Using multiple cylinders
One of the things that distinguish technical divers from recreational divers is the amount of equipment and, in particular, the number of cylinders that are carried.
Koh Tao, Thailand
A recreational diver might carry a single cylinder and for some dives may add on a pony cylinder for redundancy. Technical divers will use either a twinset or a rebreather but in addition will carry a number of stage cylinders. They are usually clipped onto the diver’s side.
The term ‘stage cylinder’ is used as a generic term to describe these cylinders but in fact there are a number of different uses for which we use these cylinders and their names reflect the purpose that the cylinder is serving at the time. Another term that is used is a ‘side-slung’ or just ‘sling’ cylinder. This comes from the fact that the cylinder is usually mounted or slung on the side of the diver. The term ‘stage’ cylinder comes from cave diving, where divers would stage or drop cylinders at various points in the cave for use on the way out. So, for example, a cave diver might swim in breathing from his stage cylinder and when he has breathed a third of it he would remove it and clip it to the guideline. He would then continue using his twinset until that was also a third used. At this point he would turn around and start swimming back out. Using this method he should get back to his stage cylinder with a third still left in his twinset. At this point he would pick up his stage cylinder, which still has two thirds left and breathe that for the rest of the exit. Using this approach he will exit the cave with a third reserve in his twinset and a third reserve in his stage. This approach works as the cave diver knows that he will be exiting along the same line that he entered and so will always be returning to his stage cylinders.
This approach is normal in cave diving but is much less common with wreck penetration diving. The two reasons for this are that firstly the penetration distances in wreck dives are not usually as long as with cave dives. Secondly the number of exits may mean that we exit the wreck at a different point to the one we entered and so we may not always get back to our stages. As a result it is much less common to stage gas on wreck penetration dives. The exception to this is that some divers remove their stages before entering a narrow restriction or hole. This allows them to get into more restricted areas but should be used with caution as if they cannot get back out or have to exit in another area then they cannot easily get back to their staged gas. For this reason it is recommended that wreck penetration divers do not stage their cylinders. The most common use of stage cylinders for wreck dives or any other open sea dives is to carry a decompression gas. When used in this way stage cylinders are usually referred to as deco cylinders. In this case they will contain a rich nitrox mix which is used to accelerate or reduce the diver’s decompression obligation.
Wreck divers may also carry a stage cylinder containing the same gas as in their twinset. This is known as bottom gas as it is breathed on the bottom rather than deco gas, which is breathed on the ascent and decompression stops. A bottom gas stage might be used to extend the bottom time of the dive if the twinset doesn’t contain sufficient gas to allow a safe reserve. It can also be used where getting subsequent fills may be difficult. In this case the diver may plan to use their bottom gas stage and a proportion of their twinset on one day and then use an additional pre-prepared bottom gas stage and the rest of their twinset for the next day. In this way they can get two dives from a single twinset and two stages of bottom gas.
The last use of stage cylinders is as a bailout stage for rebreather divers. A rebreather diver would normally plan to use their rebreather for the duration of the dive. However, if there is a problem with the rebreather then the diver would ‘bailout’ to the stage cylinders he is carrying. In this case the diver would need a bailout cylinder that they could start using at the maximum depth and would then need sufficient bailout to get to the surface completing all their decompression. Unlike the open-circuit diver the rebreather diver will not use their stages unless there is an emergency but will still need to carry them.
One of the biggest dangers facing the technical diver is breathing from the wrong cylinder and, in particular, breathing a rich nitrox mix at depth which will almost certainly lead to oxygen toxicity. For this reason it is essential that all stage cylinders are analysed before use and labelled accordingly. The maximum depth at which the gas can be breathed, known as the maximum operating depth or MOD, should be clearly marked on the cylinder in a position where the divers buddy can clearly see it. The contents and date analysed should also be marked but this can be smaller and less obvious. Every time that the diver switches to a stage cylinder he must get his buddy to check that the gas is safe to breathe at that depth. This is why the depth is marked rather than the percentage of oxygen in the mixture. It is much easier to check your depth rather than look at a percentage and work out the maximum safe depth in your head.
The main choices when using a stage cylinder are the composition of the cylinder and the size. Stages can very from five litres all the way up to 12 litres. The right choice of size will depend on a number of factors. The larger the capacity the larger the physical dimension and weight of the cylinder, and so the diver needs to consider what size cylinder they are happy to carry, both on the surface and on the bottom. The choice of decompression gas will also have an impact on the size of cylinder required. For example, if a diver uses 50 per cent for their decompression gas then they will switch to it at 21m and breathe from this cylinder for the majority of their decompression, as a result they will need a large volume of decompression gas and will need a larger cylinder to carry all of that gas. However, if the same diver uses 100 per cent oxygen as their decompression gas they will not switch until they get to 6m and, as a result, will use much less decompression gas and so can carry a smaller decompression cylinder. In the UK seven-litre stages are the most common, but in recent years with the increasing popularity of using 50 per cent as a decompression gas, it has become more common to see larger decompression stages. Although most UK cylinders are measured in terms of litres, some cylinders are now measured in cubic foot. 40 cubic foot and 80 cubic foot cylinders are becoming more common. 40 cf is approximately equal to a five-and-a-half-litre cylinder with 80cf cylinders containing approximately 11 litres. For divers using 50 per cent as a deco gas and requiring more volume of decompression gas, an 80 cf cylinder is an attractive option.
With a single stage cylinder it is common to wear this on the left-hand side. Typically the top of the stage cylinder is clipped to the D-ring on the left hand shoulder with the bottom clipped to a waist D-ring on the left-hand hip. However, when using multiple stages, either because of using multiple decompression gases, using a bottom gas stage together with a decompression stage or with multiple bailout cylinders while diving a rebreather, we need to consider where we will carry the cylinder. One option is to carry two or more cylinders all on the left-hand side. The other option is to clip one cylinder on the left and one on the right. In this case the leanest mix, the one with the lowest percentage of oxygen, is worn on the left-hand side and the richest mix, with the highest percentage of oxygen, is worn on the right-hand side. For this reason this approach is known as lean left – rich right. The best option, all stages left or lean left – rich right, will depend on a number of factors. For example if the diver is using steel stages, then the fact that they will be very negatively buoyant will make wearing them both on the left very uncomfortable as they will pull the diver over to that side. In this case wearing them lean left – rich right will be much more balanced. However if the diver is using aluminium stages then having them on the same side is perfectly feasible and can be more comfortable.
As the number of stages increases the decisions get more complicated. With three stages it is still possible to wear them all on the left hand side whereas with lean left – rich right you will need to have two stages on one side and one on the other. With four stages it is no longer possible to have them all on the same side although it is possible to have two on the left and two on the right. Beyond this additional solutions have to be used, either stages staged on the shot line, support divers carrying additional stages for use on the later parts of the decompression or the use of a leash to attach additional cylinders behind the diver. Neither of these options are desirable for open water diving. Using stages on the shot line or support divers mean that the diver is no longer self sufficient and adds significantly to the complexity and organisation required for the dive. In addition there is always the risk that the diver will have to ascend away from the shotline. Multiple cylinders can be carried by the diver if they use a leash; this is a loop of rope with a boltsnap attached. Multiple stages can be attached to the leash and then the leash clipped onto the hip D-ring so that the stages trail behind the diver. This approach will only work with aluminium stages as steel stages will be negative and will not sit comfortably on the leash. In addition only certain types of aluminium cylinder, and even then only at certain pressures, will sit comfortably. The use of a leash adds an additional level of complexity and risk to open water wreck dives. For these reasons the majority of technical divers do not use more than three or four stages.