Deco gas's containing Helium

[Depth-junkie]

But that’s not the question. The question was is it worth putting He in the deco mix. The answer is Yes if you believe current thinking. At first gas switch depth using 32% you would have a PP of Nitrogen of 1.8. When you switch to 32% your PPn would be 3.4 this is a bad thing. So to prevent this you would swap to 32/35 to give a PPn of 1.65. Thus continuing the reduction in the Nitrogen and reducing the He as well.

[color=Black]Mark, i think you are the only one on the same train of thought as myself on this one. I can see the others point of view with regards to the idea that helium in deco is only worth while if the dive is deeper requiring a 3 mix dive, or if the duration is longer(from tissue saturation point of view), but if the idea works on deeper mix dives why would it not work on dives in the 60-80m range.

my thinking behind the suggested deco mix of 36/17 was to keep the ppN2 to 2.068 @ the 34m switch, changing from the back gas of TMx 18/35 to 36/17 both contain the same percent of N2(thus avoiding a nitrogen spike) which you would normaly get if you switched to standard 36%, as the ppN2 would jump to 2.816. And we all know nitrogen is not a good thing!!!!!!!!!!!!!!

By switching to TMx 36/17 you are also reducing the helium content by 18%, And i really cant see a problem with regards staying on helium till the 9m stop, because if all the deep stops and asent are carried out correctly there should be no probs as the only concern by this point is the residule nitrogen.

I normally cut tables from v-planner for back up, and fly vr3 for the dive. i have looked at the dive plan mode again on the Vr3 for the proposed gas's of TMx 18/35,TMx 36/17 & NTx80 (as this is what i will be following during the dive) and have decided that i will extend the 21,18 & 15m stops from 1 min to 2 min. Once the Vr3 has cleared i always take a further 2min to get from 6m to 3m and then a further 3min from 3m to the surface.

As for loosing the 80% and only decoing on 36/17 this will not be a problem as we always have a drop of 100% on the trapeeze @ 6m.

So all in all i have decided to try my plan based on my laymens knowledge of deco.

Ill let you guys know how it went!!!!

[Andy Phillips]

Mark and I agree entirely on the issue of He in a deco mix, but you said 60M for 25 mins, my point is it is not worth the expense for a dive this short.

The VR3 will penalise you for the He in the deco mix, I would be inclined to not tell it and just extend your deep stops a bit plus keep a very slow assent from 6M to the surface. Once your are off the He deco and onto your final rich mix the He should fly out using 80% means you get off it at 9M, add a couple of minutes at this point, the VR3 will knock it off the 6M stop and you will give the He a better chance of getting completely out.

From the stories I have heard about He bends, the hit started in the water and hurt like buggery. Though in all cases it was self cured by going onto O2 on the boat.

Enjoy the dive,

Andrew

[graham_hk]

60 for 25 is a normal profile that I do regularly here in warm water. I would have something in the region of 15 O2 and 45 He for this dive.
Deco:
Pause (20sec or so) @ 45, 42 and 39m
1 min 36, 33, 30
2 min 27, 24
4 @ 21m on 50%
4 @ 18
4 @ 15
4 @ 12
4 @ 9 on BG
12 @ 6 on O2, 6 on BG, switch back to O2 @ 6 then creep up to surface (5-6 min).

Personally in your profile I see no benifit in switching to a deco gas (Trimix or not) so deep since the PO2 has dropped by the time you reach your intermediate stops (above 21m). If you use 50% and O2 you can have push pull deco. You pull the inert gas out of tissues with the O2 window on the first few stops of your high pO2 deco and then push the gradient with the last few (personally I like to distribute the stops equally between 21 and 9 since it makes it easy to remember and because of push pull).

Graham