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 such as 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.
Descents may be made along a shot-line, along the slope of the bottom, or in open water clear of any physical or visual cues to rate of descent other than the depth gauge or dive computer carried by the diver. Scuba divers usually dive in buddy pairs for safety reasons, and if doing so correctly will descend in view of each other in case of problems. The signal to descend is the thumb down fist. The divers will start breathing from their regulators before starting the descent, and ensure that they are functioning correctly, before releasing sufficient air from their buoyancy compensators (BCDs) to start sinking. As they leave the surface, the divers will start equalising the pressure in their middle ears to prevent barotrauma, and will add air to their dry-suits, if using them, to prevent squeeze. Air may be added to the BCD as needed to control rate of descent. They may stop at a pre-arranged depth to do a final equipment check for air leaks. Equalisation will continue as needed all the way down, and depth must be monitored using a depth gauge or dive computer so that they can inflate their BCDs to neutral buoyancy in time to stop before colliding with the bottom, or overshooting the planned depth if diving a wall or steep slope. Speed of descent can be as fast as the divers can comfortably equalise, or as slow as convenient, though a longer descent reduces the useful bottom time.
Ascents may be made along a shot-line, by following the upward slope of the bottom, or in open water clear of any physical or visual cues to rate of ascent. Use of a vertical line as a visual cue or to physically control ascent makes management of the ascent rate considerably easier.
A commonly used procedure for ascent in open water when not ascending along a shot line or anchor cable is to use a delayed surface marker buoy, which may be inflated and released to notify any vessel in the vicinity of presence of the divers as well as helping to control ascent rate. Deployment 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, which can save some time.
The divers of the group are informed of the intention to ascend, using the thumb up hand signal, and if not already neutrally buoyant, will adjust their buoyancy, and hold the inflator mechanism ready to dump excess gas from the BCD as it expands during the ascent. Increased buoyancy of the BCD and dive suit due to gas expansion could cause a runaway ascent so air is vented as the divers ascend to retain approximately neutral buoyancy. The divers frequently look upwards while ascending to avoid any obstacles. A competent diver will ascend with little or no need to fin upwards, and can stop and achieve neutral buoyancy at any depth. The diver handling the buoy may choose to remain slightly negative during the ascent to keep a small amount of tension on the line as it is wound onto the reel or spool. Ascent speed is limited to the requirement of the decompression schedule in use - (commonly kept below 10 meters per minute) so that dissolved inert gases can be eliminated safely. 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 last stop, the final ascent is done, often very slowly. Before surfacing the divers check for approaching vessels. When the divers reach the surface, they usually inflate their BCDs to establish positive buoyancy and signal the surface team or boat that they are well 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 healthy diver who allows the air to escape freely from the lungs.
An ascent in which the diver loses control of the ascent rate is an uncontrolled ascent. If the ascent rate is excessive the diver is at risk of decompression sickness and barotrauma of ascent, both of which can be fatal in extreme cases. This can occur in cases of suit blowup, BCD blowup, or loss of diving weights.
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. Divers using standard diving suits were constrained to slower descent rates, due to limitations on air supply, and the risk of suit or helmet squeeze, in extreme cases, and carbon dioxide buildup in milder cases. The USN maximum descent rate for this equipment was 75 feet per minute.
Suit blowup was a serious hazard for divers using standard diving equipment. This occurs when the diving suit is inflated to the point at which the buoyancy lifts the diver faster than he can vent the suit to reduce buoyancy sufficiently to break the cycle of ascent induced expansion. A blowup can also be induced if air is trapped in areas which are temporarily higher than the helmet exhaust valve, such as if the feet are raised and trap air. A blowup can surface the diver at a dangerous rate, and the risk of lung overinflation injury is relatively high. Risk of decompression sickness is also raised depending on the pressure profile to that point. Blowup can occur for several reasons. Loss of ballast weight is another cause of buoyancy gain which may not be possible to compensate by venting. The standard diving suit can inflate during a blowup to the extent that the diver cannot bend his arms to reach the valves, and the overpressure can burst the suit, causing a complete loss of air, and the diver sinking to the bottom to drown.
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.
Most non-competitive freediving is done with some positive buoyancy at the surface, and the diver fins downward to descend. The diver's buoyancy will decrease with depth as the air in the lungs and the wetsuit is compressed. At some stage the diver may become negatively buoyant. To ascend, the diver fins upward, generally assisted by buoyancy as the surface is approached. In competitive freediving the techniques for descent and ascent are largely specified by the rules of the specific discipline, and are quite varied, and range from unaided swimming, to pulling oneself along a shotline, to descending using a heavy weight and ascending using a lift bag.
Skandalopetra diving is a freediving technique that dates from ancient Greece, when it was used by sponge fishermen, and has been re-discovered in recent years as a freediving discipline. It consists of a variable ballast dive using a flat stone of 8 to 14 kg, with smooth, rounded corners and edges, tied to a rope, which the diver held to increase the rate of descent. During descent the diver can use the stone as a drag brake, to steer, and as ballast. The attendant monitors the depth of the diver, feels when they slow down to equalize, when they leave the stone on arrival at the bottom, and when the diver is ready to ascend.At the end of the dive, the diver stands on the stone and is pulled to the surface by the attendant.