If you had to choose, 80% or 100% for deco gas and why.

[doctormike]

But I do appreciate the stylistic advice, Dan..! As someone new to the world of tech diving, I want to look good...

That reminds me... what mix should I have in my Spare Air? Will I need a second one if I do trimix training?

 

[lowviz]

Now don't be busting on Dan V. You should feel honored that you made it to "Rule 6".

I, on the other hand, almost made it to Rule 2. *once* Team foxturd: DIR Rules. Explained

 

[Akimbo]

True statement. Is that your only consideration?

Given that a PPO2 of 2.0 was the upper limit most of my life, I find the concern over depth variation due to swells a gross overreaction. Just pop a SMB to ride the swell if the seas come up. I would do that more to save my ears than concern over PPO2 peaking.

Same with the “CNS clock”. The current standard is a reasonable and very conservative guideline for recreational divers, who are making the dive for entertainment. However, I consider staying within these guidelines far less of a risk to my health than getting bent.

You have to look at the big picture though. I would never make a dive on SCUBA (the acronym) that required enough decompression for the CNS clock to become a major concern — especially without a chamber onboard. IMHO, that is a job for surface supplied divers.

 

[PfcAJ]

Even if one were to accept your less than expert opinion on the utility of CNS for a given dive, there's the question of loading over a period of repetitive deco diving. You guys KFTC, though. Maybe you could post the GI3 Baker's Dozen​ for us, just to round out the topic?

Ask and you shall receive.
Baker's Dozen 80/20

 

[Dr. Lecter]

Cleaner is cleaner. 100% works for this, 80% is defective for this intent.

While I should know better than to ask for a rational discussion of this, I'm not going to let this ipse dixit just slide by without you backing it up. The fact that 100% works or that there is 20% N2 in 80 vs. 0% N2 in 100 isn't what I'm questioning - it's whether the relative efficiency has any significant difference. Having personally used both mixes on similar OC dives in the 200-300 range, I've never noticed any difference in how I felt afterwards. With 100 I took air breaks, had a higher CNS, and got out a few minutes sooner; with 80, fewer/no air breaks, lower CNS, got out a few minutes later.

The only chart in a textbook that I've ever seen shows surprisingly few differences among EAN mixes in terms of relative deco efficiency. So, where's your evidence 80% is significantly less effective than 100%?

 

[Ste Wart]

100%

Otherwise the sticker on my O2 bottle would be wrong

 

[PfcAJ]

While I should know better than to ask for a rational discussion of this, I'm not going to let this ipse dixit just slide by without you backing it up. The fact that 100% works or that there is 20% N2 in 80 vs. 0% N2 in 100 isn't what I'm questioning - it's whether the relative efficiency has any significant difference. Having personally used both mixes on similar OC dives in the 200-300 range, I've never noticed any difference in how I felt afterwards. With 100 I took air breaks, had a higher CNS, and got out a few minutes sooner; with 80, fewer/no air breaks, lower CNS, got out a few minutes later.

The only chart in a textbook that I've ever seen shows surprisingly few differences among EAN mixes in terms of relative deco efficiency. So, where's your evidence 80% is significantly less effective than 100%?

So if we re-write your post and delete the CNS argument (since its demonstrably bogus), it would read "With 100 I took air breaks and got out a few minutes sooner; with 80, fewer/no air breaks, got out a few minutes later."

That's why 100% is better, plus some other reasons.

 

[danvolker]

While I should know better than to ask for a rational discussion of this, I'm not going to let this ipse dixit just slide by without you backing it up. The fact that 100% works or that there is 20% N2 in 80 vs. 0% N2 in 100 isn't what I'm questioning - it's whether the relative efficiency has any significant difference. Having personally used both mixes on similar OC dives in the 200-300 range, I've never noticed any difference in how I felt afterwards. With 100 I took air breaks, had a higher CNS, and got out a few minutes sooner; with 80, fewer/no air breaks, lower CNS, got out a few minutes later.

The only chart in a textbook that I've ever seen shows surprisingly few differences among EAN mixes in terms of relative deco efficiency. So, where's your evidence 80% is significantly less effective than 100%?

The place where you are going to see the big differences in literature, are use of 80/20 versus 100% for extra surface deco.....
As to whether you could "plan" on prophylactic surface O2 being supplied by the boat, without a DAN or Coast Guard report being filed...or without the Captain getting annoyed.....you guys might want to ask some boat captains what they would do if a tech diver on their boat wanted to suck some of the boat's O2 as an extra safety measure--either for last dive, or for an up-coming successive dive...My guess is that most boats will not give the answer most of you guys want

 

[lowviz]

... My guess is that most boats will not give the answer most of you guys want

C'mon, 80% works just peaches for up to 20 minutes of deco. I'll even throw in an extra five for good luck.

 

[Dr. Lecter]

The place where you are going to see the big differences in literature, are use of 80/20 versus 100% for extra surface deco.....
As to whether you could "plan" on prophylactic surface O2 being supplied by the boat, without a DAN or Coast Guard report being filed...or without the Captain getting annoyed.....you guys might want to ask some boat captains what they would do if a tech diver on their boat wanted to suck some of the boat's O2 as an extra safety measure--either for last dive, or for an up-coming successive dive...My guess is that most boats will not give the answer most of you guys want

That's what the HP149 of O2 is for, Dan. No need to carry the damn thing with you, though.

Since AJ and I (and any peer-reviewed scientific paper of which I'm aware) have different understandings of the word "demonstrably", I'll just let this drop. Though maybe I should ask if the DIR distaste for high ppO2 and END isn't also bogus, since it's "demonstrably" not a problem for plenty of us who dive deep air?