriented dives (Bounce dives)
Surface supplied divers frequently work heavily weighted, to give them a firm footing while working on the bottom. This makes it difficult or impossible to achieve neutral buoyancy. However, as they are connected to the surface control point by the umbilical, they can be lowered to the bottom by the umbilical. For greater depths, they can be lowered on a platform known as a diving stage, or in a wet bell. These are lowered from a diving support vessel or shore installation using a man-rated winch, which allows good control of depth and speed of descent and ascent, and allows these procedures to be controlled by the surface team.
Saturation divers are lowered to the working depth and raised back to the surface in closed diving bells, which are pressurized to the same pressure as the dive depth. The diver is transferred to and from the hyperbaric accommodation after adjusting the bell pressure to match the storage pressure.
- Brittain, Colin (2004). "Practical diver training". Let's Dive: Sub-Aqua Association Club Diver Manual (2nd ed.). Wigan, UK: Dive Print. pp. 45–47. ISBN 0-9532904-3-3. Retrieved 9 January 2010.
- Monty Halls (2007), Go scuba dive, DK ADULT; ISBN 978-0756626273
- Ascent Rates, Deep Safety/Deco Stops & Time Limits Ascent and Decompression and Ascent Strategies - Brian R. Morris