Off on a CCR dive? You will need to take some bailout with you then. It’s not so deep so air should be good, yes? Maybe it is a bit deeper and you will have trimix as your diluent. To keep things simple you could just have the same onboard gas as your deep bailout gas. That’s probably OK but there could be a better way. A friend of mine who is a mathematician told me it was a more fun way too but the rest of the course students were not entirely in agreement!
Don’t worry it isn’t too difficult. Let’s start with a few simple guidelines first.
What ever gas you have as your bailout, it needs to be a life supporting fraction of oxygen (FO2) for the depth of the dive your are planning. For the majority of open ocean dives in the shallower than normoxic trimix depth range this often means just a single cylinder, providing guideline number two below is considered.
You need to have life sustaining volume. Bailing from your CCR on to your used (you used it during the descent) 2 or 3 litre onboard air diluent at 40 msw/131 fsw is not going to get you back to the surface. Even a second 3 litre strapped to the rebreather might not do it, especially if you have incurred any deco.
So as well as choosing the right FO2, have enough of it with you.
One of the ways in which we might choose the ‘right FO2’ is to consider another couple of general guidelines.
The first of these is that ideally you don’t really want your bailout gas to have any undesired physiological impact. By way of an explanation, let’s look at what we are actually breathing when on the loop of a CCR, breathing a setpoint of 1.3 PO2, using air diluent at a depth of 36 metres (118 feet). The actual fraction of inspired O2 would be a touch over .28, or 28%. For some divers or in some diving environments, bailing out from a warm, moist breathing loop or 28% on to a cold dry regulator supply air could bring on an uncomfortable level of narcosis.
Just to recap, divide the PO2 by the depth as an absolute pressure will give you the fraction of inspired oxygen (FiO2)p
So rather than air as a bailout gas, we could take Nitrox 28. It is a life supporting FO2 for our depth and will perhaps lessen any physiological effect of bailing out on to open circuit.
We will have a look at choosing diluent and using an off-board cylinder plugged in to your rebreather in a future post.
The final guideline dovetails nicely with the previous one, in that our bailout gas should’t have any adverse affect on your decompression obligation or no-decompression limits. In some diving circumstances it would be easy to go from a no-stop dive in to a decompression obligation just by bailing out to a poorly chosen gas. In the event that the reason for the bailout becomes or is unrecoverable the subsequent ascent is never usually immediate. Time can often spent at depth initially trying to resolve the problem, then alert your buddies or dive team, maybe send up a surface marker buoy; all of these can easily take a dive from no-stop in to deco or add to an existing deco requirement, avoidable with careful gas selection.
So, our bailout gas should have no appreciable effect on our no-stop or decompression time and no adverse physiological effects.
If you are using trimix, then choosing your bailouts and any necessary decompression gasses can be a bit more involved but nothing too taxing. Yes, another post for the future.