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. Ascent and descent are historically the times when divers are injured most often when failing to follow appropriate procedure.
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.
Descent rates are usually limited by equalisation issues, particularly with ears and sinuses, but on helmet dives can be limited by flow rate of gas available for equalising the helmet and suit, by carbon dioxide buildup caused by inadequate exhalation, and for divers breathing heliox at great depths, by high-pressure nervous syndrome. Ascents of divers breathing at ambient pressure are normally limited by decompression risk, but also to a far lesser extent, by lung overpressure injury risk. Historically there has been considerable change in the recommended maximum ascent rate, mostly to limit risk of decompression sickness.
Freedivers are less limited by equipment, and in extreme events may use heavy ballast to accelerate descent, and an inflatable lift bag to accelerate ascent, as they do not normally stay under pressure long enough to be affected by decompression issues. Atmospheric pressure suit divers are physiologically unaffected by the external pressure. Their rates of ascent and descent are limited by equipment deployment and recovery factors.
Scuba diving
Controlled descent
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 often dive in buddy pairs for safety reasons, and if following standard protocol, 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.
Uncontrolled descent Uncontrolled descent is usually caused by failure to control buoyancy during the descent. Consequences depend largely on the depth. In shallow water the diver will inevitably be stopped by reaching the bottom, at which point corrections can be made, and in the worst case weights can usually be dropped. There is a risk of ear or sinus squeeze if the diver is unable to equalise sufficiently. In deep water it is an emergency and may require immediate ditching of weights to stop the descent if inflation of buoyancy compensator and/or dry suit do not suffice.
Controlled ascent
Three stages can be distinguished in the procedure for a controlled ascent. The beginning, where the decision and preparations are made, The initial ascent, and the decompression stage.
The beginning may be when a specified decompression or time to surface limit has been reached, breathing gas has reached a limit, the task of the dive has been completed, or the dive has been terminated for some other reason. The procedures are intended to limit risk of barotrauma of ascent and decompression sickness, while maintaining an efficient ascent profile. One of the most important criteria for a controlled ascent is control of the ascent rate, both during the periods of active ascent (pulls), and periods of staged decompression (stops).
Maintaining a steady ascent rate at less than 60 feet per minute is challenging to scuba divers without an accurate depth reference. As of 1990 recreational scuba divers were mostly unable to reliably ascend in mid-water at the recommended rate of less than 60 feet per minute. Scientific divers were faced by the same problem. Commercial divers largely avoid it by using surface supplied equipment or ascending along a jackstay or shotline to the worksite. Accurate depth measurement and ascent rate alarms provided by a dive computer make this problem more tractable.
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 the more recently developed delayed surface marker buoy, or decompression buoy, inflated and deployed at the start of the ascent to notify any vessel in the vicinity of presence and location of the divers as well as helping to control ascent rate and stabilise depth. Deployment may be done before starting the ascent, or at any time during the ascent. It is usually easiest to deploy at the bottom, and can then aid ascent control for the full ascent, but some divers prefer to deploy at the first decompression stop or at the safety stop, which can save some ingassing time at depth. Ascending on a decompression buoy while slightly negatively buoyant makes accurate depth and ascent rate control relatively simple, by maintaining light tension on the line as it is reeled in, at the cost of carrying the equipment, a slightly longer time at depth during deployment, a few litres of gas for inflation, and the competence to deploy the DSMB safely, which is taught in advanced recreational diver training, along with precise buoyancy control and decompression procedures, which may include use of decompression gases. It is usual for a buddy pair or team to ascend at a single decompression buoy to reduce the risk of entanglement of the lines. Other team members can then use the diver with the reel as a reliable primary depth reference, and their own depth gauge or dive computer as a secondary reference.
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. At the end of the dive the BC should contain only a small amount of gas to support the unused gas in the cylinders. The divers look upwards while ascending as applicable 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. This reduces the risk of entanglement of the line, and gives the diver warning when buoyancy is about to become positive by reduction in line tension. 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, and if worn on the wrist can usually be monitored while winding in the line, allowing for precise and reliable depth control, and where the computer has an ascent rate indicator, also accurate control of ascent rate.
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 about 1–3 minutes may be made at about 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, sometimes very slowly. Before surfacing, the divers check for approaching vessels by looking upward around themselves and listening. 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.
Emergency ascent
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. Middle ear and sinus barotrauma are also possible if ascent rate is too fast and gas cannot escape from the affected body cavity fast enough.
Uncontrolled ascent An ascent in which the diver loses control of the ascent rate is an uncontrolled ascent. It is usually the consequence of excessive buoyancy. 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. The risk of pulmonary barotrauma in a healthy person who is medically fit to dive is very low if the airway remains open during the entire ascent, as is shown by experimental work and experience with submarine escape training, but after significant pressure exposure at depth, the risk of central nervous system decompression sickness increases with ascent rate. Uncontrolled buoyant ascent can occur in cases of , , or insufficient or lost diving weights.
Surface supplied diving In surface-supplied ambient pressure diving, two very different modes are in use, and the procedures and problems associated with them are also very different. These are surface oriented dives, where the diver starts and ends the dive at atmospheric pressure, and saturation dives, where the diver remains under pressure close to that of the working depth before, during, and after the underwater dive exposure, and is compressed before a series of dives, and decompressed at the end of the series of dives.