No Technical Training for Me.

[Rick Murchison]

There are a number of reasons why 80% is a lousy choice. The biggest being that 80% is used "like" O2, except that it has N2 in it.

No, 80% is used like 80%, just as 50% is used like 50%.

N2 is what you are trying to get rid of. Additionally, it is used starting at 30 ft, which isn't even a PO2 of 1.6, so you aren't getting full benefit from the O2 window.

So what? at 30ft it's better than either 50% or 100%, isn't it?

At the 20ft and 10 ft stops on 80%, which is the most critical phase of the dive, the PO2 is roughly useless for decompression (1.2 and 1.0). The O2 window is always wide open regardless of depth since there is no inert gas.

Useless? You guys slay me with your absolutist, meaningless and misleading terminology - just see your very next statement

Ask yourself...why do we breathe 100% O2 to treat a DCS hit?

Let me guess, because the PO₂ of 1.0 is "roughly useless for decompression?" You said it, so it must be so! The fact is that the main driving force for inert gas elimination is the inert gas gradient between the body and the inspired gas. Going to 80% at 30ft is better than staying on bottom gas. It even opens up the "oxygen window" earlier.

There is no argument for using 80% that has any merit. It is an antique concept.

There is no argument if you have both 50% and 100% available, true. But... if you have only a single bottle, 80% will get you out of the water sooner, and that's a very good argument for it indeed.
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Just in case y'all haven't noticed, our friend Soggy fills his arguments with innuendo and implication; terms like "lousy choice," "no argument," "antique concept" and "knowledgeable people" are there to keep you from thinking; from discussing the data and the facts, by implying that if you don't agree then you must be outdated or not knowledgeable or otherwise "strokey."
Don't buy it!
Run your own numbers; make your own decisions. There are sound reasons to use almost any gas or combination of gasses under the circumstances that favor them. In this specific case, if you're stuck with a single deco gas and your choices are either 80% or 100%, then 80% will often get you safely out of the water sooner.
If you want to use 100% that's fine, and I sure don't have anything against it.
I will often choose 80% for myself.
Rick

 

[Thalassamania]

There's an optimum mix for every depth and an optimum depth for every mix.

 

[Soggy]

The situations he offered were those where there are considerable waves or swells ... or a deco along walls or pinnacles where downcurrents are common.

How is a downcurrent on 80% at 30ft is safer than on O2 at 20ft?! This sounds suspiciously like the buoyancy argument, which is a bunch of BS.

Beyond the increased deco time, what are the disadvantages?

What are the advantages?

I never said it was a safety issue. I just said it was a dumb choice and I cited reasons why.

 

[Soggy]

Just in case y'all haven't noticed, our friend Soggy fills his arguments with innuendo and implication;

Actually, I *want* people to think about it because it's obvious that many people do not. You can dive whatever you want. I still think 80% is a silly choice.

There is no argument if you have both 50% and 100% available, true. But... if you have only a single bottle, 80% will get you out of the water sooner, and that's a very good argument for it indeed.

Yeah. I guess if you do a dive for 60 minutes to 120' on 30/30 and only have one gas available for deco, 80% will get you out 1 minute faster than 100%.

 

[Soggy]

There's an optimum mix for every depth and an optimum depth for every mix.

Yes, that's true. Unfortunately, in the process of diving best mix on everything, you lose all the advantages of standardized gas mixes. I know one guy who will dive 25% to 160', and uses something like 67% for deco, because he's figured out that for one specific dive, that specific mix will get him out of the water 2 minutes faster.

 

[Thalassamania]

Yes, that's true. Unfortunately, in the process of diving best mix on everything, you lose all the advantages of standardized gas mixes.

My point is that "standardization" is in the eye of the beholder. Who died and left GUE to decide what's standard and what's not?

For my own diving I agree with you about 80%, I have no use for it (but then I was diving 100% at 30 feet all the time, way back when), but I can see where others might and I don't feel the need to find a denegrating reason to explain their choice of "standard."

 

[TSandM]

I read the original post, and my first reaction was to feel extremely empathetic toward the student. Maintaining buoyancy control in doubles under significant task loading is NOT easy for some of us. The IANTD tech instructor who was out on the boat with us yesterday said, "Bag shoots . . . One of those things you can do perfectly 99 times, and the hundredth it's a complete disaster."

I am very lucky to have had access (and to continue to have access) to classes which are designed to introduce and polish these sorts of skills at depths and in situations where errors aren't likely to be lethal (or even injurious in some classes). I think the path I've chosen, taking the road to technical diving (if I ever get there) in bite-sized pieces, would be an excellent strategy for a great many people.

In this case, the instruction and instructional plan seems faulty, but I would expect a student who is ready to do this kind of training to have done the reading and study required to recognize those faults. I obviously expect too much of people

 

[Soggy]

My point is that "standardization" is in the eye of the beholder. Who died and left GUE to decide what's standard and what's not?

No one. I don't follow all of GUEs recommended gases either. Who said anything about GUE here? GUEs choice is 50% for a single deco gas in the ocean. All you people who think I'm a koolaid drinking gooey lemming need to read my profile.

 

[Thalassamania]

No one. I don't follow all of GUEs recommended gases either. Who said anything about GUE here? GUEs choice is 50% for a single deco gas in the ocean.

Sorry, hell hath no fury as an unwarranted assumption.

 

[grazie42]

Here´s an example-profile where 80% is "better":
55m(~180ft), 30mins BT, BG 21/35, v-planner (nominal):
Deco (ean 40 & 80): RT: 65mins, OTUs: 88, CNS%: 34,4
Deco (ean 50 & 100): RT:65mins, OTUs: 93, CNS%: 40,4

Now Soggy may feel that standard gasses are more important than getting out of the water "cleaner" (less OTUs & CNS%) but if he wanted to, for the same effort and cheaper, he could...