I was wondering
- Thread starter wolfpack
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The short answer is no. One dives with helium mixes not to prolong one's bottom time (as is usually the case with EANx mixes), but to manage the risks of oxygen toxicity and inert gas narcosis. When one dives with trimix, one is not trying to stay within the NDL of recreational (sport) diver tables.
Hope this helps
DD
The use of 30/30 is now being advocated for recreational dive profiles by GUE (it is only a matter of time before the other agencies jump on the bandwagon). There really is no such thing as a NDL dive. While a guy called MHK (a GUE instructor) is claiming that you will have longer bottom times using 30/30 vs 32% I do not believe that assertion.
I base this on the fact that helium on gasses and off gasses at a greater rate than nitrogen. A common value you see quoted is 2.65 based on the solubility differences of He vs N2. I have not seen any studies that have determined that this is the actual rate. There are a couple that indicate the off gassing rate of He is 2-2.5 times greater than N2.
This is an important facet of diving with He because ascending from a dive at a fast a rate means inadequate decompression. A proper rate (~30 fpm) will minimizes bubble growth and expansion and controls desaturation better when compared to faster rates.
However, there are numerous other benefits to using 30/30 vs 32%, such as reduced breathing effort and an associated reduction in CO2 production & retention, a lower narcotic level, and in my opinion you just feel better. The latter may just be a result of doing a proper ascent. As an aside my wife and I currently dive 30/30 the same as 32% (single tank) and have not had any problems.
omar
I base this on the fact that helium on gasses and off gasses at a greater rate than nitrogen. A common value you see quoted is 2.65 based on the solubility differences of He vs N2. I have not seen any studies that have determined that this is the actual rate. There are a couple that indicate the off gassing rate of He is 2-2.5 times greater than N2.
This is an important facet of diving with He because ascending from a dive at a fast a rate means inadequate decompression. A proper rate (~30 fpm) will minimizes bubble growth and expansion and controls desaturation better when compared to faster rates.
However, there are numerous other benefits to using 30/30 vs 32%, such as reduced breathing effort and an associated reduction in CO2 production & retention, a lower narcotic level, and in my opinion you just feel better. The latter may just be a result of doing a proper ascent. As an aside my wife and I currently dive 30/30 the same as 32% (single tank) and have not had any problems.
omar
WYDT
Contributor
Omar, I can definitely vouch for the benefits of "shallow" Helium. I've used 30/30 on several of my recent cave dives where the depths ranged from as little as 40ft to around 100max. I've previously done these dives on 32% (EAN) and can tell you there is a marked difference when doing the same dives on 30/30.
I feel more alert and see things I didn't see before. I never felt "narked" before on the 32% but definitely see a difference with 30/30... Helium GOOD!
I love helium but I can't see divers who cry over a $5 tank of air ever paying for trimix for the same dive.
Even mixing it yourself it gets expensive not to mention time consuming.
Even mixing it yourself it gets expensive not to mention time consuming.
WYDT
Contributor
true.... I'm hoping as it becomes more widely accepted that the price will drop some.
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Helium is pretty expensive here in the Lower Keys. For me to fill a set of doubles (200 FT3) costs me about $30 in helium alone.
As far as decompression obligation goes, my calculations indicate a 60 minute dive to 100fsw on 30/30 requires 43 minutes of decompression. While the same depth/Bottom time on EAN32 requires only 31 minutes of decompression. Both instances the back gas is used for deco.
The additional cost coupled with the additional deco time does not seem to warrant the use of Helium at 100fsw.
The benefit is certainly virtually no narcosis. A 30/30 mix at 100 fsw has an Equivalent Narcotic Depth of 34 feet. So your mental state is the same as diving air at one atmosphere.
As far as decompression obligation goes, my calculations indicate a 60 minute dive to 100fsw on 30/30 requires 43 minutes of decompression. While the same depth/Bottom time on EAN32 requires only 31 minutes of decompression. Both instances the back gas is used for deco.
The additional cost coupled with the additional deco time does not seem to warrant the use of Helium at 100fsw.
The benefit is certainly virtually no narcosis. A 30/30 mix at 100 fsw has an Equivalent Narcotic Depth of 34 feet. So your mental state is the same as diving air at one atmosphere.
What software are you using for your calculations showing the 100 foot dive on ean vs he? Just curious.
Thanks everyone for the response, the reason I ask is that I saw where MHK was stating on another news group that 30/30 would increase bottom time. It was my opinion that it did not, although I am barely versed on this subject. As I read further everyone else disagrees with mhk and says that it does not add bottom time. MHK is a gue instructor so I am not really sure where he is coming from.???
Thanks everyone for the response, the reason I ask is that I saw where MHK was stating on another news group that 30/30 would increase bottom time. It was my opinion that it did not, although I am barely versed on this subject. As I read further everyone else disagrees with mhk and says that it does not add bottom time. MHK is a gue instructor so I am not really sure where he is coming from.???
nradov
Guest
The question as stated makes no sense, or seems to be based on a flawed assumption. You can stay on the bottom as long as you want with either gas. Then you'll have to do the correct deco. For a short bottom time it would be about the same deco with either gas. 100ft is a little deep for nitrox anyway.
SimonN
Guest
$30, is that all?
To get twin faber 12Ls (100cuft fabers) filled with 45% He costs AU$175 (~US$100), just for the helium.
It would be AU$117 (~US$70) for 30%.
Could explain the lack of DIR divers (who actually do any real diving) and the fact the most of the tech divers (actually doing the dives) are using CCRs here in Oz.
Wolf pack - there is anecdotal evidence that helium is less damaging to the bodies tissues than nitrogen and offgasses 'better'. This could have been translated as more bottom time for the same amount of deco?
Personally, there is no way you'd get me diving on 30/30 unless someone else was paying, I know I am no where near narced enough at 30M (100ft) on air/nitrox to miss anything worth not missing.
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