Rebreathers: are they right for you?
The use of rebreathers has become very common amongst technical divers.
Koh Tao, Thailand
In this article we will look at the advantages of a rebreather and some of the reasons why people may want to switch to diving one. We will also look at the types of people who are suited to rebreather diving and those who aren’t.
One of the biggest factors that comes into the decision to switch to a rebreather is costs. This has two aspects: the costs of the rebreather, including buying and maintaining it, set against the cost of open circuit diving. One of the biggest disadvantages of open circuit trimix diving is the cost of the gas. A twinset of trimix can cost anything from £30 for a relatively weak mix for use in 40m to over £100 for a mix suitable for diving deeper than 100m. This makes each trimix dive an expensive proposition. On the other hand a rebreather uses much smaller cylinders because the gas is reused rather than wasted; as a result we use much less and the gas costs are much lower.
We might only be spending £10-£20 for the same mixtures discussed above. As a result there is a significant gas saving when compared to open circuit diving. This can look very attractive when you are spending considerable amounts on each open circuit fill. However this must be set against the costs of the rebreather. Depending on the model a rebreather is likely to cost between £4-8,000. If you go for an older second-hand model then you might get one for less than this. In addition to the initial cost of the rebreather you will need to factor in training on the rebreather which is likely to add on another £1,000.
In addition there will undoubtedly be additional costs to add on extra equipment to the basic rebreather. As a result it is not uncommon for the initial start up costs on a rebreather to be between £6-10,000. Obviously you will need to do a lot of trimix diving in order to save enough to justify this initial outlay. The vast amount of technical divers do not do enough diving to clearly justify buying a rebreather based on savings in gas costs. Unless you are doing 20 or more trimix dives a year then it is not cost effective. If you also do a significant amount of recreational diving then each dive may actually cost you more on a rebreather.
On open circuit you may only need to pay for an air fill but on even the shallowest dive you will still need to use pure O2 in one of the cylinders and use carbon dioxide absorbent in the rebreather. The annual replacement of the three O2 sensors and handset batteries also needs to be taken into account. This means a shallow recreational dive may only cost £5 for an open circuit diver but £10-£15 for a rebreather diver. Unless you are doing significant numbers of deep trimix dives a year with little or no recreational diving then the gas savings from using a rebreather will not outweigh the initial start up costs. However there are still a number of other reasons why a rebreather is an attractive option.
Every time we breathe in we only use a small proportion of the gas we inhaled, the rest is breathed out. On open circuit this is exhaled into the water and lost. However on a rebreather the exhaled gas is fed back through the rebreather, carbon dioxide is removed and the oxygen we used up is replaced. As such it makes much more effective use of the gas we have and we don’t need to carry anywhere near as much gas. As we have already seen this makes each fill much cheaper. It also has a number of other advantages. The amount of oxygen the body uses in each breath is roughly the same independent of depth.
This means that we use the same amount of oxygen at 100m as we would at 10m and so our oxygen cylinder will last the same amount of time at 100m as at 10m. On open circuit we breathe much more at depth due to the effect of pressure and so as we go deeper and deeper we have to take larger and larger cylinders. Despite taking these large cylinders they will still be used up very quickly and so the amount of available gas becomes the most critical part of dive planning. On a rebreather our gas supply is used up at the same rate irrespective of depth and so the amount of gas we are carrying is no longer the limiting factor. Instead the critical factors become the amount of decompression we are incurring, the duration of our carbon dioxide absorbent and our risk of oxygen toxicity.
Of course this assumes that the rebreather will always work as intended. Just in case there are any problems with the rebreather we also need to carry open circuit gas to get us safely to the surface. In this case we would need a bailout cylinder that we could start using at the maximum depth and would then need sufficient bailout to get to the surface completing all our 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. The fact that we are only using smaller cylinders makes the logistics of filling much easier. The volumes of gas needed are much smaller when filling a 3L cylinder rather than twin 12s and so it becomes feasible to take enough gas with you for even a week’s diving.
This contrasts with the situation when using open circuit where significant volumes of helium and oxygen will be required as the majority will be breathed out and wasted. This can make a big difference if you don’t have a local dive shop that can fill trimix. If you have to drive 50 miles there and back to drop off a twinset for a trimix fill and then do the same thing next day to pick it up then the logistics of this can be significant. If you are diving somewhere without easy access to a dive centre that can fill trimix the ability to take your own gas simplifies the planning and logistics.
Even if you wont save money and don’t need a rebreather for logistic reasons there are still other reasons to switch. Diving is a hobby and so doesn’t always have to be justified on cost reasons. The cheapest option is not to dive but most of us don’t consider this a possibility. In the same way that some people spend their money on motorbikes, horses, home cinemas, model helicopters or any other hobby there is no reason why someone shouldn’t spend their money on a rebreather just because they want one. Other people may switch to a rebreather to challenge themselves to learn something new.
There are a number of reasons why many divers want to switch to a rebreather. However, the risks of rebreather diving mean there are some people who are better suited to rebreather diving than others. Rebreathers are significantly more complicated pieces of equipment than an open circuit scuba set. While rebreathers do not require a huge amount of effort they do require more care and maintenance than open circuit. For divers that throw their kit into the back of the car or into the garage and then don’t look at it until the next dive this can cause a problem. This type of person is not really suited to rebreather diving unless they can discipline themselves to ensure they maintain the rebreather.
On the other hand there are many divers who enjoy cleaning and maintaining their equipment almost as much as the dive itself. They get pleasure from adjusting the kit until it is just right and it is viewed as part of the hobby rather than an added chore they must do. This type of person is ideally suited to rebreather diving. Do you have the right mindset? In addition to cleaning and maintenance rebreathers require discipline. There is a certain mindset that is required to ensure that the unit is assembled correctly each time and that all of the pre-dive checks are rigorously followed. Most rebreather accidents are caused by the divers not following the correct procedure. This includes not diving the unit if there is any problem with.
Many divers become complacent and will dive with known problems with their rebreather. They are confident that they can overcome the problem and in the majority of cases they manage to deal with the known problem. However if there is any problem during the dive the impact of the initial problem can be significantly increased by subsequent problems. It requires a significant level of discipline to call a dive for what might appear to be a minor problem but becoming complacent about these failures is one of the most common causes of accidents. During the dive the diver must constantly monitor the unit to ensure it is operating correctly. This is summed up by the golden rule of rebreather diving ‘Always know your partial pressure’.
It doesn’t matter if the diver is at 10m or 100m the level of monitoring is the same and so a 10m dive must be approached with the same mindset as a 100m dive. As such there is no such thing as a casual rebreather dive. Not all divers have the mindset to adjust to rebreather diving but without this mindset they should not consider rebreather diving.
In order to be a safe rebreather diver there are a number of skills that need to be mastered over and above the basic open circuit skills. Some of these are related to the normal operation of the rebreather and some are related to emergency situations. Like any skill it takes practice to master these skills and practice to maintain them.
When moving from open circuit to a rebreather there are skills, like buoyancy control, which must be re-learnt. This takes time and effort. For an experienced diver this means the frustrating process of going back to basics and building up their experience. Unless you are prepared to put in the time to master the basic skills you will always be diving on a base of weak rebreather skills, even if you were previously a very experienced open circuit diver. These skills also need to be practiced regularly in order to ensure that they are maintained. This means that it is essential to dive a rebreather regularly in order to maintain the appropriate skill levels.
For some people a rebreather is a desirable and in some cases an essential way to progress their diving. For others the advantages do not necessarily outweigh the disadvantages. For this reason it is a personal decision and not one to be taken lightly. You must decide whether there is a good reason to dive a rebreather and then whether you have the right mindset to be able to dive it safely. Like many things the correct decision will vary between individuals.