BSAC published new gas density recommendations

[inquis]

In this case the helium in the back gas starts to re-enter the tissues faster than the nitrogen is leaving it.

Been thinking about this a little more... By saying the helium "re-enters" the tissue, there's an implicit assumption the tissue loading was below ambient to start with. That's almost certainly true for the fast tissues, but they're well away from critical tension anyway. At MOD3 depths, you're really hammering on those tissues, and the ppHe in the controlling tissue (likely with 1-3 hr half-times) may well be above ambient at 20 ft/13% He (i.e., pretty low). In that case, He does NOT enter, rather the rate it exits merely slows down.

I verified in Subsurface for a 100 m dive, bailout at 5 or 30 mins, that the highest 9 tissue ceilings did not increase at all, even when doing air breaks on 12/65. That tells me that even with a helium fraction far larger than your 13% deco mix, helium was still off-gassing in those critical tissues (i.e., no ICD).

 

[Nick_Radov]

It's good to see a focus on minimizing gas density in order to keep WOB within reasonable limits and prevent hypercapnia. But I'm not thrilled with the target PO2 of 1.4 bar for bottom mix. Seems a little high. We prefer to target more like 1.2 bar for bottom mix so that you don't run up the oxygen exposure during the working phase of the dive. That prevents vasoconstriction and loss of lung vital capacity. Then you can crank up the PO2 a little more during deco.