Quote: (Originally Posted by
uri)
I've been having a debate with a mate about the use of 18/40 to depths of 90 metres or so. It follows on from a thread on Rebreather World where Dave Sutton suggested that a couple of minutes on a ppo2 a few tenths of a bar above 1.6 is less risky than using a diluent that would be hypoxic on the surface.
The gist of my viewpoint is that the only time one actually gets the full pp02 from the dil is if one has to do a full loop flush and that would be rapidly breathed down added to the fact that one would probably ascend anyway to reduce the pp02.
The other view is that one is injecting the 18/40 all the way down anyway to keep minimum/constant loop volume and that this would tend to keep the pp02 too high for too long.
It seems to me that with a low setpoint, it's very unlikely that injecting 18/40 on the way down (even to 90m) would spike the pp02 above 1.6 letalone keep a pp02 that was above 1.6 (or even close to it) for any length of time. I also can't see why topping up the loop volume on the bottom with 18/40 would cause serious problems since the bulk of the gas already in the loop at this point would be at high setpoint of say 1.3 and the amount added would be a small perecentage of that already there so the effect on the total loop would be minimal.
Does anyone have any views on which is the more accurate perspective?
U.
I dive with a low O2 mix (10/50tx) on deep dives for three reasons:
1) Practical application - It allows me to dive a standardized mix regardless of whether I'm diving to 200' or 400'. I have the same bailout tables, and I know the schedules roughly in my head. If I'm at 250' and see somthing a bit deeper, I can drop down without having to worry about a PO2 spike. The difference in OC bailout times between 10/50 or 18/50 are minimal, especially if you carry another nitrox mix.
2) A hypoxic mix doesn't bother me on CCR - It makes no difference to me if the mix is hypoxic at the surface because I can pre-breathe with it at my low setpoint with no problems, and I can go from the surface to the bottom on the mix. If I were to bail out, it wouldn't matter that I was using a hypoxic mix because I would likely have a nitrox mix with me as well (50%). Even if I lost that tank, I could use the 10/50 tx to 33 feet, and then switch to O2 while I worked my way up to 20 feet.
3) PO2 Spikes & Diluent Flushes - I have tried flushing down my loop in a high O2 situation with 18/50 tx, and it takes forever. The only realistic option is to go OC and blow into the loop. If you were to have a solenoid stuck open, or hit your manual O2 add by accident, flushing is nearly useless.
I just don't see the benefit of using a high O2 diluent. Yes, we can push the CNS clock, probably without incident, but why?
Aloha,
Charlie