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But I want one... Switching to rebreathers after diving open circuit scuba

Many of the pieces of equipment used by technical divers look different to the equipment used by recreational divers. However, for most of the time, the basic principles are the same.
But I want one...   Switching to rebreathers after diving open circuit scuba
Published in X-Ray Issue: 51 - Nov 2012
Authored by: Mark Powell | Photography: Peter Symes | Translation:
However, the use of rebreathers in technical diving completely changes many of the basic principles that govern recreational diving. Some limits are removed almost completely but other complications are introduced.

A rebreather is simply a way to reuse the gas breathed out by the diver in conjunction with a method of removing the carbon dioxide produced by the diver. The main advantage of a rebreather is that it is much more efficient on gas usage. When breathing normal air at the surface we use up about 4% of the oxygen in the air for our metabolism and breathe out the rest of the oxygen as well as all the nitrogen. This means that 96% of the gas we breathe escapes with each breath.

At depth this is made even worse by the fact that we are breathing gas under much higher pressure and so each breath contains a much higher volume of air. At 40m we are breathing 5 times the volume of air with each breath compared to the surface and at 90m we are breathing ten times the volume that we would be on the surface.

With every breath this gas escapes from our second stage and bubbles to the surface. This is inefficient as the body still only needs the same amount of oxygen but we are wasting ten times as much gas with each breath in order to obtain it. This is one of the reasons why for open circuit divers gas planning and breathing rate is so critical.

The limiting factor for an open circuit dive to 90m is almost certainly going to be the amount of gas that needs to be carried for the dive. A rebreather avoids this problem by re-using the gas breathed out by the diver, as the majority of the gas isn’t being wasted the diver doesn’t need to carry as much gas in order to do the same dive.

A rebreather works by catching this exhaled breath and instead of allowing it to escape it reuses it in the next breath. The small fraction of Oxygen that was used up by the body is replaced with more Oxygen. This means that rather than wasting the vast majority of each breath we reuse all of the gas.

By reusing our gas this provides three key benefits to the technical diver.

1 - Gas Duration
Firstly we don’t need to use anywhere near as much gas as an open circuit diver. The amount of Oxygen required by the body is roughly the same at any depth and so our Oxygen will last the same at 100m as it will at 20m. The balance of the gas in the breathing loop, known as the diluent, is reused rather than wasted and so again we use the same amount at 100m as at 20m. This vastly decreases the amount of gas we will need to complete the dive and rather than twin 20L cylinders we can use two 3L cylinders.

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 our 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.

This means that running out of gas on a rebreather is much less of a concern than for an open circuit diver. 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.

2 - Gas Costs
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 week 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 £5-£10 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 four and eight thousand pounds. 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 thousand pounds.

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 and 10 thousand pounds. Obviously you will need to do a lot of trimix diving in order to save enough to justify this initial outlay.

The majority of 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 oxygen in one of the cylinders and use carbon dioxide absorbent in the rebreather. The ...