Helium VS. Air NDL

[amascuba]

It would be better to have a proper understanding of the decompression properties rather than relying blindly on a computer.

 

[rjack321]

No,

Your NDL times aren't going to change. You will just need to have enough buoyancy control to make a proper and controlled ascent. Helium is a smaller molecule than Nitrogen, therefore it dissolves into tissues faster, but also forms into a bubble more quickly on the ascent.

Here is an example generated by a dive-planner.
Both dives have 21% oxygen content.
Both dives are 100ft for 20 minutes.

The first dive is on air:

Settings: GF=30-90 ppH2O=2.041 ftsw
===================================================
DESC:100ft for 01:40 [  2] on Air, SP: 1.3, END: 81ft
DIVE:100ft for 18:19 [ 20] on Air, SP: 1.3, END: 81ft
ASC : 30ft for 02:20 [ 22] on Air, SP: 1.3, END:  0ft
DECO: 30ft for 00:40 [ 23] on Air, SP: 1.3, END:  0ft M-Value: 56% [02], GF: 50%
DECO: 20ft for 01:00 [ 24] on Air, SP: 1.3, END:  0ft M-Value: 62% [02], GF: 70%
DECO: 10ft for 01:00 [ 25] on Air, SP: 1.3, END:  0ft M-Value: 68% [01], GF: 90%

The second dive is using 21/35 Trimix. 21% air, 35% helium, and the remaining content is nitrogen:

Settings: GF=30-90 ppH2O=2.041 ftsw
===================================================
DESC:100ft for 01:40 [  2] on Trimix 21/35, SP: 1.3, END: 31ft
DIVE:100ft for 18:19 [ 20] on Trimix 21/35, SP: 1.3, END: 31ft
ASC : 30ft for 02:20 [ 22] on Trimix 21/35, SP: 1.3, END:  0ft
DECO: 30ft for 00:40 [ 23] on Trimix 21/35, SP: 1.3, END:  0ft M-Value: 60% [03], GF: 50%
DECO: 20ft for 01:00 [ 24] on Trimix 21/35, SP: 1.3, END:  0ft M-Value: 66% [04], GF: 70%
DECO: 10ft for 01:00 [ 25] on Trimix 21/35, SP: 1.3, END:  0ft M-Value: 72% [04], GF: 90%

Notice that both of these dives actually have "deco time" even though 20mins on air at 100ft is used in various "no deco" tables and as a 1 dive "no deco" limit in many Haldane based air computers.

The whole concept of "no deco" is nonsensical in the context of helium as an inert gas because you must be controlling your ascent to a far greater degree than the "no deco" limits would ask for. Every dive is a deco dive regardless of mix used. Some are just more tolerate of a relatively fast ascent to the surface than others.

 

[Chernish]

Tomek,

Use a Trimix-computer. Pretty please. With a cherry on top.
The reasons are:
1) If we could do that, nobody would spend all this money on trimix computers, would we?
2) Nitrogen and Helium do not go in and out of the compartments at the same speed and a "single gaz decompression" calculation cannot account for that.

I am surprised that the NDL doesn't change. I thought it would in fact decrease with the mix. I will double-check (not now as I am in the office though ).

Cheers!

 

[MookieMoose]

I would expect that NDLs would increase with heliox vs. air, just like is indicated in the tables Thalassmania posted. Helium is a smaller molecule than nitrogen, but it is also less soluble in both water and fatty tissue. Both of these properties play important roles in determining how quickly, and tho what extent it moves in and out of tissue. Based on teh two above properties, helium will ongas and saturate tissue faster than nitrogen, but at saturation, there would be less helium in the tissue than there would be nitrogen. Since helium will move in and out of tissue faster than nitrogen, and less of it will accumulate in tissue at teh same depth I would expect NDLs to increase slightly. Also, as others have mentioned, becuase helium moves in and out of tissue faster, it is much less forgiving of erors on ascent and during decompression. You need to be very careful about ascent rates and stop depths with helium in your mix.

**This information is for informational purposes. I in NO WAY advocate the use of helium as an inert gas by recreational divers for the purposes of extending bottom times.**

 

[Chernish]

I fooled around with MV-Plan and found these results:

For a dive at 40msw/130fsw on air, the NDL time is 10 minutes

Configuration : GF=95-95 PpH2O=0.627 mem
==================================================
DESC: 40m for 02:00 [ 2] on Air, SP: 0.0, END: 40m
DIVE: 40m for 08:00 [ 10] on Air, SP: 0.0, END: 40m
ASC : 0m for 04:00 [ 14] on Air, SP: 0.0, END: 0m

The same 10 minutes with a 21/79 Heliox (ie air where Nitrogen was replaced by Helium) gives 7 minutes deco stops (2 minutes at 6msw/20fsw and 5 minutes at 3msw/10fsw)

DESC: 40m for 02:00 [ 2] on Heliox 21/79, SP: 0.0, END: 0m
DIVE: 40m for 08:00 [ 10] on Heliox 21/79, SP: 0.0, END: 0m
ASC : 6m for 03:23 [ 13] on Heliox 21/79, SP: 0.0, END: 0m
DECO: 6m for 01:35 [ 15] on Heliox 21/79, SP: 0.0, END: 0m M-Value: 89% [04], GF: 95%
DECO: 3m for 05:00 [ 20] on Heliox 21/79, SP: 0.0, END: 0m M-Value: 95% [04], GF: 95%

With 21/79 Heliox, the NDL time is 5 minutes

DESC: 40m for 02:00 [ 2] on Heliox 21/79, SP: 0.0, END: 0m
DIVE: 40m for 03:00 [ 5] on Heliox 21/79, SP: 0.0, END: 0m
ASC : 0m for 04:00 [ 9] on Heliox 21/79, SP: 0.0, END: 0m

For quicks, the NDL with a 21/39 Trimix (ie air where only half of the Nitrogen has been replaced with Helium) is 8 minutes, ie right in the middle of the air and the 21/79

DESC: 40m for 02:00 [ 2] on Trimix 21/39, SP: 0.0, END: 15m
DIVE: 40m for 06:00 [ 8] on Trimix 21/39, SP: 0.0, END: 15m
ASC : 0m for 04:00 [ 12] on Trimix 21/39, SP: 0.0, END: 0m

 

[Dr Deco]

Hello Readers:

Helium should have longer NDLs than nitrogen [with the same percentage of oxygen in each] because it less soluble. The rapid diffusion overcomes this, however. In the limiting case, a gas with zero solubility would have limits that were infinitely long for all depths. Helium is the least soluble gas but it does have a small solubility. The low solubility shows up with saturation at shallow depths in an increased "drop out depth."

At the other end of the spectrum, very short deep dives will have a shorter NDL than nitrogen because it enters tissues much more quickly. Fast diffusion trumps solubility is this case.

Dr Deco :doctor:

 

[nereas]

Hi
If I dive with Helium would my NDL times be shorter comparing to Air ?
What is the physical explanation for that ? Is it true for both shallow and deep dives ?

First, if you plan to dive with helium mixes, you need to purchase either deco software or a helium dive computer or both. I recommend V-Planner as the software, and the Nitek HE or the VR3 as the dive computer.

When you program these with your planned dive, you will see that there are required stops and required slow ascent rates with the helium mixes.

In many cases the dive times will be the same for similar mixes, such as EAN 30 compared with TMX 30/30, or Air (21%O2/78%N2/1%Ar) compared with TMX 21/40. However, helium as a perfect inert molecule will react more quickly to decreasing pressure, or so we have observed, and therefore your typically slow recommended-ascent rate for nitrox or air now becomes a required-ascent rate for trimix/helitrox, and if you come up too quickly, you may suffer DCS (bubbling in your arterial blood system) leading to a cerebral stroke and paralysis or death.

Therefore, if you are asking questions like this, it would be like playing with your life to go out and try this on your own, without first getting professional training in decompression procedures and helium mixes.

Thus, second, you surely should take the training as well, before you try it.