80% or O2 DECO

[wedivebc]

I think there would be a lot of people who would disagree with this statement.

Explain to us why it's a non factor. Maybe we can use the more appropriate term "partial pressure vacancy" while discussing it's merit or lack of.

I'm all ears.

I too have asked this question and every time the answer comes back to GIs rant to BAUE about oxygen window. The fact that increasing the O2 PP to the maximum amount that can be safely tolerated by a diver is nothing new or miraculous. It benefits the diver by reducing the N2 in the breathing mix and also increasing the N2 gradient (partial pressure vacancy) to the max using a nitrogen 'vacant' gas. The question I want to know the answer to from a thinking diver, not a GI sock puppet is why is 80% at 30ft any less valuable than 50% at 70ft or any other gas breathed at it's highest acceptable ppo2? There seems to be the idea that GUE standard gasses open some magic window that allow some special process to take place.
The principal is pretty standard in all deco schedules regardless of whether the words oxygen window are asssociated with them or not, reduce N2 reduces inert gas intake during deeper stops and increases inert gas gradient to promote offgassing.
Is there more to it than that?

 

[onfloat]

Isn't the point of standard gases so everything is the same? Creating standard proceedures? So all the divers in the water on the same page and not having divers on different mixes with different schedules in the water at the same time. And for big projects ( i.e. WKPP) the ease of preping everything for dives without being confused about who is using what tanks.

 

[do it easy]

I think the oxygen window is created by the asymmetry with how the body handles O₂ and CO₂ that is different than how it handles inert gasses like He and N₂.

At any rate, it is my opinion that the oxygen window is a misplaced focus- maximizing the oxygen window is not the direct cause of efficient off-gassing, but a side effect of it. In other words, I think, that maximizing (inspired) O₂ enlarges the oxygen window, not the other way around.

from Johnny E. Brian, Jr., M.D. Gas Exchange, Partial Pressure Gradients, and the Oxygen Window, page 12

Although enlarging the oxygen window may not directly affect tissue gas removal, it does directly affect tissue on-gassing during decompression, which affects the amount of time required to decompress the tissue.

Regarding partial pressure vacancies in the venous blood:
from Johnny E. Brian, Jr., M.D. Gas Exchange, Partial Pressure Gradients, and the Oxygen Window, page 13

Although no direct experimental data exists on this topic, oxygen breathing may limit venous blood supersaturation, prevent venous bubble formation, and thus speed tissue gas removal.

Again, I see this as a side effect of maximizing inspired O₂ and minimizing inspired inert gasses.

This is my interpretation of it...

 

[ageddiver]

Isn't the point of standard gases so everything is the same? Creating standard proceedures? So all the divers in the water on the same page and not having divers on different mixes with different schedules in the water at the same time. And for big projects ( i.e. WKPP) the ease of preping everything for dives without being confused about who is using what tanks.

Yes, exactly. This is what it's all about for OC divers - compromise. The more "perfect" gradient is only practical with rebreathers. When diving OC, it's always a compromise between the "right" gases for the dive and the real practicality of getting the team in sync. and for mixing gases / cutting tables. My own personal experience has resulted in what I can only call the 50ft. change. For dives to 200ft. I have no problems with 50% and 100%. From 200 to 250ft. I feel much better after the dive if I use 36/10 and 100%. From 250 to 300ft. I use 3 gases - 32/20, 50% and 100%. BT makes no difference. The changes were made to wean me off my bottom gas relatively early in the ascent, get me out of the water in a reasonable time, keep a "standard" set of mixes, and just make me feel better after the dive (deco is far from an exact science, and you've got to listen to your body).

Back to the original question - why might 80% be better? Look at the CNS after a long deep dive with extended stays in and around 20ft. using 100% and compare against 80% - that might make a difference if you were planning to do another in 4 hours. Just another reason why you might want to keep an open mind depending on the situation.

As an aside, even if team members use slightly different deco gases, there's little separation on the line coming up - not enough IMO to sacrifice safety. I think it's actually a bit of an advantage since we stagger RT's slightly and don't have divers getting on the boat at exactly at the same time.

 

[limeyx]

Are you just joking or did you really hear this?

It sure doesn't make sense to me since this would mean you would be (over)extending intermediate stops. I thought the whole concept behind a deco gas (exclude 100% bottle) and the S-curve was to utilize the oxygen window at the deepest part of the deco gas range where it is effective and to push the gradient at the lower stops. All one is doing if they extend the intermediate stops is loading of slow tissues. There seems to be no benefit to this.

Really heard it, but in a place where I have agreed not to repost from.
From people more qualified than me to discuss it also.

 

[limeyx]

I too have asked this question and every time the answer comes back to GIs rant to BAUE about oxygen window. The fact that increasing the O2 PP to the maximum amount that can be safely tolerated by a diver is nothing new or miraculous. It benefits the diver by reducing the N2 in the breathing mix and also increasing the N2 gradient (partial pressure vacancy) to the max using a nitrogen 'vacant' gas. The question I want to know the answer to from a thinking diver, not a GI sock puppet is why is 80% at 30ft any less valuable than 50% at 70ft or any other gas breathed at it's highest acceptable ppo2? There seems to be the idea that GUE standard gasses open some magic window that allow some special process to take place.
The principal is pretty standard in all deco schedules regardless of whether the words oxygen window are asssociated with them or not, reduce N2 reduces inert gas intake during deeper stops and increases inert gas gradient to promote offgassing.
Is there more to it than that?

One question to ask regarding the 80% @ 30 feet would be how long you spend at 30 feet (and I dont really know the answer to this)

with 100% you spend a long time at PPO2 of 1.6 (the 10 foot + 20 foot stop)
If you use 80% then the benefit of 1.6PP O2 is lost as soon as you move to the 20 foot stop.

There was more to the O2 window to that in our Tech 1 class. However, how much of it is medically proven/understood by me might reduce that somewhat
(something to do with haemogobin, buses, milk trucks and plasma)

 

[JeffG]

nt...

 

[rjack321]

Are you just joking or did you really hear this?

It sure doesn't make sense to me since this would mean you would be (over)extending intermediate stops. I thought the whole concept behind a deco gas (exclude 100% bottle) and the S-curve was to utilize the oxygen window at the deepest part of the deco gas range where it is effective and to push the gradient at the lower stops. All one is doing if they extend the intermediate stops is loading of slow tissues. There seems to be no benefit to this.

I have heard the same thing, the premise being that doing a linear ascent (i.e. from 70 to 30) is simpler than reshaping. I have done linear acsents in that depth range on 50% and noticed zero difference from a S-shaped deco.

 

[Doc Intrepid]

The question I want to know the answer to from a thinking diver, not a GI sock puppet is why is 80% at 30ft any less valuable than 50% at 70ft or any other gas breathed at it's highest acceptable ppo2?

80% is theoretically less useful than 100% due to the efficiency of offgassing gas held in solution. Note that 30 fsw has little to do with it. While ppo2 1.6 is achieved with 80% at 30 fsw, and earlier than on 100%, the point is that to release gas held in solution it is necessary to ascend.

The rate at which molecular gas transfusion occurs across a semipermiable membrane is driven (primarily - forgive my simplification) by relative molecular 'density' on both sides of the semipermiable membrane. There are two factors that account for a great deal of that relative 'density'. These are the ambient pressure, which determines the rate at which gas comes out of solution or tissues; and the gas being breathed - which determines how much N2 exists on one side of the semipermiable membrane.

In order to theoretically achieve the "cleanest decompression possible", one would want to reduce N2 to zero on one side of the semipermiable membrane, while reducing ambient atmospheric pressure at a rate that maintains as high a molecular density on the other side of the membrane as possible without actually incurring DCI. This is the "gradient" referred to in the RGBM model. The magnitude of the variance between the two sides of the membrane determines how 'steep' the gradient is. The greater the magnitude of the variance, the more rapid and 'complete' the offgassing, or movement of gas molecules from one side to the other.

The former is accomplished by breathing a gas without any N2 in it. The latter is accomplished by controlling rate of ascent, and in particular, ascent during the 50% reduction from 2 ATA to 1 ATA (or from ~33 fsw to the surface). As the diver ascends, gas continuously comes out of solution - too slow an ascent is as unhelpful with respect to cleanest decompression as too fast.

(Thus, breathing 80% at 30 fsw with ppo2 1.6 would be valuable at that depth. To achieve "cleanest decompression", however, one would eventually need to ascend to release gas held in solution. At which point an 80% deco gas would no longer provide an optimal gradient...)

In other words, to optimize efficiency of offgassing of what Weinke refers to as "a critical volume of bubbles or separated gas" (2003, p.274) it is essential that the diver maintain N2 coming out of solution at high volume (on the arterial side of the membrane) and the N2 level at lowest volume in the lungs (which would result from breathing a gas that does not contain N2).

Note that I've mentioned the word "theoretical" a number of times.....

While AFAIK O2 does indeed provide a cleaner decompression in a shorter time, it is not without it's drawbacks.

Most software, to include V-Planner and D-Plan, will yield runtime increases of less than 4-5 minutes overall resulting from the use of 80% instead of O2 (for the depths and runtimes that the majority of SB divers would be doing. Very few of us, if any, are routinely going "downtown" with George...).

Use whatever you want to use. In the 1980's I routinely decompressed on air using Canadian military decompression tables when diving in Lake Superior. Astonishingly, I continue to live. Very few people will give a crap what you use to decompress with.

The single exception is if you are diving with a team. If you are diving with a team it makes a great deal of sense, logistical and otherwise, to all use precisely the same gasses throughout the dives. If you find this problematic, then don't dive with the team.

Hope this helps.

Doc

P.S. The above generally comes from Weinke, B.R. "Basic Decompression Theory and Application" Best Publishing 2003, in particular, pages 266-275. Any mistakes in comprehension are naturally my own. I recommend that anyone interested buy the book and read it. Even if you are not into the math, just by carefully reading the text you can pick up what Weinke is saying. While Weinke is not without his critics, AFAIK he represents cutting edge in terms of explicating theoretical behaviors of gas in solution, bubble formation, and the process of on-gassing and off-gassing while diving. FWIW. YMMV.

 

[karstdvr]

Interesting discussion that I knew would be charged with emotion. I had a similar discussion this past weekend with some of my Missouri cave diving brethern. They use an 80/20 or 70/30 because that can be the difference in decoing in warmer (relative term) spring water versus very cold lake water. I've used 80/20 in a cave where the restricitions are so tight and flow so strong that it would be crazy to deco at 10 or 20',but 30' feet is a good location with more room and less flow.Sometimes you have to do what makes sense.