Deco gas's containing Helium

[Depth-junkie]

I am planning a 60m dive for next week and am thinking of using a trimix deco gas of 36/17 for the first gas switch to keep PPN2 to 0.893(47%). This will be the same ppN2 as my back gas of TMx 18/35 at the 34m switch. I will then go on to deco at 9m on 80% nitrox.

I have done this dive many times before using TMx 18/35 and nitrox 36 and 80 for deco, but have been intrested in the recent talk of keeping the ppN2 the same for the first switch and it seems to make sense, so thought i would give it a try.

Have any other YD out there tryed helium in deco mixes(Bar CCR divers)?,did you feel any better for?,it seems to only add an extra few min of deco compared to normal nitrox deco.Any Thoughts on this plan or advice?

The proposed plan is 60m for 25min on TMx18/35
switch's will be @ 34m TMx36/17 and @ 9m on Nitrox 80
Stops will Be:
40-2
31-2
21-1
18-1
15-1
12-6
9-6
6-32

[Bob Cooper]

Here's what I'd do for 60m for 25mins using 18/45, 50% and 100%

60-25 (18/45)
45-1
42-1
39-1
36-1
33-1
30-1
27-1
24-1
21-5 (50%)
18-5
15-2
12-2
9-6
6-15 (100%
then 5 mins to surf

Would be interested to hear what others say......

Bob

[Andy Phillips]

Hi,

I can't see much point in He on a 60M dive,

Tend to dive the VR3 plus added deep stops, but my slated plan would be;

60M 25 mins 18/35

39 - 1
36 - 1 Switch to 32%
33 - 1
30 - 1
27 - 1
24 - 1
21 - 1
18 - 1
15 - 3
12 - 4
09 - 5 Switch 80%
06 - 21

plus 5 mins to surface.

The VR3 would get me out a bit quicker. In addition to this, odds are I would stick in a 1 min pause at 45 and 42.

Andrew

[And]

Hi

Looking at your planned profile you would screw yourself up with He in the deco gas. The stops after that switch are too short. Helium demands a better (ie longer) ascent profile than that IMHO.

Helium in the deco gas is usually only recommended with longer bottom times for a two deco gas dive.

Sorry, to make that clearer, with only two deco gasses you don't need helium, but with more than two then you would, unless your bottom time was longer than an hour say.

Andy

PS Oh, and I'd do what Bob did

[Depth-junkie]

Hi all thanks for the replies,

I guess i will stick to my original gasses that i have used many times before for this depth(ie 36 & 80% for deco). I can see what you guys mean about only needing helium in deco if bottom times are a lot longer or the dive is a 3 deco mix dive, it was just a thought to see if it may improve the profile!!!!!!!!

Bob as for your profile i can not get any of my software to match your plan.
I use proplanner(VR3 diveplan) & v planner to cut profiles, what do you use?
it looks interesting!!!! i see that you do your deep stops starting deeper and also start 1 min every 3m from there on up. I dont start the 3m incriments till 21m.

[Andy Phillips]

You won't be able to, it's a GUE "Deco on the Fly" profile. Don't bother asking just search the site for DOTF and you will see the many discussions we have had on this subject.

Andrew

Originally Posted by Depth-junkie

Hi all thanks for the replies,

I guess i will stick to my original gasses that i have used many times before for this depth(ie 36 & 80% for deco). I can see what you guys mean about only needing helium in deco if bottom times are a lot longer or the dive is a 3 deco mix dive, it was just a thought to see if it may improve the profile!!!!!!!!

Bob as for your profile i can not get any of my software to match your plan.
I use proplanner(VR3 diveplan) & v planner to cut profiles, what do you use?
it looks interesting!!!! i see that you do your deep stops starting deeper and also start 1 min every 3m from there on up. I dont start the 3m incriments till 21m.

[Mark Chase]

For what its worth

25 @ 60 on 18/45

5m/min ascent to...

1 @ 39 gas switch to 32%
2 @ 18
4 @ 15
5 @ 12
6 @ 9 gas switch to 80%
25 @6

Decoplanner 20/80GF

5 min ascent to the surface added by me

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.

This is claimed to greatly improve the decompression at a sub clinical level and to me it makes sense. If you put the He into the profile it increases the deco but the divers actually running these gases appear to be ignoring the He.

Bobs profile has a switch to 50 % at 21m with a PPn of 1.15. The switch to 50% puts him on a PPn of 1.55. The shock to the system is far less and therefore the profile looks better than a swap deeper to 32%.

If it weren’t for my personal preference of bail out gas I would have preferred to run 50% for deco. As it was I have been doing a lot of research into adding He to my 32% and if I had stayed OC that’s the option I would have gone for on my longer deep dives (60+).

Food for thought and I certainly don’t have enough medical knowledge to adequately argue the case but on a layman level it seems pretty obvious that hiking the PPn is a bad thing.

ATB

Mark Chase

[Mark Chase]

The proposed plan is 60m for 25min on TMx18/35
switch's will be @ 34m TMx36/17 and @ 9m on Nitrox 80
Stops will Be:
40-2
31-2
21-1
18-1
15-1
12-6
9-6
6-32

Using your numbers:

Gas switch at 34m from 18/35 = a ppn of 2.07

Switching to Nx36 = ppn of 2.8

Switching to 36/17 = ppn of 2.33

beter to switch to 36/20 ppn of 1.94 and roughly a 30% drop in He so good all round.

ATB

Mark Chase

[NotDeadYet]

I haven't checked the profile but I'd guess if your EAN80 went wrong then only having the choice of two trimixes would make for a very long hang.

[Mark Chase]

If you did all the deep stops and the ascent OK then the Nitrogen is the problem not the He. You can off gas He in air no problem. Helium bends can be treated with surface presure 02 so I would stick to my Nitrogen deco shalow stop profile and get on o2 on the boat.

But all this is unnecessary if the 10ltr 80% drop tank is properly deployed

Oh yes and Helium bends hurt like hell so be prepaired for a world of pain till the 02 kicks in.

ATB

Mark Chase