Current best practice for deco gas: 75% vs 100%

[boulderjohn]

why is 100% considered so much better when it only gives you an additional ~4 min of deco taking 80%? That seems pretty insignificant if you ask me. Unless I’m missing some other significant benefit?

Here is a benefit that I really like.

When I am doing the 20 foot stop on pure O2, it is something of a PITA. If I allow myself to go to 19 feet, my computer gets mad at me for missing my deco stop. If I go below 21 feet, my computer gets mad at me for a high PPO2. When I am done with the 20 foot stop, it is a very relaxing feeling, because there is no benefit to going much shallower than that. The PPN2 will be 0.0 at any depth, so I don't need to get near 10 feet to maximize my gradient. Where I do most of my diving, our 10 foot stops are typically about 30 minutes, and rather than hover and stare at each other that time, we usually go for a very casual swim, and it is very nice not to particularly care how deep we are during that time.

If I were on 80%, I would want to do that entire 30+ extra minutes as close to 10 foot as I could, with my computer once more getting angry with me if I went to 9 feet. I could do it deeper, but that would not maximize the gradient, and I would have to have a longer stop.

 

[TrimixToo]

You lot are useless really. I ask a question and all you can do is pedantically attack my question.

So, hands up who uses 100% in
1) flat calm conditions inland
2) the sea at all
3) in the sea with some swell.
4) in the sea without an 'anchor line'

As in, what conditions do you actually dive in while needing a strong deco gas?

All of the above if I will incur a deco obligation.

 

[rjack321]

Unrelated to the previous question but on a dive like that you are probably taking 2 deco gases. So if you take say 50% and 80%, instead of 50% and 100%, why is 100% considered so much better when it only gives you an additional ~4 min of deco taking 80%? That seems pretty insignificant if you ask me. Unless I’m missing some other significant benefit?

You're thinking of this backwards
What value does 20% N2 provide? None zero zippo, that's the gas (even on a trimix dive) that you are trying to eliminate.
And when/if you're bent you want 100% to decompress with, you might as well use it for all your shallow deco.

There are 2 additional reasons people use 80% that haven't been talked about in this thread:
1) to be contrarian and pissoff GI3 even though he stopped diving a decade ago
2) their shallow buoyancy sucks and they can't hold a 20ft stop without going to 19ft and everything flashing red on their computer

 

[TrimixToo]

Nope. Sometimes it's close, but there are plenty of other variables at play, especially once you start getting to longer depths and run times.

As an example, GF 50/80, 60m dive for 45 minutes, 18/35 bottom gas, all stops are the same until 6m.

50% calls for a 72 minute 6m stop for a 166 minute total run time.
100% calls for a 45 minute 6m stop for a 139 total run time

If O2 were twice as fast. the 6m stop would be 36 minutes.

That's just the deco, no gas, CNS, or other considerations.

100% has theoretically double the rate of transport across the blood/air membranes in the lungs than 50%. That reduces the inert gas in the blood and helps speed the transport from the tissues to the blood. But the rate of transfer from the (theoretical) tissues to the blood varies not only as a function of dissolve PPinert in the blood returning from the lungs but also as a function of that tissue's half-life. I believe that explains the difference between "twice as fast" and "not twice as fast, but still faster."

Someone please jump in to correct me if I've got this wrong...as if I'd have to ask this of others on SB (grin).

 

[PfcAJ]

Does anyone really give a **** if they’re at 19 or 21ft during the oxygen stop? Come on guys. Get it together.

 

[rjack321]

Does anyone really give a **** if they’re at 19 or 21ft during the oxygen stop? Come on guys. Get it together.

me dammit! I have an image, in my head, I am trying to uphold

 

[TrimixToo]

Does anyone really give a **** if they’re at 19 or 21ft during the oxygen stop? Come on guys. Get it together.

I once had an instructor chide me publicly for dropping all the way down to 22 or 23' for less than 20 seconds on O2. I didn't even know what to say!

Edit: He was not my instructor and I was not taking a class from him.

 

[KenGordon]

100% has theoretically double the rate of transport across the blood/air membranes in the lungs than 50%. That reduces the inert gas in the blood and helps speed the transport from the tissues to the blood. But the rate of transfer from the (theoretical) tissues to the blood varies not only as a function of dissolve PPinert in the blood returning from the lungs but also as a function of that tissue's half-life. I believe that explains the difference between "twice as fast" and "not twice as fast, but still faster."

Someone please jump in to correct me if I've got this wrong...as if I'd have to ask this of others on SB (grin).

No, this is not right. What matters (if you believe Buhlmann, Haldane etc) is the difference between the ppInert in the blood and in the lungs. The ppInert in the tissues will be something like 3 at the last stop at 6m. With o2 the lungs will be at ppInert of 0 so the delta is 3, with 50% it will be 0.8 so the delta is 2.2. So to begin with the advantage is only about 25%, as time goes on that advantage gets bigger, but eventually yo surface before it gets to 100% (ie twice as fast) due to double the delta. (I am probably wrong about the 3, but it still applies even at say 2, when the delta is 2 vs 1.2, so faster but not twice as fast)

 

[KenGordon]

Here is a benefit that I really like.

When I am doing the 20 foot stop on pure O2, it is something of a PITA. If I allow myself to go to 19 feet, my computer gets mad at me for missing my deco stop. If I go below 21 feet, my computer gets mad at me for a high PPO2. When I am done with the 20 foot stop, it is a very relaxing feeling, because there is no benefit to going much shallower than that. The PPN2 will be 0.0 at any depth, so I don't need to get near 10 feet to maximize my gradient. Where I do most of my diving, our 10 foot stops are typically about 30 minutes, and rather than hover and stare at each other that time, we usually go for a very casual swim, and it is very nice not to particularly care how deep we are during that time.

If I were on 80%, I would want to do that entire 30+ extra minutes as close to 10 foot as I could, with my computer once more getting angry with me if I went to 9 feet. I could do it deeper, but that would not maximize the gradient, and I would have to have a longer stop.

You’d only get one minute extra deco for doing your 80% final stop at 6m compared with doing it at 3m. This is because inspired ppN2 is only 0.32 is 0.26 vs a much higher tissue ppN2.

And you could look at stuff at 10m too.

 

[KenGordon]

2) their shallow buoyancy sucks and they can't hold a 20ft stop without going to 19ft and everything flashing red on their computer

This is my reason. Best to be realistic.