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

[tbone1004]

Thanks @rjack321 and @tbone1004 for your informative posts.

If you use O2 at 6m will it be twice as fast to complete deco than 50%? I know this is kind of a stupid question but I have no idea.

see my post #22 in this thread. It all depends on the deco curve required for that dive and that's why I said it was a rough example of how the concepts of gradients work. There are factors such as "slow vs. fast" tissues so you have 16 different sets of pumps working with different flow restrictors in place, etc etc.

 

[KenGordon]

Hands up who uses 100% in the sea in choppy conditions?

 

[Ryan Mcshane]

Hands up who uses 100% in the sea in choppy conditions?

 

[boulderjohn]

Hands up who uses 100% in the sea in choppy conditions?

Sure. If you are down 15 feet or so, you shouldn't have any real problems with it.

A lot of people forget that if you are on pure O2, there is no real benefit to doing your final stop at 10 feet. Your PPN2 will be zero at any depth. If you have N2 in your mix, then you will want to minimize that PPN2 by staying as close to 10 feet as you can.

 

[PfcAJ]

Hands up who uses 100% in the sea in choppy conditions?

Sup

 

[seeker242]

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.

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?

 

[rjack321]

Hands up who uses 100% in the sea in choppy conditions?

Hi!

Choppy? Like surface chop? Those short wavelength "waves" don't extend down to even 15ft nevermind 20ft.

Swell? The depth of influence of even a 8ft swell is not a big deal plus here's a special hint, if you aren't hanging on the anchor line you go up and down with the swell too

 

[KenGordon]

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?

 

[boulderjohn]

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?

What is wrong with the responses?

I dive in all 4 of your situations, and I would use 100% in all of them. The only reason I know to use 80% (and I think it is valid) is if you are in a situation where you can't get enough pure O2 in your deco bottle and have to top it off with some air.

 

[rjack321]

What is wrong with the responses?

I dive in all 4 of your situations, and I would use 100% in all of them. The only reason I know to use 80% (and I think it is valid) is if you are in a situation where you can't get enough pure O2 in your deco bottle and have to top it off with some air.

Same as John except I would add

There are a few specific caves where the 20ft stops sucks and you can (and should) do most of your deco deeper. I'd bring 80% or something similar for those.