Not diving to greater than 30m/100ft unless with helium

[Storker]

Like I said, I know nada about the behavior of He. If He behaves like N2 WRT tissue loading, I'd still consider weak tmx as an alternative for deepish dives if He were as cheap as O2. Because narcosis and because I'm usually min gas limited when I'm on nitrox.


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[Kevrumbo]

From Bruce Wienke, Technical Diving in Depth, Reduced Gradient Bubble Model (RGBM) In Depth:

Helium NDLs are actually shorter than nitrogen for shallow exposures . . . Reasons for this stem from kinetic versus solubility properties of helium and nitrogen, and go away as exposures extend beyond 150 fsw, and times extend beyond 40 min or so.

Helium ingasses and outgasses 2.7 times faster than nitrogen, but nitrogen is 1.5 to 3.3 times more soluble in body aqueous and lipid tissue than helium. For short exposures (bounce and shallow), the faster diffusion rate of helium is more important in gas buildup than solubility, and shorter NDLs than nitrogen result.

For long bottom times (deco and extended range), the lesser solubility of helium is a dominant factor in gas buildup, and helium outperforms nitrogen for staging. Thus, deep implies helium bottom and stage gas. Said another way, transient diving favors nitrogen while steady state diving favors helium as a breathing gas.
Helium offgassing rate


Translating BRW's conceptual take on why shallow Helium has shorter NDL's than Air or Eanx 32 into practical and proper decompression profiles is not difficult: the NAUI RGBM Deco Tables have been out since 2003. Below (linked) are the comparative NDL's for Ean 32 and Helitrox; and example deco profiles for bottom times at 30m and 33m, for Ean 32 and Helitrox w/ & w/o O2 :
http://www.scubaboard.com/forums/te...358026-recreational-trimix-7.html#post5564996

The greater diffusivity of Helium over Nitrogen means it can potentially load free phase bubble seeds/bubble nuclei more readily than Nitrogen to produce a Boyle expansion pathology resulting in DCS upon ascent --even if you were diving trimix within air or nitrox NDL's. . .

So standard Helium blends ease work-of-breathing and mitigates CO2 & N2 narcosis; however resulting NDL's for recreational depths & times are less than that of Nitrox or Air, and IMO/IME requires Oxygen deco for effective Helium washout at least on the last & final dive of a repetitive set of recreational Trimix dives.

 

[scubafanatic]

Being PADI I have never thought of using helium. My deep diving to 120+ feet has been very rare, but on those occasions I haven't felt any noticable effects of narcosis (I know we are affected somewhat and do things slower but maybe not notice). Not that it's at all important compared to one's safety, but I imagine helium costs a lot. Never heard of it above 130'.

Not only does Helium have ever increasing cost issues, the vast majority of dive locations don't have helium available at any price.