Mixed Gasses - Basic Question

[The Kraken]

OK, guys, all I wanted to know was how the N and He percentages were determined for the diving gas mixtures.

I had no intention of starting a war on decompression gasses . . .

 

[BigJetDriver]

Folks,

The subject of mixing gases breaks down to 99% physics, and 1% voodoo.

If you are talking about tri-mix, logic dictates the following order:

(1) Oxygen is the most critical component. It must be breathable at your target depth, and must not exceed a critical maximum PPO2. This leads to the realization that, if your target depth is deep enough, your mix will not posess enough oxygen to be breathable near the surface, hence, you must have another mix to utilize in that region.

(2) The next most critical component is nitrogen, both in terms of the narcosis effect, and inert gas loading computations. If you look at the narcosis effect first, you can say to your self: "Self, I know that I feel pretty alert and not "narced" at 100 fsw, so I will put in a fraction of nitrogen that will give me the same PPN2 at my target depth as if I were at 100 fsw on air.

(3) Since you now know the required fraction of oxygen, and the required fraction of nitrogen, the rest ot the mix will be helium.

The rest of the story, as Paul Harvey would say, is more complicated, and is the subject of numerous books. Suffice it to say in very general terms that (a) helium is a light gas which is "fast in, fast out" and (b) nitrogen is a heavier, slower moving gas.

There are various decompression algorithms that deal with inert gas loading and un-loading in your body in different ways. Some are more efficient in dealing with the presence of helium than others, because they were created largely for that purpose, hence some of the disparate decompression time gradients.

Hope that helps!

 

[O-ring]

Please note that msandler said "The reason you do not want to go overboard with He is, it will actually prolong Deco on shorter dives" (emphasis added).

If you search on "mass transport limit" you might find further info and perhaps an explanation by BRW.

Thanks for the reference, Charlie.. Here's one gem I unearthed from reading Bruce's post:

In the region 100 -140 fsw, and beyond air
NDLs, the use of HELITROX (enriched heliair)
is really advantageous from the point of view
of staging, repets, and hang time.

So according to Bruce, this isn't an issue except in <100' dives, where most of us aren't using helium anyway or are using lower helium mixes (30/30 for example).

Here's another good snippet:

For short,
shallow exposures, the slower diffusivity
of N2 results in less total gas buildup in bulk
tissue, and hence shorter NDLs than He.

So what the original poster meant to say was "short and shallow", not just short. Besides, with the length of hangs you are talking about in these shallow dives, one could argue that a bit less "NDL" but more He could be much more beneficial for a myriad other reasons. Basically, are we really looking at getting the best deco or getting the shortest deco?

Edit - I found another good one:

All,

Tec and rec diving come at the question of nitrox vs
helitrox vs heliox from different vantage points, as Dr D
noted in his post. It's economics, not optimality of
mixes. And availability.

But on the optimality side, consider the following.

In the 90 -140 fsw range, helitrox wins especially with
repets. Beyond that, heliox wins especially on RBs for
long(er) exposures. Trimx with high He is great, too, especially
with pure O2 washout in the shallow zones. The latter is
widespread across tec diving from all reports. So is pure
O2 in the shallow zone with deco on just about any mix
or diluent. The above are deco efficient, but maybe more
important is the fact that divers report feeling better after
He rich dives vs N2 rich dives, especially deco repets. And
caes other than single, no-deco penetrations.

 

[BigJetDriver]

OK, guys, all I wanted to know was how the N and He percentages were determined for the diving gas mixtures.

I had no intention of starting a war on decompression gasses . . .

No worries, mate! See my post above for the general guidelines.

The rest of the stuff is for the experts like BRW, and Edmonds, etc. to write books about.

Cheers!

P.S.---Do you realize that all of the moves you fling-wing guys make are "backwards"?

 

[dagodiver]

It's PPN2

Thankyou i was typing way to fast, like almost 5 words per minute.....

 

[BigJetDriver]

I had no intention of starting a war on decompression gasses . . .

Actually, if viewed as a war, the dual-phase or RGBM guys are well ahead, if for no other reason than that the other guys (Haldane, Buhlmann, etc.) are all dead!!! :11:

P.P.S.---Did you know that helicopters do not really fly? The beasts are so ugly that Mother Earth just naturally repels them!

 

[Charlie99]

Thanks for the reference, Charlie.. Here's one gem I unearthed from reading Bruce's post:

So according to Bruce, this isn't an issue except in <100' dives, where most of us aren't using helium anyway or are using lower helium mixes (30/30 for example).

Helium will have longer deco times for much deeper dives also, but only if the dive time are short.

You can see the effect on dives in the 100'-140' range easily by just plugging a few dive profiles that generate 5 or 10 minutes of deco into just about any decompression program, then changing the He fraction. Not a lot of difference between EAN32 and 30/30, but you actually pay a penalty when increasing the fraction He higher.

Mild deco in the 100-140' is a fairly common profile -- so "Helium is your friend" is a statement that, while generally is true, should not be taken too literally.

An easy way to visualize the effect is to realize that since He diffuses faster, the limiting tissue using Heliox will be about 2 or 3 times slower than for Nitrox. Slower tissues have lower M-values.

 

[novadiver]

OK, guys, all I wanted to know was how the N and He percentages were determined for the diving gas mixtures.

I had no intention of starting a war on decompression gasses . . .

No war here. get a good reliable deco planner with a mix blender softwaer and your all set to run the numbers and find the " best mix" for the job.

 

[O-ring]

Helium will have longer deco times for much deeper dives also, but only if the dive time are short.

You can see the effect on dives in the 100'-140' range easily by just plugging a few dive profiles that generate 5 or 10 minutes of deco into just about any decompression program, then changing the He fraction. Not a lot of difference between EAN32 and 30/30, but you actually pay a penalty when increasing the fraction He higher.

Mild deco in the 100-140' is a fairly common profile -- so "Helium is your friend" is a statement that, while generally is true, should not be taken too literally.

An easy way to visualize the effect is to realize that since He diffuses faster, the limiting tissue using Heliox will be about 2 or 3 times slower than for Nitrox. Slower tissues have lower M-values.

OK, ok..we are on the same page then. I must have misunderstood something earlier on.. We all love helium for various reasons and even though you may be able to shave a couple minutes here or there by not using it you wouldn't dream of it, right?

 

[BigJetDriver]

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Yo, where are ya, bud? How can I toss around anti-helicopter comments without someone to banter with???