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.