Triox & V-Planner

[Roatan Tec Team]

In regards to HPNS, in V-planner I have a dive planned to 345feet using TMx 12/57. V-planner won't allow a gas switch with nothing less than a triox 32/23. Know I completely understand about HPNS and its effects, what I want to know is, are there some divers out there following V-planner strictly, or disregarding the helium that high in the deco mix, and adding only small amounts, eg; 10%.....

 

[GrimSleeper]

In regards to HPNS, in V-planner I have a dive planned to 345feet using TMx 12/57. V-planner won't allow a gas switch with nothing less than a triox 32/23. Know I completely understand about HPNS and its effects, what I want to know is, are there some divers out there following V-planner strictly, or disregarding the helium that high in the deco mix, and adding only small amounts, eg; 10%.....

Erm, not entirely convinced you've got the right end of the stick with this one...

HPNS (High Pressure Nervous Syndrome) is a potential consequence of high inspired ppHe, somewhere in the region of 11-12atm. At 345 feet, you shouldn't encounter HPNS even if you were breathing Heliox, although you'd be getting towards the depth where you'd have to think about it. A reduction in inspired ppHe (ie. ascending) would reduce the risk of HPNS, so it's not a concern during the ascent phase.

The suggested switch to 32/23 as a first deco gas is intended to lessen the risk of Isobaric Counterdiffusion (ICD), which has caused DCS hits on gas switches where there is a big spike in inspired ppN2. By keeping 23% He in the first deco mix, you would avoid jumping your ppN2 from 1.55atm to 3.4atm at the 40m switch. As an additional benefit, you give yourself less N2 to off-gas later in the schedule, which theoretically gives you a 'cleaner' decompression as the Helium dissolves out of solution much faster than the Nitrogen you would be replacing it with if you just switched straight to Nx32 at 40m.

On the other hand... The risk of ICD is marginal on a switch at 40m, if you're desperate to save the $$ on He you might well get away with it. PLEASE DO NOT READ THAT AS MY SUGGESTION THAT YOU DO SO. Whenever I have dived a profile in the depth range you are looking at, my gas selection has been TMx 12/57 with TMx 32/25, Nx50 and O2 for deco. Given that the gas for a dive like this is insanely expensive anyway, I figure it's not worth risking an ICD hit and possibly spending the rest of the ascent puking at every breath (as described vividly by both Sheck Exley and John Bennett, amongst others) for the sake of not putting some He in my deco mix.

Hope that makes sense...!