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Alternative supply (AS)

  • Every diver should have an AS on every dive.
  • An AS could be an octopus (an additional second stage attached to the first stage of your main regulator) or an independent alternative supply (such as a pony cylinder with its own regulator).
  • If you are using an octopus regulator, you should make sure that the first stage will provide enough gas for two divers in an emergency. Regulators that confirm to the EN250 standard should be able to do this. Ensure that the manufacturer certifies use of the regulator at suitable temperatures for the dive.
  • If you are using a pony cylinder as an AS, it should have a capacity of at least three litres.
  • Auxiliary cylinders having a capacity of less than three litres and BC mouthpieces (with/without an integrated second stage regulator) are not considered an adequate AS.
  • For dives below 30m BSAC strongly recommends an independent AS such as a ‘pony cylinder’ or separate regulators attached to separate cylinders. If a manifold is fitted to the pair of cylinders it should allow the diver the ability to isolate each cylinder/regulator assembly should a failure occur.
  • Sometimes the AS is on a long hose, often 2m long. If you and your buddy are not holding on to each other, there is a risk that the donated regulator could be pulled from the receiver’s mouth. The receiver should hold the donated regulator (or its hose) to reduce this risk.
  • You should brief your buddy before the dive on how to use your AS in case they need it. A good way of doing this is to give a demonstration of the out of gas response (a ‘dry-run’).  

Buoyancy compensator (BC) cylinders and delayed surface marker buoy (DSMB) cylinders

  • Some BCs and DSMBs can be inflated using a small dedicated cylinder, typically 0.1 or 0.2 litres in capacity. These can be refilled from larger cylinders.
  • The testing regulations for mini-cylinders are the same as for your main cylinder. As you will be filling the mini-cylinder yourself (from your main cylinder) you might not think that it is necessary for it to be tested. However, you should get mini-cylinders tested in accordance with current BS EN test standards as they could harm you if they fail while being filled. 
  • You should make sure that you do not overfill mini-cylinders. Some only have a working pressure of 207 bar, and you need to be careful if you are filling these from a 232 bar or 300 bar cylinder.
  • Try to avoid completely emptying mini-cylinders, as water can work its way into the empty cylinder and cause corrosion. 
  • You should not use rich nitrox mixes to fill a mini-cylinder unless it is in oxygen service, adhere to manufacturer’s recommendations. 


  • A checklist can help you prepare your equipment thoroughly and in a logical way, meaning you are less likely to make mistakes in assembling it.
  • The use of appropriate checklists is especially important if you dive with rebreathers as they require specific pre-dive checks.

Cylinder labelling

  • When diving with nitrox, BSAC advises that you mark your cylinder with the nitrox mix and maximum operating depth (MOD).
  • If you are carrying different gas mixes in different cylinders, for example when using rich nitrox mixes for decompression, you should take extra care to analyse and if necessary remark your cylinders with gas mix and MOD as appropriate.
  • Breathing the wrong gas at depth can have very serious consequences. When diving with multiple cylinders, BSAC recommends you should use a system to prevent you confusing the cylinders, such as colour and shape coding; mouthpiece guards; and cylinder placement conventions such as rich-right/lean-left. 

Cylinder pressures

  • You should not fill a cylinder higher than its working pressure. 
  • BSAC recommends that you check your cylinder pressure before the dive, and at regular intervals throughout the dive.

  • BSAC recommends that you surface with a minimum of one-third of your cylinder’s capacity left as a reserve. For example when diving with a 232 bar cylinder you should surface with around 75 bar left as a reserve.

  • However, there are some circumstances (for example deep or long dives, and no clear surface dives) where this is not an adequate reserve. 

  • For decompression gases a reserve of 50 per cent is recommended. 

Cylinder testing

  • Cylinders must be in test according to the standards set down by the relevant authority and should be stamped with the date and type of the most recent test, which enables divers to know the next test date.
  • Cylinders not ‘in test’ must not be filled.
  • If your cylinder passes its test, the cylinder shoulder should be stamped and you should also receive a certificate.
  • If you plan on putting oxygen or nitrox in your cylinder, (including by partial pressure blending nitrox), your cylinder must be inspected and cleaned every 15 months. Your cylinder should be marked to show that it’s in oxygen service.

Delayed surface marker buoys (DSMBs)

  • Deploying a DSMB is one of the most common things to go wrong on a dive, and so you should practise this skill regularly.
  • Do not attach the reel to yourself or to your equipment because if it jams you will be dragged to the surface. Instead make sure you can jettison the reel quickly in case you need to.
  • There is a convention that a single DSMB means “situation normal” but a red DSMB and a yellow DSMB on the same line indicates a problem. If you plan on using this system you should make sure that everyone on the boat, including the skipper or coxswain, is aware of the situation.
  • If you plan to use an alternative signalling method ensure all are aware of the meaning and the action to take if it is deployed.

Dive computers

  • Dive computers offer accurate and automatic recording of depth and time and continuously calculate the diver’s decompression requirements according to the depth and duration of the dive. 
  • Computers are also available with advanced features such as the ability to calculate decompression requirements for a variety of nitrox mixes and mixed gases, and also to monitor available gas and gas consumption rates. 
  • The use of a dive computer is no substitute for proper dive planning, including proper attention to gas requirements and dive time. Divers should learn how to use the planning function on their own dive computer and apply this practice prior to every dive.
  • Individual susceptibility to decompression illness varies and can be affected by a number of factors, for which no computer or decompression table is able to allow. Divers should be aware of this and avoid pushing computers beyond their limits. 

 Gradient Factors

For dive computers using gradient factors to modify Buhlmann ZHL-16C algorithms, the following values should reduce the risk of DCI to tolerable levels for most people.










 The rationale for the differing GFs between trimix and Air/Nitrox is due to the types of dive undertaken using the respective gases. Air/Nitrox dives being generally shallower and shorter than those of trimix.


Diver propulsion vehicles (DPVs/scooters)

  • Ensure you obtain training and practice in the use of DVPs.
  • Buoyancy control can be more difficult when using a DVP. Make sure that you are not using its power to compensate for being negatively or positively buoyant.
  • When using a scooter, you should consider what would happen if the scooter failed. This is particularly important on dives where you are planning on returning to the shot, or need to return to a particular entry point when shore diving. 
  • A flooded scooter can be very negatively buoyant, so you might want to carry a lifting bag to recover it to the surface.

Lifting bags

  • If you lift a heavy object using your own buoyancy from the seabed and drop it, then there is a good chance you will have a rapid ascent. You should, therefore, avoid using your BC or drysuit to carry heavy objects (for example when moving the shot weight to closer to the wreck), but instead use a lifting bag.
  • If you plan to send an object all the way to the surface by making it positively buoyant, then you should make sure you are not directly under the object. If there is any current, you should also make sure that there are no divers downstream of the object.
  • If you are lifting very large objects, you should take extra care, for example by using multiple lifting bags, and/or breakout bags so that the lift is as controlled as possible. If you do not have experience of this yourself, you should think about involving someone who does have experience of this, or going on a course to learn more.
  • The recovery of objects from underwater may be subject to legislation and all divers should ensure they comply (see Legislation).

Manufacturers’ recommendations

  • In addition to the guidance given in this document the manufacturers’ recommendations for their equipment should always be adhered to (for example, frequency of servicing).

Rebreather, closed-circuit (CCR)

  • You must obtain a formal qualification from a recognised agency to dive with a rebreather on a club dive (except if you are doing a rebreather try-dive).
  • If your buddy is unfamiliar with rebreathers, there are a number of issues that you will need to explain to them:
    - How to lift you to the surface. This is more complex with a rebreather as your buddy will have to dump gas from your counterlungs as well as your drysuit and wing. 
    - How to close the mouthpiece in case you pass out and it falls out of your mouth, flooding the unit. This should be done well before the buddy check.
    - A breathing loop flood will cause a significant loss of buoyancy requiring more action to establish positive buoyancy than otherwise anticipate.
  • You should follow the manufacturer’s recommendations regarding maximum scrubber duration and not exceed it. While it may be acceptable to do multiple dives on one scrubber fill, you should be scrupulous about recording how long a scrubber has been used for. If in any doubt, you should change the absorbent.

  • You should make sure your scrubber is correctly packed. You can weigh the scrubber to make sure that it is holding the right amount of absorbent. Remember that long journeys or bumpy boat rides can increase the risk of the absorbent settling and causing “channelling”. 

  • You should change your oxygen cells at the manufacturer’s recommended intervals, if not sooner. 

  • You should use the type of battery specified by the manufacturer and change them at the manufacturer’s recommended intervals or sooner. Some rebreathers give a warning when the voltage drops to a critical level, at which point you must change the batteries. 

  • You should disinfect the breathing loop at the manufacturer’s recommended intervals. Make sure that you are using the correct disinfectant in the right concentration, and that you let it remain in contact with the loop for enough time. 

  • You should be careful when using a rebreather on the surface as there is a risk of hypoxia, especially on manual rebreathers or if you are using hypoxic diluent. 

  • Use of a Mouthpiece Retaining Strap (MRS) may reduce the risk of an unconscious diver drowning.  They also lessen jaw fatigue on long dives

Rebreathers, semi-closed circuit (SCR)

  • If you are diving a semi-closed rebreather you should make a slow ascent. If you ascend too quickly there is a risk that the oxygen level of the loop will fall quickly and the loop could go hypoxic.
  • You should always check the flow rate of a semi-closed rebreather before each dive. A blocked jet can make the loop dangerously hypoxic.
  • If you plan on swimming on the surface while diving a semi-closed rebreather, BSAC recommends that you switch to an open-circuit regulator or a snorkel, to stop the loop becoming hypoxic.
  • If in doubt about the partial pressure of oxygen loop, manually flush with breathing gas.
  • Use of a Mouthpiece Retaining Strap (MRS) may reduce the risk of an unconscious diver drowning.  They also lessen jaw fatigue on long dives


  • If you dive a manifolded twinset, you should be able to shut it down very quickly in case you have a free flow. 
  • You should practise shutdowns regularly. This will not only make you quicker, but will reassure you that you can still perform this important skill.
  • If you cannot shut down a manifolded twinset quickly enough, then keep the manifold closed and dive your twinset as two independent cylinders.

Surface detection aids

  • BSAC recommends that you think about carrying one or more surface detection aids to make you more visible to boats, and to rescue aircraft. 
  • As well as a DSMB, useful detection aids include 

    - Torches 
    - Strobes
    - Fold-up flags 
    - Signal mirrors (polished stainless steel – not glass)
    - High-power whistles (various mouth-powered ‘storm whistles’, and direct-feed-powered air horns)
    - Radio distress beacons

  • There are several types of radio distress beacons available for diver location in an emergency
    - Personal locator beacons (PLBs)
    - Emergency Position Indicating Radio Beacons (EPIRBs)
    - Submersible transmitter/receivers

Weights and weight belts

  • If you are correctly weighted for buoyancy and trim it will make your dive safer, more enjoyable, and you will use less gas.
  • You should weight yourself so that you are able to hold a safety stop, horizontally, with near empty cylinders. 
  • If you are diving with new or unfamiliar equipment, then you should do a proper weight and trim check to make sure you are correctly weighted and can hold horizontal trim without finning or sculling. 
  • Remember seawater provides more buoyancy than freshwater and so you will need to add weight when moving from freshwater to seawater.
  • In the event of an incident it may be necessary on reaching the surface to release weights and the emergency method of weight release should be covered in a buddy check.
  • Ensure that any weights or the weight belt cannot be snagged by other equipment. It may be necessary to release other equipment such as BC crotch straps before releasing a weight belt.

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