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Altitude diving (and flying)

  • Beware of diving at altitude, and of flying or travelling to altitude after diving. At altitude atmospheric pressure is lower than at sea level, even in a pressurised aircraft. The reduced pressure has implications for your decompression obligations. It also means that you may not be able to travel at altitude, or travel in an aeroplane, if you have been diving recently.
  • BSAC ‘88 tables have four levels that allow you to understand both the decompression obligation of diving at altitude, and whether you can safely travel to altitude or fly after a dive.
  • While BSAC ‘88 tables indicate divers can fly in a pressurised aircraft on a code B, BSAC suggests you wait 24 hours between diving and flying if possible.
  • Some dive computers have different altitude settings. You should make sure that you understand how your dive computer works, and that it is set to the correct setting.


Ascent rate

  • The safe ascent rate is a trade-off. Ascend too quickly and you will have an increased risk of decompression illness (DCI). But if you ascend too slowly you will absorb additional inert gas (nitrogen and/or helium), which could give you an increased decompression obligation.
  • BSAC tables are compatible with a slow ascent rate, as the ‘dive time’ includes descent, bottom time, and ascent time. However you will need to factor in the slow ascent when planning your dive.
  • Dive computers will automatically recalculate if your ascent rate is slower than planned, but they may require you to do extra decompression stops as a result.
  • You should make sure that you are in control of your ascent rate at all times. You should be able to stop quickly at any time.
  • You should always ascend slowly from 6m to the surface. This should take at least one minute. 
  • Using a shotline or other fixed datum will help you to control your ascent rate. Do not watch other divers to gauge your ascent rate. If they are ascending too fast, or too slowly, you will automatically follow them.

BSAC tables

  • BSAC considers BSAC decompression tables (’88, nitrox & ox-stop) to be safe diving tables. If you use a different set of tables or a dive computer, then BSAC recommends they are at least as conservative as BSAC tables. This means they give the same or shorter no-stop time for a given depth, or give the same or longer decompression stops for a given profile, compared with BSAC tables.
  • Because BSAC tables work on the basis of a ‘dive time’ that includes ascent time, rather than ‘bottom time’ BSAC tables are compatible with a slow ascent. However, if you plan to take advantage of this, you must start your ascent sooner.
  • BSAC recommends that if it is safe to do so, you should do a safety stop of at least one minute at around 6m. Longer safety stops are even better. However, safety stops are optional, and there may be some circumstances when it is more sensible to omit the safety stop and ascend directly to the surface, for example if you are running low on gas or if sea conditions make maintaining accurate depth control difficult. 
  • If you are using BSAC tables, BSAC recommends you should not conduct more than three dives in one day, and that if you are diving deeper than 30m you should take a 24-hour break after diving for four consecutive days.
  • You should be aware that the advice given under Decompression (below) applies to all dives, including those using BSAC tables. 


  • When you dive your body will absorb inert gases, and will start to release them as you ascend. It is important to know whether your dive profile is safe, so BSAC recommends that you plan and monitor your dive with either a computer or with decompression tables.
  • BSAC also recommends that you produce a back-up plan in case something goes wrong on the dive.
  • Age, fitness, exertion, obesity, smoking, alcohol consumption, fatigue, dehydration and injuries can all make decompression illness more likely. You should be aware of these factors and take them into account by diving conservatively and not pushing the limits of your chosen decompression computer or table.
  • You should try to avoid sawtooth profiles.

Decompression illness

  • You should monitor yourself and other divers in the hours following a dive as while DCI symptoms can appear immediately, they may also take several hours to appear.
  • Minor symptoms, such as tingling and numbness, can indicate a more serious problem, leading to potentially greater disability than more dramatic symptoms such as intense joint pain, so anyone showing even minor symptoms should be treated as if they are suffering from serious DCI.
  • You should administer oxygen and fluids to anyone suspected of DCI as soon as possible.
  • You should contact the emergency services as soon as possible. They will advise on how/whether to evacuate the casualty, and how/whether they should be recompressed.
  • In the UK you can contact the emergency services in the following ways

    - VHF radio channel 16 (at sea)
    - Dial 999 (on land)
    - In England, Wales, and NI, the National Diver Helpline on 07831 151 523 
    - In Scotland you can call the Aberdeen Royal Infirmary Hyperbaric Medicine Unit on 0345 408 6008

  •  This information is updated on the DCI helpline - National Decompression Illness helpline pages on this website.

  • When abroad make sure you know the local emergency procedures.

Decompression stops

  • You should plan any decompression stops before your dive.
  • For dives with long decompression stops you should think about decompressing as a group on a trapeze or lazy shot. This is because there will be plenty of gas and other support around if you or another diver has a problem. 
  • Conduct your decompression stops in a horizontal position so that all parts of your body are decompressing at the same depth.
  • You should avoid vigorous exercise while decompressing, however light and gentle movement is encouraged.

Missed decompression stops

  • If a diver misses any decompression stops, they should not re-descend. 
  • No attempt should be made to enter the water again in order to complete any decompression stops. In this situation the diver is increasing the risk of decompression illness and merely placing a possible casualty in a hostile environment (see Decompression illness).
  • You should treat the diver as if they are suffering from DCI, even if they are not symptomatic. This means you should administer oxygen and fluids.
  • You should call the emergency services immediately, and follow their advice.

Re-entry decompression

  • If a diver misses planned decompression stops, no attempt should be made to enter the water again in order to complete them (called re-entry decompression). In this situation the diver is increasing the risk of DCI and merely placing a possible casualty in a hostile environment.

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