How much gas in case of accidental deco.

[ktomlinson]

I think that the structure of the training most of us in the US take for recreational SCUBA create this unnecessary mystique about deco diving. We are all deco diving, just in rec diving we make sure that our ascent rate will clear our deco obligation. I don't think that it has been mentioned, but the table you learned in OW should have included a contingency for if you overshoot your NDL. Since you know your SAC you should be able to figure your gas requirements using that table.

I am curious if after having read through this thread if the OP still intends to get a stage bottle?

 

[freeclimbmtb]

I had a nicely written response to a few questions...when i noticed there were 3 pages of replies (!!!) First and foremost I want thank people for not being toooo hard on me. Suggesting slinging an 80 was more of me typing without really thinking about what I was writing. Yes that would be super excessive, bulky, etc. A 40 would have been a more well thought out choice. A lot of great replies and good advice, and plenty of food for thought. I was really just thinking back to the time I DID get an obligation and thought to myself, "what if..." and the wheels started turning.

As for a sling, yes, still planning to get one. (I was thinking maybe a small propane tank though...80-90 gallon should do...properly clean up to 40% of course) Just kidding. A 40 seems like the smart choice.

For the intentions behind wanting it, not so much exclusively solo diving, but why not be self sufficient AND dive with a buddy. (Particularly an instabuddy you just met on a dive boat...just stuffy my overactive imagination thinks about)

As for the SAC, that particular dive was 55 minutes at an average of 35 feet using 1700 from a HP80. (looks closer to .32...i was a little off.) Its not a SAC I would use for any sort of calculations though...not until I have some repeatability.


And I don't drag cars behind me on road trips, thats what AAA is for. DAN on the other hand sorta depends on you making it to the surface...

 

[TSandM]

I think a 40 makes much better sense. And it's a lot of gas -- Using GUE's ascent strategy, which incorporates one minute stops from half-maximal depth up (using a lot more gas than just going up to 15 feet and hanging) a single diver, stressed, will need about 20 cf to get up from 100 feet. And you certainly would never use 40 cf doing your mandatory 8 minutes stop for violating the PADI tables by less than 5 minutes . . . And before you tell me that you wouldn't USE the 40 in that setting, remember that the slung bottle is emergency gas. Emergency can be due to gas loss on your primary setup, or an unforeseen need for more gas than you planned. The slung bottle is there for either situation.

Of course, the RIGHT answer is to plan your dive and dive your plan -- Know roughly how much time you have at the depth you're proposing, and monitor your gauges so that you don't end up violating your tables or your computer. That's part of situational awareness, which is a big safety thing for divers.

 

[Diver0001]

I'm not sure the best place for this topic...move as you see fit.

If you were on a no deco dive and for whatever reason (inattention, dealing with a situation, etc) ran your NDL down and clipped the edge of the deco envelope, how much gas should you have to safely clear your obligation without pushing an out of air situation?

I'm not sure if you're PADI trained but if you are then you'll remember from your open water course that they said something about "emergency decompression" for cases where you "clip" the NDL. What the PADI materials say about that is the following :

Emergency Decompression – If a no decompression limit is exceeded by no more than 5 minutes, an 8-minute decompression stop at 5 metres/15 feet is mandatory. Upon surfacing, the diver must remain out of the water for at least 6 hours prior to making another dive. If a no decompression limit is exceeded by more than 5 minutes, a 5 metres/15 feet decompression stop of no less than 15 minutes is urged (air supply permitting). Upon surfacing, the diver must remain out of the water for at least 24 hours prior to making another dive.

So in the scenario you're describing where you "clipped the edge of the deco envelope", which I take to mean no more than 5 minutes over the NDL, then the tactic to back out of it is to extend your safety stop (which is now mandatory) to a minimum of 8 minutes. The amount of "extra" gas you'll need, therefore, is enough to spend at least 8 minutes at 5 metres. Of course that tactic assumes you're using the table so if you have a computer you should be aware that it can (will) use a different algorithm and you should make sure that the computer is "clear" as well before you surface (air supply permitting).

I should also put in a warning about "don't try this at home" and all that blah blah, but being a person with a normallly functioning brain I'm sure you already know that.

R..

---------- Post Merged at 01:06 AM ---------- Previous Post was at 12:33 AM ----------

...snip... if your reg free flows before you clear that obligation, now whats your backup plan?

Your buddy is wearing it on his back. That's the reason you dive with a buddy unless your gear is fully redundant (and you have the experience and all that other cover-your-bases stuff)

Or if you and your buddy spend a little too much time teasing the lobsters and both rack up an obligation, what if he (she) has a free flow?

You, likewise, are wearing your buddy's backup plan on YOUR back.

Plenty of divers look at air remaining over NDL remaining.

Just don't be one of them and you won't have that problem.

as you gain experience and comfort in the water, your air lasts longer, and suddenly, your planning your dive off the gage, not the timer, and you run out.

Actually, as you gain experience and comfort in the water you also gain the presence of mind (free attention) to watch your gauges so you DON'T run out. In my experience, the vast majority of divers who I've ever seen run out of air do so for two main reasons: (1) they didn't watch their gauges and (2) they didn't watch their gauges. (yes entrapment and gear problems happen but in reality they're a distant 3rd).

Look.... all of what I've written in the 2nd part of this post is to point out one thing, which is that you're doing something that should never be done... looking to solve a skills problem (not watching gauges) by carrying extra gear.

You're unnecessarily complicating things and carrying extra gear STILL doesn't solve the main concern you have which is that you need to *pay attention* as you go. It's clear to me that you have some concerns in this area and you're looking for solutions, which is excellent and I commend you for that, but for your own good you'd be better off, in my opinion, looking for solutions to *avoid* getting in a "situation" rather than solutions to get the genie back in the bottle once its escaped.

R..

 

[lowviz]

-What Diver0001 said.

Just carrying extra gas without increasing your core skills at managing that extra gas just gets you into deeper, you know...

 

[DA Aquamaster]

I agree.

When I started diving, we used the PADI version of the US NAVY table and it included blocks for "unintentional" decompression. It essentially included all of the times, depths (down to 140') and stops for profiles that did not require a stop deeper than 10'. When I considered it, it became obvious that you'd have to grossly exceed you targeted depth/time to require more than 15 minutes of deco. Most of the possible "unintentional" profiles required a 3 to 5 minute stop with a few more requiring 7 to 10 minutes and only a couple requiring 12-15 minutes. What that told me is that a "mandatory" safety stop of 5 minutes would cover a lot of sins.

The tables and computers in use today have shorter NDL's but the same holds true - in most cases, if your computer wanders into decompression at recreational depths and calls for a few minutes at 10', in most cases it will clear before you reach the 10' stop depth, and will probably be cleared before you get to 30 ft or so. In that regard, then if you slip past the NDL by a minute or so, just immediately start your normal ascent, pay attention to the ceiling, make any stop required, if you still have one and then add a 5 minute safety stop.

Assuming you have a SAC of .75, you will be breathing 1 cu ft per minute at 10 ft, so a 3 minute deco stop and a 5 minute safety stop on top of that will only require 8 cu ft of gas. That would be covered (barely) by the 12.8 cu ft you get with 500 psi in an AL80 with some left overs to allow ascent to the surface from 10' some gas for your BC and a bit of gauge error, but you'd be cutting it thin.

Thus, I'd recommend a slightly larger reserve, but hauling around an AL 40 as a contingency is a bit much, and worse will lead to some "creep" as you start cutting your reserve thinner as you have it along "just in case" but then pretty soon it starts working it's way into the gas plan and that will get you into trouble without proper training.

 

[DaleC]

I think that the structure of the training most of us in the US take for recreational SCUBA create this unnecessary mystique about deco diving. We are all deco diving, just in rec diving we make sure that our ascent rate will clear our deco obligation.

I agree with the first sentence but not the second. Decompression diving, by it's classical definition, involves a need to decompress before surfacing directly. Rec diving, within the limits, allows for direct ascent, currently 30'/min but just as validly 60'/min in the past with a SS being only suggested. Only the values bordering the NDL's have a required SS (which should really be considered "clipping the edge of deco" to coin the phrase). This suggests that sufficient N has not accumulated within the diver to cause DCS symptoms thus, the diver need not decompress in the water.

I understand what you were intending but suggesting all dives are decompression dives muddy's the waters unnecessarily for the beginning rec diver IMO. Conventional wisdom might suggest that technifying rec diving would make it safer but part of the safety of rec diving is that it is not tech diving. Some believe that placing artificial barriers on rec diving actually acts to take away some of that safety.

Doing half stops and 10'/1min ascent times being another good example (along with diving thirds) of a tech strategy transplanted into rec diving without any real benefit being shown (specifically for rec diving). In some cases, taking that long to ascend could create more problems than the theoretical ones it attempts to address (buddy/boat separation while ascending through low vis or fast current being two). It's great if one just wants to practice that skill for tech diving down the road but it should not become a "necessity" that clouds the mind to more practical strategies.

Donning fire proof pants now...

 

[lowviz]

...//... Conventional wisdom might suggest that technifying rec diving would make it safer but part of the safety of rec diving is that it is not tech diving. Some believe that placing artificial barriers on rec diving actually acts to take away some of that safety.

Such is the problem, thanks for bringing it up with this twist.

Restated: If you are skilled enough and disciplined enough to never exceed NDL, you are a "safe" rec diver. So should the less skillful recreational divers rely on tech diving procedures?

 

[RonFrank]

Accidental deco is like accidental Sex, it does not exist IMO. If you accidentally go into deco then your a moron and should quit diving. Sometimes one can get in a situation where you have to stay at depth longer then planned which does happen, but is rare. I had it happen to me when my buddy got fouled at 110' but that is a rare case.

You preach keeping 1/3 of your gas for reserve, but in the next breath your talking about a pony? My advice, get a good buddy, Plan your dive and dive your plan. If you dive within the rec limits (easy to do) you should never run out of air but if you do you have a buddy and the surface is 2 minutes away or less. Rec diving is not that hard and staying out of deco is easy. There are very few exceptions.

 

[lowviz]

That would be the right answer.

So why do four or more agencies hold back to see who will answer first? I still say having a small pony is a good thing, never know when you may need to clip it onto someone. People in distress get freaky underwater, don't usually follow the textbook.

I'll most gladly assist you from behind where you can't reach my stuff...