Emergency ascent


An emergency ascent is an ascent to the surface by a diver in an emergency. More specifically, it refers to any of several procedures for reaching the surface in the event of an out-of-gas emergency, generally while scuba diving.
Emergency ascents may be broadly categorised as independent ascents, where the diver is alone and manages the ascent by themself, and dependent ascents, where the diver is assisted by another diver, who generally provides breathing gas, but may also provide transportation or other assistance. The extreme case of a dependent ascent is underwater rescue or recovery of an unconscious or unresponsive diver, but this is more usually referred to as diver rescue, and emergency ascent is usually used for cases where the distressed diver is at least partially able to contribute to the management of the ascent.
There are three basic mechanisms for ascending through the water column: Positive buoyancy, hydrodynamic thrust, and force directed through a static external object. Any combination of these can be used.
An emergency ascent usually implies that the diver initiated the ascent voluntarily, and made the choice of the procedure. Ascents that are involuntary or get out of control unintentionally are more accurately classed as accidents. An emergency ascent may be made for any one of several reasons, including failure or imminent failure of the breathing gas supply.

Reasons for making an emergency ascent

An emergency ascent implies that the dive plan has been abandoned due to circumstances beyond the control of the diver, though they may have been caused by the diver, as is often the case in out-of gas emergencies in scuba diving. Out of gas emergencies are generally the most urgent contingencies in diving, as the available time to deal with the emergency can be measured in minutes or seconds, while most other non-traumatic emergencies allow more time.
Other reasons for emergency ascent may include:
  • Failure of a rebreather requiring bailout to open circuit – This is not always considered an emergency ascent, though it is usually urgent, and is considered a sufficient reason to abort the dive.
  • Compromise of diver buoyancy control due to loss of ballast weight.
  • *Tethered-ascent – where the diver has unintentionally lost full control of buoyancy due to a loss of ballast weight, and controls ascent rate by use of a ratchet dive reel with the end of the reel line secured to the bottom.
  • Lost buoyancy ascent – where the diver loses the ability to establish neutral or positive buoyancy without resorting to ditching weights. This can be due to a major buoyancy compensator failure or a major dry-suit flood.
  • Injury or illness occurring during a dive may make an ascent for access to treatment urgent.
  • Scuba equipment failure leading to non-catastrophic but rapid loss of breathing gas.
  • Sudden loss of thermal protection due to dry suit leak or loss of heating water supply in a hot-water suit. The urgency of a dry suit leak depends on the rate of heat loss and how long the ascent will take, but a long decompression obligation can make it an emergency. Heating failure in a hot-water suit is generally more urgent and can be life-threatening.
  • Inability to read instruments due to damage or loss of mask or severe damage to helmet faceplate. It may not be possible to accurately monitor depth, rate of ascent or decompression stops. This can be mitigated if a dive buddy can monitor control the ascent, or if the diver's computer has audible alarms for fast ascent and exceeding a ceiling. Ascent on a tangible reference such as a DSMB line, shotline or anchor line is also helpful.
  • Flooding of helmet or full-face mask that cannot be rectified.
  • Entanglement requiring abandonment of breathing apparatus.
  • Entrapment of bell or failure of bell recovery system.
  • Entrapment of umbilical, or damage to umbilical resulting in main gas supply failure.

    Techniques for achieving ascent

Three basic mechanisms are available which can cause a diver to ascend in the water. Any combination of these may be used, and the exact combination will depend on the circumstances of the emergency and to some extent, on the choice of the diver:
  • Positive buoyancy is the imbalance of buoyancy forces on the diver and their equipment as a whole. The rate of ascent depends on the magnitude of the buoyant force and the drag force resisting upward motion. Buoyancy of the diver is controlled by adjusting the mass and volume of the diver and their equipment, and drag is a function of velocity and the shape and area of the diver passing through the water. A scuba diver should normally be able to achieve positive, neutral or negative buoyancy by adjusting their buoyancy control equipment, and should at all times be able to achieve positive buoyancy, either by adding gas volume or dropping ballast weight.
  • Hydrodynamic thrust is the force imposed by swimming and finning. When directed downwards, and when sufficient to overcome any negative buoyancy, it will propel the diver upwards at a speed balanced by the drag.
  • Upwards force caused by reaction to pulling or pushing downwards against an external object which can resist the force, that is sufficient to overcome any negative buoyancy.

    Techniques for slowing ascent

  • Flare to increased drag
  • Dump air from BCD or dry suit
  • Hold onto the upline

    Terminology for emergency ascents

The terminology is diverse, and not always used consistently.
  • Alternate air source ascent is an ascent in which the diver ascends breathing from an alternate air source regulator, either of their buddy or of their own gas supply. It is an ambiguous term, and is not necessarily an emergency ascent depending on which alternative air source is used, and may be independent or dependent also depending on whose gas supply is used. PADI use the term for an emergency ascent using air supplied by the buddy via an octopus DV.

    Independent action

Emergency ascents where no assistance from another diver is given.
  • Bailout ascent is an ascent where the diver breathes from their own bailout set carried to provide an emergency breathing gas supply for this kind of emergency.
  • Blow and go is a free ascent where the diver exhales at the bottom before starting the ascent. The breath may be held during part of the ascent, as the lungs are emptied before starting. This procedure is considered unnecessarily hazardous by many recreational training agencies, and paradoxically has been implicated in a greater number of lung barotrauma incidents than other styles of free ascent. It may also make the diver negatively buoyant at the start of the ascent, necessitating greater effort to ascend, and reduces the oxygen reserves of the diver, making a loss of consciousness due to hypoxia during the ascent more likely.
  • Buoyant ascent is any ascent where the diver is propelled towards the surface by positive buoyancy.
  • Buoyant emergency ascent is an ascent where the diver achieves positive buoyancy by ditching some or all of theur ballast weight. It is usually irreversible, and often the ascent rate is only partly controllable.
  • Closed circuit bailout is bailout to a second rebreather carried for this purpose.
  • Controlled emergency swimming ascent is an emergency swimming ascent which remains under control and which is performed at a safe ascent rate, with continuous exhalation at a rate unlikely to cause injury to the diver by lung overexpansion. The second stage regulator is retained in the mouth and may be breathed off if gas becomes available as the ambient pressure drops.
  • Emergency free ascent is like a controlled emergency swimming ascent without a regulator in the mouth. It is considered unacceptably hazardous for training purposes by some agencies.
  • Emergency swimming ascent is a free ascent where the diver swims to the surface at either negative or approximately neutral buoyancy.
  • Exhaling ascent is an ascent where the diver continuously exhales at a controlled rate during the ascent. This may apply to an emergency swimming ascent/free ascent or a controlled emergency swimming ascent, and distinguishes it from a blow and go procedure.
  • Free ascent is the name of the procedure used in US Navy submarine escape training. However the term is also used for other emergency diver ascent procedures where breathing gas is not available to the diver during the ascent. Free ascent basically implies that no additional breathing gas is available during the ascent, in much the same way that freediving implies that no additional breathing gas is available during the dive.
  • Open circuit bailout is a change from breathing off the rebreather loop to open circuit, either by physically changing from the rebreather dive/surface valve to an open circuit bailout demand valve, or by switching the rebreather bailout valve from closed to open circuit. This action is taken both when there is a recoverable problem with the rebreather loop, in which case once the problem has been corrected, a reversion to closed circuit is usual, or when the loop has failed irrecoverably, in which case an ascent is made on open circuit, which is generally regarded as an emergency ascent.
  • Reserve air ascent is an ascent using the gas in the main cylinder after actuating a reserve valve to release the gas trapped by the reserve valve mechanism. A reserve air ascent is not traditionally considered an emergency ascent, as it was the standard procedure before the use of submersible pressure gauges became widespread.

    Dependent action

Ascent in an emergency with assistance provided by another diver.
  • Buddy breathing ascent is where the diver is provided with breathing gas during the ascent from the same demand valve as the donor, and they breathe alternately.
  • Assisted ascent, sometimes octopus assisted ascent is where the diver is provided with breathing gas during the ascent by another diver via a demand valve other than the one in use by the donor during the ascent. This may be supplied from the same or a different cylinder, and from the same or a separate 1st stage regulator. The divers' breathing is not constrained by each other, and they may breathe simultaneously.