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A question that came up during a recent conversation was why do rebreather divers carry a cylinder with them.  We’re not talking about the cylinders for the rebreather or dry suit, but the one that’s usually on the side. For open circuit divers this is usually associated with “going deep” but rebreather divers usually have them irrespective of depth. So why ?

There are two reasons to carry one.  The first is not everyone is on a rebreather.  What happens to someone using  open circuit equipment and they need an alternate air source.  There is no octopus on a rebreather, there may be something that looks like one but it isn’t.  So one reason is to provide an alternate air source for those on open circuit.

However this is not the primary reason. The main reason is that it’s an alternate air source for the rebreather diver themselves.  Rebreathers recycle the gas being breathed and are very efficient.  They have much small cylinders with 3 litres being common, as opposed to usual 12 or 15 litre used in recreational diving.  If a rebreather fails underwater, the equipment is usually configured so that the diver can change to an open circuit mode and breathe from one of the cylinders.  But let’s have a think about this.  As an open circuit diver a 3 litre cylinder will happily get you from 30 meters, with a safety stop.  But, and this is a really big but, that’s with a full tank. This is where it is different for a rebreather diver.  Rebreathers are inefficient on descents and ascents.  In actual fact the only time the rebreather tends to use the gas in this cylinder is during the descent.  Therefore the chances of this cylinder being full during the dive are zero.  Based on what is the likely amount of gas used during the descent, an open circuit ascent from 30 metres using the rebreather cylinder is achievable, just.  From 40m the margins are very tight and both of these scenarios assume you aren’t going to use any gas for buoyancy once you get the surface.  So by having a separate cylinder that doesn’t get used unless there is an issue, a rebreather diver with a “bailout” cylinder has a huge amount of contingency for their ascent, in the unlikely event that things go wrong.