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| Originally Posted by Sheff diver Point taken. I was refering to thirds on the scrubber. My point is that if you are really going to take advantage of the RB, you will have to carry a sh1t load of OC as bailout, or a second RB. Thinking of some of the long dives at 70m or 90m (or deeper) here. Agree that dil usage can be a lot higher in caves, but you can still get an awful long way on a 12 of dil, even if you decide to work on thirds. Scrubber duration is the limiting factor, not dil (or O2), so as Stuart says, the dive should be planned according to this. When we were discussing this, and I asked about extending the duration of a scrubber, the response was "how likely are you to need more than 3 hours heading in?" You can take spare sof with you to recharge the scrubber on the other side of the sump (thinking of cavers here, so the sole aim is to pass the sump and go caving on the other side), and have OC or even a spare rebreather staged / brought in for the deco on the way out. |
Essentially the duration of the rebreather (limited by scrubber or drive gas) should match the planned duration of the dive (as a minimum). Bailout should be sufficient to exit from the furthest point though there are no protocols that I am aware of to decide what excess of bailout over the "shit or bust" bare minimum required. It is common for rebreather divers in caves to pool and/or stage bailout - this means that they dive together and exit if one rebreather fails (a problem would arise if both divers' rebreathers failed...).
There comes a point when you put your trust in your kit and trim your safety margins accordingly (this is akin to diving a short sump on a single set).
I've mainly used my rebreather for dives where it gave me a lot of working time (for surveying or relining) - not far in or deep but just handy to be able to spend time doing stuff rather than watching my guages. The lack of bubbles was handy too. Many dives are best done on bailout alone - i.e. OC, and, as stated earlier, many dive profiles are not rebreather friendly.
The last couple of Wookey dives were done solo using two rebreathers because of the limitations in providing two divers with adequate bailout. In the Ressel, the divers carried in spare sorb.