Drawbacks to Helium??

[Sas]

The drawbacks to me seem to be the fact that it costs a bajillion dollars.

 

[waterpirate]

The biggest single drawback of helium is the lack of knowledge about it and it's acceptance-rejection by agencies. Helium is the new voodoo gas that nitrox used to be. The notion that it is only for tech dives is one of favorite urban myths. Is it expensive? Yes. Like everything else you have to "pay to play"
Eric

 

[w ripley]

One of the advantages of diving a rebreather is just how little HE you do use. Not trying to say dive a RB to save money (it won't happen) but a little HE goes a long ways with one.

I use to use several "K" bottles of HE a year on open circuit (every time you exhale you throw away the inert stuff), but now with a RB a single bottle will last me a year or two (as you rebreathe it over and over again in the closed loop). If you are going deep a lot, you might look into a RB. If you go that direction at least you will be able to cut down on the quantity of HE you use.

 

[netmage]

Looking through various agency's standards for diver qualification, it seems to me that some agencies are a little gun-shy of giving divers access to Helium, and quantities of it at certain levels (eg. TDI's AN/DP Helitrox add-on that limits mixes to 20% He) while other agencies are far more liberal with Helium for dives on the same level (eg. GUE Tech1 certifying for 21/35) and some of these additions have been recent (TDI) while other programs involving Helium have been established longer.

Why is this? Why do some agencies keep He levels low, while others want it early, often, and in larger amounts? Is there some danger to using elevated fractions of Helium that beginning technical divers may not be ready to handle? I'd really like to understand the difference between agency's motives.

NAUI's rationale for the 17% limitation of Helitrox was that if a diver had issues controlling buoyancy, the He % would not be a contributing factor in any sort of DCS issue....

GUE's program has a much stricter buoyancy requirement earlier in the training program, hence when someone has moved into Tech 1, buoyancy control is pretty much a non-issue. Also, the whole "He is your friend", is one of the cornerstones of their philosophy.

 

[Blackwood]

NAUI's rationale for the 17% limitation of Helitrox was that if a diver had issues controlling buoyancy, the He % would not be a contributing factor in any sort of DCS issue....

Do they have any model-based justification for saying 17% He won't contribute to DCS but 18% will?

 

[waterpirate]

Do they have any model-based justification for saying 17% He won't contribute to DCS but 18% will?

This is where the rubber hits the road. I am not aware of any studies or data that will support one way or the other. My use of helium is governed by the effects it has in relation to schedules and sobriety. The bolt to the surface arguement agianst helium speaks to a divers skill set and lack of training and or redundant gas supply, not to the helium. I would argue that if bolt to the surface is an option in your dive plan you should not dive deeper than you can skin dive and breathe nothing but air.
Eric

 

[Kevrumbo]

Originally Posted by kanonfodr
Looking through various agency's standards for diver qualification, it seems to me that some agencies are a little gun-shy of giving divers access to Helium, and quantities of it at certain levels (eg. TDI's AN/DP Helitrox add-on that limits mixes to 20% He) while other agencies are far more liberal with Helium for dives on the same level (eg. GUE Tech1 certifying for 21/35) and some of these additions have been recent (TDI) while other programs involving Helium have been established longer.

Why is this? Why do some agencies keep He levels low, while others want it early, often, and in larger amounts? Is there some danger to using elevated fractions of Helium that beginning technical divers may not be ready to handle? I'd really like to understand the difference between agency's motives.

Shallow depth NDL's on Helium can be less than Nitrox, or even Air diving:

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.

http://www.scubaboard.com/forums/ask-dr-decompression/32090-helium-offgassing-rate-2.html

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.

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

Sometimes even within a prescribed general deco profile, you have to tailor it to your own individual physiology. The GUE rule-of-thumb of diving 30/30 triox like Eanx32 utilizing MinDeco/ascent rules resulted in the "chokes" for me, after two repetitive dives to ave depth 24m with a 90min SIT a few years ago. I now use Oxygen for deco on the last repetitive dive when using recreational triox...

See also this thread:
http://www.scubaboard.com/forums/technical-diving-specialties/358026-recreational-trimix.html

 

[netmage]

Do they have any model-based justification for saying 17% He won't contribute to DCS but 18% will?

You'd have to pull that rationale from good ole mr open himself, weinke....

 

[lowviz]

I thought that the issue was breathing hypoxic mixes at shallow depths. 0.17 is the lower limit for 1 ATA and 0.16 is the "first indications of hypoxia" per Vance Harlow's Oxygen Hacker's Companion page 137. -I think that he got this from NOAA, not very clear. (but then this IS the "Oxygen Hacker's Companion")

-yes, I mean the PPO2, not He

 

[netmage]

I thought that the issue was breathing hypoxic mixes at shallow depths. 0.17 is the lower limit for 1 ATA and 0.16 is the "first indications of hypoxia" per Vance Harlow's Oxygen Hacker's Companion page 137. -I think that he got this from NOAA, not very clear. (but then this IS the "Oxygen Hacker's Companion")

-yes, I mean the PPO2, not He

We are referring to He %.....