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Bailout plans

  • It is good practice to plan how you will surface safely if you lose a lot of gas from your cylinders during a dive, or if your rebreather fails underwater.
  • Your bailout plan will depend on the dive. For simple dives on a single cylinder, your buddy could be your bailout plan. For very deep dives using multiple gases, your bailout plan will be much more complicated.
  • You should write down complex bailout plans on a slate or waterproof notebook before the dive.
  • For dives with long decompression stops, you should consider whether to have spare decompression gas available, for example attached to a decompression trapeze, or available as a “drop bottle”.
  • If you are using a drop bottle, make sure that everyone on the boat, especially the skipper, knows the relevant signal and knows what they are expected to do when a drop bottle is called for.

Carbon dioxide

  • The signs and symptoms of a carbon dioxide toxicity incident are not always obvious immediately, but if you find yourself breathing quickly, or feeling confused or panicked, then it may be that you are having a carbon dioxide problem. If you think that this is the case, you should stop (after signalling your buddy) and try to regain control of your breathing. If you cannot regain control, switch to another gas source and abort the dive. 
  • You should be aware of the potential for carbon dioxide retention on deep dives. This is because, although you may not recognise it when you are relaxed and breathing easily, your body will have to work increasingly harder at depth, and you might not be able to ventilate your lungs adequately to get rid of all the carbon dioxide. 
  • You can minimise the risk of carbon dioxide retention by avoiding working hard underwater, breathing normally, and using well-maintained regulators and equipment. Breathing trimix instead of nitrox or air reduces the gas density and consequently work of breathing (see Work of breathing). 


  • In order to be covered by BSAC's liability insurance, you must have formal training in order to operate a compressor.
  • Take care when positioning the air intake. Do not put it in a place where there are lots of fumes or other potentially dangerous gases in the air.
  • In particular, if your compressor has a petrol or diesel engine, make sure that the air intake is upwind of the engine exhaust.
  • Make sure the air produced by your compressor meets the required purity standards. Air used for partial pressure blending of nitrox needs to meet higher purity standards, and everything downstream of the final filter needs to be oxygen clean.


  • If you are diving a rebreather, you should plan carefully which diluent gas you use. 
  • Always analyse your gas before putting equipment together.
  • If you have too high a percentage of oxygen, a diluent flush won’t work very well if you need to reduce the partial pressure of oxygen quickly. However, if you are using a hypoxic diluent this increases the risk of hypoxia at shallower depths, especially if your rebreather is fitted with an automatic diluent valve (ADV).
  • If you are diving deep, then you should think about using trimix as your diluent to reduce the effects of narcosis. BSAC suggests that you have an equivalent narcotic depth (END) of 30m, or less if you are undertaking a complex dive. 

Gas density

  • Increased gas density at depth means that it requires more energy to breathe at depth. This, together with other effects of depth means that it becomes increasingly difficult for your body to remove carbon dioxide.
  • The effect starts to become increasing important for dives on nitrox or air below 30m and poses increased risk below 40m.
  • You can reduce this effect by using trimix instead of air or nitrox.
  • You can also reduce the risk by avoiding situations which could cause you to breathe hard at depth, such as swimming into a strong current. 
  • It is recommended the density of the breathing gas should not be greater than 6.3 g·l−1; the ideal density is less than 5.2 g.l-1.  The information in the BSAC gas density tables should be used when selecting the breathing gas.

Gas mixes

  • You should have a suitable qualification if you plan to dive with a nitrox mix or trimix. 
  • Using a breathing gas with a high partial pressure of oxygen increases your risk of oxygen toxicity. You should not exceed a partial pressure of oxygen of 1.4 bar on the bottom phase of the dive, or 1.6 bar while on decompression stops (when you will be at a shallow depth and also not working hard).
  • You should always analyse your gas before putting your equipment together.
  • In appropriate depths you could add an extra element of safety if you dive nitrox, but keep your computer set to air or a weaker nitrox mix.

Gas mixes for deeper dives

  • If you are diving deep, then you should consider using trimix to reduce the effects of narcosis and improve work of breathing. BSAC recommends choosing a gas that gives you an equivalent narcotic depth of 30m or less, especially if you are undertaking a complex dive. However, a high level of helium can also increase the amount of decompression time some algorithms ask you to do, which can add other risks. You need to plan your dive carefully taking into consideration the risks and choose an appropriate gas for your plan.
  • There is a risk of passing out from hypoxia (lack of oxygen) when the partial pressure of oxygen is below 0.15 bar, and so you should be very careful when diving with hypoxic gases, and use travel gases if necessary. 
  • You should also be very careful when using hypoxic diluents when diving on a rebreather and monitor the partial pressure of oxygen at all times.
  • Guidelines for gas mixtures for open circuit diving


Gas reserves

  • You should plan your dives beforehand, including calculating an adequate gas reserve. 
  • Gas consumption can vary enormously between divers, and even the same diver can have different gas consumption rates depending on cold, exertion, experience, and depth. You should, therefore, monitor your own and your buddy’s gas supplies throughout the dive to ensure your reserves are still adequate. The size of an adequate reserve will depend on the capacity of the cylinders, the planned dive, and both your breathing rate and that of your buddy. 
  • BSAC recommends that a good rule of thumb is to surface with one-third of your primary cylinder(s) left as a reserve (around 75 bar in a 232 bar cylinder).
  • You might decide that under some circumstances a reserve of a third might not be enough. You should plan the dive beforehand and decide on a larger reserve if this is the case. Remember that a stressed diver can breathe at a surface breathing rate of 50 litres/minute or more during the first stage of an incident, and potentially not relax until their first decompression stop. In particular, if you are doing a deep dive with lots of decompression you should carry a reserve sufficient to cope with a failure.
  • A reserve is to cope with unforeseen circumstances happening to you or your buddy. It should not be used to extend dive time.  

Nitrogen narcosis

  • Narcosis can slow down your reactions and impair your decision-making ability. This means that if you have a problem while you are suffering from narcosis, you are less likely to resolve it successfully.
  • You should plan deep dives especially carefully, and think about how you will deal with an incident if you are suffering from narcosis.
  • You can use trimix to lessen the effects of nitrogen narcosis.
  • On dives using trimix, BSAC suggests using an equivalent narcotic depth of 30m.

Oxygen toxicity

  • The oxygen in breathing gases is essential for life but can become toxic at partial pressures above that found in air at the surface (a partial pressure of 0.21 bar).
  • When mixed gases and nitrox are being used the partial pressure of oxygen should not exceed 1.4 bar for each mix used for either travel (descent and ascent) or bottom phases. 
  • For divers holding an appropriate qualification an oxygen partial pressure of 1.6 bar may be used for decompression purposes down to a maximum depth of 10m. All divers who do not hold a suitable qualification should not exceed a 1.4 bar for any chosen gas mix.

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