Helium VS. Air NDL

[G~Diver]

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 ?

[amascuba]

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%

[G~Diver]

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.
[/code]

Would the NDL time will be the same if the mixture is 21% O2 and 79% Helium?

[amascuba]

Yes..

[TSandM]

It is my understanding that the precise kinetics of helium are somewhat controversial. Although it is a smaller molecule, it is far less lipid soluble, and lipid solubility accounts for a lot of the absorption of gas. Some decompression programs will penalize you for helium in the mix with longer decompression times. I would assume those models would also give you less no-deco time on helium, as well, for the same reasons.

GUE's Rec Triox class teaches the same limits for helium mixes as for Nitrox. MDL time is determined by total inert gas in the mix, regardless of its identity.

[Tomeck]

With helium, we must go up more slowly than nitrogen, because saturation and desaturation are slower?

[amascuba]

With helium, we must go up more slowly than nitrogen, because saturation and the desaturation are slower?

Helium is a smaller molecule than nitrogen. Therefore on the descent it will dissolve into the tissues quicker than nitrogen will. On the ascent helium will also come out of a solution quicker than nitrogen will. What does this mean? Helium is less forgiving on improper ascents. If you shot to the surface from depth that you would more likely have a DCS incident using helium than you would nitrogen. It's also not so much that the ascent is slower with helium than nitrogen, it's more that the ascent needs to be controlled with a higher degree of accuracy.

[Thalassamania]

Helium and nitrogen are NOT THE SAME. By way of comparison, here are the US Navy Tables no-D limits for 0.7 ata Constant Partial Pressure Oxygen, in Nitrogen, and in Helium

Depth--Nitrogen--Helium
50-----143-------205
60------74-------133
70------51--------81
80------39--------51
90------32--------37
100-----27--------29
110-----24--------22
120-----19--------18
130-----16--------13
140-----13--------11
150-----11---------9
160------9---------8
170------8---------7

[Charlie99]

It is my understanding that the precise kinetics of helium are somewhat controversial. Although it is a smaller molecule, it is far less lipid soluble, and lipid solubility accounts for a lot of the absorption of gas. Some decompression programs will penalize you for helium in the mix with longer decompression times. I would assume those models would also give you less no-deco time on helium, as well, for the same reasons.

It's not as much that it is controversial, but that helium in a mix has different effects on deco depending upon the depth and time.

He will shorten NDL times slightly --- due to the rapid ongassing of helium.

OTOH, because it has lower solubility, on dives much past air NDLs, the deco time on helium mixes will be less than with air.

Differences in "quality" of inert gas saturation - He vs. N2 has some comments by Bruce Weinke about the significant advantages of helium for deep/long dives, and the minor reduction in no-stop times for short, deep dives.

[Tomeck]

Helium is a smaller molecule than nitrogen. Therefore on the descent it will dissolve into the tissues quicker than nitrogen will. On the ascent helium will also come out of a solution quicker than nitrogen will. What does this mean? Helium is less forgiving on improper ascents. If you shot to the surface from depth that you would more likely have a DCS incident using helium than you would nitrogen. It's also not so much that the ascent is slower with helium than nitrogen, it's more that the ascent needs to be controlled with a higher degree of accuracy.

If I have a computer with three mixtures like D9, can I use the percentage of helium as nitrogen or muss it a special computer for trimix?