RumBum:
Okay, I am not trying to learn how to dive mixes from this thread, but I totally find it interesting. I like the physics of diving, but I have no desire to go deep enough to use a trimix or heliox. I probably should read some books on it, but I find technical reading very difficult.
The original question for this thread was
caribou:
at recreational depth (say 120 feet), if we were to breath 21% oxygen, 79% helium (or any other inert gas, excluding nitrogen), asides from the problem of getting such a fill, would it eliminate NDL ?
Or would we see the same problem, i.e. the helium goes into solution in the bloodstream, and creates bubbles as the pressure decreases, prompting the need to create tables again...
if not then, since 21% oxygen isn't toxic at this depth (120 feet), what else could prevent us from staying down as long as the gas supply allows and then ascend slowly back up.
so (beginner disclaimer) the answer would be YES there are mixes of other types of gas that you can breath for diving. It not reasonable to dive helium at a depth of 120 feet due to the long deco time you would incur for a very brief dive.
Ok.... the answer to the first question, "would it eliminate NDL ?" is no.
The biggest reason to not use Heliox at recreational depths is more that the cost-to-benefit doesn't compute, as well as the impact on decompression time. As an example, using one currently popular decompression algorithm, a 20 minute bottom time at 120 feet on air would require stops totalling 17 minutes. Using Heliox at 21% Oxygen would add 12 more minutes - a total of 29.
But... let's look quickly at some other alternatives, both cheaper than Heliox-
With a PO2 max of 1.4 ATA, your best mix for such a dive using Nitrox would be EAN 30, and your required deco obligation would be 5 minutes.
And for the "best of all worlds" we could reduce our narcotic depth to about 75 feet (using the N2+O2 method)
and reduce our deco requirement to 2 minutes (a normal safety stop would suffice) by using 30/30 Trimix (That's "Triox" for you NAUI trained folks).
RumBum:
So helium has faster on-gassing and slower off-gassing properties than nitrogen?
The "faster ongassing" common knowledge is, even as we type, undergoing research and revision, with the latest research showing some fascinating counter-intuitive results. The jury's still out but it
may be that we'll have to do some major rethinking on Helium on and off-gas rates.
RumBum:
Nitrox mix, however does extend your NDL. Is there a table that shows the speed at which different gasses build up in tissue?
There are
theoretical tissue models from which tables can be produced, but every real cell in a real living organism will have its own rate on any given day under any given set of circumstances - so a table isn't practical for real live tissue(s).
RumBum:
Ox-tox is not an issue at recreational limits as long as you are breathing 21% O2; but you can survive with as low as 12%, right?
At one atmosphere.. so you're really asking about survival at .12 ATA oxygen. The answer is "highly variable" - and there's a difference in survival and functioning. Most folks can function well down to about .16 with moderate exertion.
RumBum:
I was just now trying to extrapolate the formula for oxygen partial pressure at depth, but got myself confused.
Let's keep it simple. PPO2 is the fraction of oxygen (FO2) times the actual pressure in atmospheres (ATA). In feet, you get atmospheres with depth/33 + 1. So at 99 feet you have 99/33 +1 or 4 ATA, and with air you have PPO2 of .21X4=.84
RumBum:
I am probably trying to get too technical again for my current knowlege level. Like I said before, though, I just think this stuff is really interesting. I think I am ready to take Nitrox classes now.
It
is fascinating. Have fun with it.
Rick
(Make mine an Appleton's)
