Helium Fraction and Standardized Gases

[Kevrumbo]

What's the deepest deco gas you would use on a 300 foot dive?

Depends on the exposure. 120 or 190 deco gas (35/25 or 21/35).

Generally 21/35, I tend not to use 35/25 until the exposure is long enough that the 36-24m stops get longer than about 3-4 mins.

Or if I'm in a cave where the profile requires more gas in that region.

HTH
John

Why do you use an intermediate deco gas that has a higher fN2 than your bottom mix (12/60 or 10/70 trimix in this case, dive to 90m/300')?

Think about it! The much better & consistent strategy is to utilize deco gases that titrate down, or at least hold the fraction of Nitrogen nearly constant (i.e. no significant fN2 increases as you ascend through the deco stops); that means using a "best mix" deco blend over standard mix.

Here's an excerpt of a deco gas planning write-up for some of the deeper longer cave dives of the WKPP (utilizing a dry habitat for Oxygen deco):

A trimix of 10.5 percent oxygen/ 80 percent helium was selected owing to the average bottom depth of 280'/85m. Considerations in this selection were:

Since many tissue compartments will reach saturation and decompression will take longer than a few hours, the high helium content has advantages for off-gassing effficiently later in the dive. The amount of time helium takes to reduce its partial pressures in tissues by one-half are about 2.7 times faster than the half-times for nitrogen. . .

As decompressions times lengthen to two and a half hours or more, counterdiffusion of excessive amounts of nitrogen can become a real problem. It can have the effect of doing a deep air dive in the middle of decompression. As shallower stops are made near the end of deco, the diver's body can be loaded with enough nitrogen that it offsets any advantages gained in eliminating helium. Because of nitrogen's greater molecular weight, greater solubility in body tissues and slower half-times, it can take longer and be more difficult to eliminate than helium. This is a special concern at the final deco stop where oxygen is used to remove inert gas from the slowest tissue compartments. . .

[Non-standard, intermediate] decompression mixes that achieve an acceptable balance of these factors are a trimix of 19 percent oxygen / 50 percent helium at 240'/73m; trimix 25 / 35 at 190'/58m; trimix 35 / 25 at 120'/36m; trimix 50 / 15 at 70'/21m; 100 percent oxygen at 28'/8.6m [in a dry habitat], with periodic breaks using trimix 15 / 45.

This selection allows the fraction of helium to gradually taper off while the fraction of oxygen gradually increases and the fraction of nitrogen remains nearly constant. Helium off-gases efficiently with the reduction in pressure and the increasing oxygen fractions. Nitrogen loading during deco is kept below target limits upon arrival at the [oxygen] dry habitat stop. . .

From Erik C. Baker, Decompression Strategies Enable Deep, Long Explorations of Wakulla Springs, Immersed Magazine p.30, Fall 1999.
See also Erik Baker and the Varying Permeability Model: Technical VPM Publications

Study and also refer to this thread below:
http://www.scubaboard.com/forums/ask-dr-decompression/366427-inner-ear-dcs.html#post5682237

 

[Kevrumbo]

Ironically, I chose to post my 300 foot dive question in the DIR forum because my (limited) exposure to DIR led me to believe that in it there is a very strong emphasis on standardization, therefore there would be one single standardized response to my question. It is reassuring to see that, even in standardization, there is room for personal judgement. I am therefore surprised to see (in this forum) arguments in favour of best mix rather than standard mix...

Decompression Strategies Enable Deep, Long Explorations of Wakulla Springs
(Jarrod Jablonski in consultation witn Erik C. Baker)

Granted, this is a very specialized case of deep and long saturation/exposure. Most of the time though, for Tech 1 and 2 dives, standard GUE mixes work well. But once you start approaching these extreme depths, you've got to start seriously considering these issues like inert helium and nitrogen loading & off-gassing rates in your deco gas strategy. . .

 

[lamont]

My $0.02:

Having read that article I think it needs to be read in the context of the prevailing ideas in the 90s being that helium was scary and that you should get completely off of it as soon as possible and switch to 21% as a deep deco gas. This is bad for multiple reasons, including IBCD inner ear DCS risk. The use of 21/35 and 35/25 standardized gases is consistent with the paper. If you want to do perfectly accurate math and pick a slightly different fHe for your 190 or 120 gas then as long as its in the direction of more helium, its still a standardized gas and is not best mix (but the results will probably not be as useful as the switch from air to trimix at those depths). I certainly don't see any way that this paper can be extrapolated into a statement that JJ believes in "best mix".

 

[Kevrumbo]

No Lamont! It's a simple quantitative arithmetic discrepancy with regards to Decompression Strategy:

The gist of it is --that if you're trying to off-gas Nitrogen loading from your bottom mix, why are you switching to a intermediate "standardized deco gas" with significantly more Nitrogen than your bottom mix??? (i.g. 10/70 bottom mix has fN2 of 20%; and you switch to a 21/35 intermediate deco mix that has a significantly greater fN2 of 44%???)

The article goes on to explain and implies why this is a bad practice --albeit the article's context is for an extreme depth & exposure. The principle behind it IMHO though is valid & applicable, and begs the above question.

HELIUM "BEING SCARY" HAS NOTHING TO DO WITH THE QUESTION OR THE PREMISE OF THE ARTICLE!!!

(btw, Moderator's mistake on the thread split-off Title --Should have been, "Nitrogen Fraction in GUE Standardized Intermediate Deco Mixes. . ."

 

[danvolker]

No Lamont! It's a simple quantitative arithmetic discrepancy with regards to Decompression Strategy:

The gist of it is --that if you're trying to off-gas Nitrogen loading from your bottom mix, why are you switching to a intermediate "standardized deco gas" with significantly more Nitrogen than your bottom mix??? (i.g. 10/70 bottom mix has fN2 of 20%; and you switch to a 21/35 intermediate deco mix that has a significantly greater fN2 of 44%???)

The article goes on to explain and implies why this is a bad practice --albeit the article's context is for an extreme depth & exposure. The principle behind it IMHO though is valid & applicable, and begs the above question.

HELIUM "BEING SCARY" HAS NOTHING TO DO WITH THE QUESTION OR THE PREMISE OF THE ARTICLE!!!

(btw, Moderator's mistake on the thread split-off Title --Should have been, "Nitrogen Fraction in GUE Standardized Intermediate Deco Mixes. . ."

If you guys don't mind, let me "turn" this thread a little.....I did hundreds of dives with George Irvine and Bill Mee in the late 90's, depth range 260 to 290, in ocean.
Different parameters than cave diving for sure...you CAN NOT plan to have more bottles waiting for you, due to the strong potential for currents to blow you off of any potential ascent line ( which we would rarely use in our own diving), it is nice to have safety divers, and smart practice to have them check on you at the 50 foot stop, but you plan the dive assuming you will be on your own till back at the surface.
As the ocean current will be the biggest challenge on getting to the wreck or reef feature, and in moving around on the target area, being slick in the water is paramount. We would use double 80's with a high helium mix -- if memory serves, it was usually 50% He, and O2 would have been either 10 or 15% ( also if memory serves...I will check with Bill on this next time I see him--whatever George and Bill were using, I would just be happy to use it, and no matter how radical the profile, I never once had even the tiniest DCS event or anything)..... And we would have a 30 cu foot or 40 cu ft bottle clipped stage style with 100% O2.
We would always breathe backgas up to our final stop for O2. We would never want an intermediate mix requiring another high drag tank to drastically interfere with the the need to be as slick as possible thorughout the dive.

I'll say this again...if you want to be good at challenging deep ocean dives, you need to be slick in the water. The best dives I have ever been on, are high current dives, where the fish are thick over the structure, and the current concentrates them. You will not see me with a travel mix, ever

One of George's big concerns was that if someone was ever injured on a 280 foot ocean dive, that we would need to be able to blow off most of the deco in getting them to the surface--if needed. Similarly, if we had done our standard 25 min bottom time, and realized a huge storm was coming up, we would not be "deco weanies"....so shape of deco could be altered, and accelerated. This occured a few times, as with the body recovery we did in the student tech diving death of Jame Orenstein....where we did less 10 minutes of deco time after 280 for about 12 minutes, and with zero complications....This just pointing to shape of deco profile, and use of just the two mixes ( bottom mix or 100% O2) as being excellent for typical ocean tech diving depths and durations.

We never believed in doing 30 and 40 minute bottom times at 280 feet, because this would create a scenario where you could no longer blow off huge amounts of deco time in an emergency or dangerous weather shift, so we would not do this...And this is meaningful in choice of mixes and volume to bring as well.

 

[Jonny boy]

If you guys don't mind, let me "turn" this thread a little.....I did hundreds of dives with George Irvine and Bill Mee in the late 90's, depth range 260 to 290, in ocean.
Different parameters than cave diving for sure...you CAN NOT plan to have more bottles waiting for you, due to the strong potential for currents to blow you off of any potential ascent line ( which we would rarely use in our own diving), it is nice to have safety divers, and smart practice to have them check on you at the 50 foot stop, but you plan the dive assuming you will be on your own till back at the surface.
As the ocean current will be the biggest challenge on getting to the wreck or reef feature, and in moving around on the target area, being slick in the water is paramount. We would use double 80's with a high helium mix -- if memory serves, it was usually 50% He, and O2 would have been either 10 or 15% ( also if memory serves...I will check with Bill on this next time I see him--whatever George and Bill were using, I would just be happy to use it, and no matter how radical the profile, I never once had even the tiniest DCS event or anything)..... And we would have a 30 cu foot or 40 cu ft bottle clipped stage style with 100% O2.
We would always breathe backgas up to our final stop for O2. We would never want an intermediate mix requiring another high drag tank to drastically interfere with the the need to be as slick as possible thorughout the dive.

I'll say this again...if you want to be good at challenging deep ocean dives, you need to be slick in the water. The best dives I have ever been on, are high current dives, where the fish are thick over the structure, and the current concentrates them. You will not see me with a travel mix, ever

One of George's big concerns was that if someone was ever injured on a 280 foot ocean dive, that we would need to be able to blow off most of the deco in getting them to the surface--if needed. Similarly, if we had done our standard 25 min bottom time, and realized a huge storm was coming up, we would not be "deco weanies"....so shape of deco could be altered, and accelerated. This occured a few times, as with the body recovery we did in the student tech diving death of Jame Orenstein....where we did less 10 minutes of deco time after 280 for about 12 minutes, and with zero complications....This just pointing to shape of deco profile, and use of just the two mixes ( bottom mix or 100% O2) as being excellent for typical ocean tech diving depths and durations.

We never believed in doing 30 and 40 minute bottom times at 280 feet, because this would create a scenario where you could no longer blow off huge amounts of deco time in an emergency or dangerous weather shift, so we would not do this...And this is meaningful in choice of mixes and volume to bring as well.

That, quite frankly, is scary

 

[danvolker]

That, quite frankly, is scary

Care to elaborate?

 

[Jonny boy]

Care to elaborate?

Happy to....
It seems that you are giving your tacit approval to a dive of 280 ft (in my world that's about 85m) with double 80's and a 30 cuft bottle of 02.
Now, i would assume that you guys were at the top of your game physically, even so your breathing rate be somewhere around 15 litres per minute? Without knowing your exact deco profiles it is difficult to make an accurate assumption on gas consumption, but your bottom time alone will have consumed something like 2000 litres. AL80's can, generally, only be pumped to 207 bar thus giving you about 4500 litres of available gas - half of which you have used on the bottom and the other half has to last you until you can get onto the 02 at 6m. Doesn't seem to be alot of reserve?
Obviously it all worked out well for you as you noted that you never even got a suggestion of a hit, however i would question whether modern thinking around minimum gas, team gas and deco theory would have this as a suitable profile?
My initial comment about it being scary was not meant to be detrimental, simply of statement of my response upon reading your post.

 

[RTodd]

Dan will have better explanations about the slightly multi-level nature of the dive profiles, etc. but you are also making incorrect assumptions about the fill pressures of George's tanks.

 

[Slamfire]

(...I never once had even the tiniest DCS event or anything)...

Does this mean you've never had a DCS hit ever?