Accidental Deco

[chrpai]

I was just going with what I read here:

Ratio decompression - Wikipedia, the free encyclopedia

Ratio decompression has never been adopted by more mainstream technical diver training agencies, such as TDI or IANTD. Although the safety record of ratio deco appears to be good, it suffers from a number of limitations.

The diver is generally limited to two potential bottom breathing mixes (specific mixes of Trimix). The technique does not work for deep air diving, or if a diver elects to use a non-standard bottom mix.

Since I've never taken the class and I know GUE and UTD are proponents of standard gases, I judged this to be credible.

 

[boulderjohn]

You can use ratio deco with any mix(es). In fact the original rules were based on air with EAD applied.
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I've heard people say that.

Of course, when I took Ratio Deco from Andrew Georgitsis and UTD, he emphasized the minimum deco was only for EANx 32 and could not be used for air, and that Ratio Deco was based on standard mixes, but I guess things could change. That was a couple of years ago.

 

[tamas970]

Can you explain again why a 20 cu-ft bottle is useless for deco? I'm confused?

Because it is not enough to complete all deco obligations - maybe for a kid or for someone with very low SAC rate, but definitely not me, who empties 90CuFt on a 45min dive.

 

[PfcAJ]

I was just going with what I read here:

Ratio decompression - Wikipedia, the free encyclopedia



Since I've never taken the class and I know GUE and UTD are proponents of standard gases, I judged this to be credible.

False. False. False.

You can ratio deco with any gas. You've just got to talk the time to identify the ratios. That's gunna throw a lot of people for a loop.

1:1 and 2:1 are simple, common ratios that work well with standard gases, but I've used RD for stuff pretty far outside of the 'GUE standard' on some wonky profiles. You've just gotta fiddle with deco planner and figure it out.

 

[James R]

Every recreational computer I've owned that can calculate "oh s***" stops has no way of planning those stops. So how would you go about calculating how much more air is needed? You might use some dive planning software to get an idea but the computer isn't going to tell you. Right?

I agree with this. Not likely the computer could tell you how much gas you need because it likely has no idea how much gas you started with or how much you have left. It might be tracking how much pressure remains in the tank, and that's about it as far as gas goes. Are you diving an AL80, double AL80, sidemount AL40s, a single HP133, or whatever you can think of? The only way it could is if you can program this before the dive starts.

 

[boulderjohn]

I agree with this. Not likely the computer could tell you how much gas you need because it likely has no idea how much gas you started with or how much you have left. It might be tracking how much pressure remains in the tank, and that's about it as far as gas goes. Are you diving an AL80, double AL80, sidemount AL40s, a single HP133, or whatever you can think of? The only way it could is if you can program this before the dive starts.

There are computers that can predict the amount of gas you will need for a given dive when you go into plan mode, but those are not computers that the average recreational diver is using. Once the computer tells you how much you will need, it is up to you to strap on enough gas to get it done or change the plan.

 

[chrpai]

Once the computer tells you how much you will need, it is up to you to strap on enough gas to get it done or change the plan.

That's the chicken and egg I'm trying to point out. The typical rec computer isn't giving you the get out of jail information until you've gotten in jail.

If I plan an NDL dive (lets say square to keep it simple ) I know how to do the math to figure out how much gas I need for the dive and my reserve. But if you totally violate that plan (otherwise you would still be within NDL ) it's hard to calculate your gas needs since your computer isn't spitting out stops and times until after you've done it.

Maybe the assumption is people will typically violate the plan by going deeper rather then longer and that NDL creeps up on you sooner therefore if you had enough gas for the first plan you should likely have enough gas for the oh**** moment.

Either way, seems like a pointless feature. Hopefully I'll never find out.

 

[boulderjohn]

That's the chicken and egg I'm trying to point out. The typical rec computer isn't giving you the get out of jail information until you've gotten in jail.

If I plan an NDL dive (lets say square to keep it simple ) I know how to do the math to figure out how much gas I need for the dive and my reserve. But if you totally violate that plan (otherwise you would still be within NDL ) it's hard to calculate your gas needs since your computer isn't spitting out stops and times until after you've done it.

Well, it seems to me that one should not plan to totally violate the plan.

The reserve you mention should take care of any minor violation. If you violate your deco by 5 minutes, say, your reserve should be able to handle that easily. The simple reality is that if you only violate by a minute or two, it won't take much more than a standard safety stop to take care of your deco obligation. For almost all accidental NDL violations, the computer will call for no more than an extended safety stop. If you are getting multiple stops at multiple depths, your violation was a lot more than a little oopsie. If you are totally violating the plan, then you either did it intentionally and should have planned for it or something really serious happened as a result of some other major failure. If you got stuck inside a wreck all alone and had a hard time getting out on a recreational NDL dive, then there is a lot more to talk about in your dive planning than the amount of air you brought.