Ascending and descending (diving)
||This article needs additional citations for verification. (January 2010)|
In 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 training organizations place importance on these protocols early in their diver training programmes.
Controlled ascent 
When not ascending along a shot line or anchor line, the following steps should be taken when ascending in open water:
- Before commencing the ascent manoeuvre, the signal buoy is released and inflated to notify any vessel in the vicinity of the ascent.
- The accompanying diver (buddy) is given the ascension signal (thumb up)
- The buoyancy compensator device (BCD) is set to neutral, inflater hose is pointed upwards and both divers slowly begin to swim up. The BCD is slowly filled with air using the inflater hose. Care is taken not to fill the BCD too rapidly, as the air will expand because of the reduced water pressure, increasing the buoyancy of the BCD. The divers keep looking up to avoid any obstacles.
- Using the inflater hose to release air, the ascension speed is kept below 10 meters per minute. An ascent meter or dive computer may be used to help judge this speed.
- A safety stop of 1–3 minutes is made at 3–6 meters from the water surface.
- After the stop, the last part of the ascension is done very slowly (at no more than 5 meters per minute). The divers keep looking around and listen closely for any possible vessels.
When above the surface, the BCD is inflated and a signal is made, if a safety boat is waiting.
Controlled descent 
When not descending along a shot line or anchor line, the following steps are performed:
- 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.
Emergency ascent 
In emergencies when a person runs out of air 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 carries out a controlled emergency swimming ascent. The diver starts to swim up exhaling steadily and continuously along the entire ascent. The mouthpiece is kept in as the cylinder still contains some air and it may become available as the ambient pressure decreases. It is important to exhale continuously, to avoid over-expansion of the air in the lungs (as the water pressure decreases while swimming up), which could cause the lungs to burst. The speed of ascent has to be a compromise between too slow (and running out of air before reaching the surface) and too fast (risking a burst lung).