How much gas in case of accidental deco.

[TSandM]

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.

There are various ways of teaching people to do things. You can simply give them a set of rules and tell them not to break them. That'll work pretty well with the people who are temperamentally likely not to break rules.

You can give people a set of rules and explain why the rules are what they are, and what the consequences of breaking them are. That will induce obedience in some of the more thoughtful people.

Neither approach gives the student any tools to cope with a broken rule. And humans being what they are, rules are likely to get broken from time to time, even if that isn't planned.

But you can give the student a set of procedures and explain that these procedures apply to a subset of the possible situations the student might encounter, and that it is desirable for a student, with limited experience and training, to confine his explorations to those situations. If you also recognize that there are scenarios outside those protocols, and give the student some basic tools to cope with them, you will avoid the "OMG I BROKE A RULE" panic response, which we have seen written up on this board a number of times.

Deco isn't death. Tech divers do deco all the time. The reason recreational divers are told to stay out of deco is not because deco is horrible, but because recreational divers don't have the planning tools to know that they can do the deco they incur (gas, thermal protection, redundancy) and they may very well not have the diving skills to actually DO the deco (hangs in midwater with no visual reference, or a stepped profile that requires precise buoyancy control and timing). It is my understanding that CMAS includes some backgas deco at their higher levels of recreational diving; GUE teaches a "deco stop" approach to recreational dives, to make the transitions from no-deco to mandatory deco a pretty minor change.

I don't think the OP should plan to exceed his no-deco limits. But he is asking the right question -- IF you end up with a deco obligation, how do you know how much gas you need to execute it? I'd much rather see a recreational diver look at his computer and say, "Oh, drat, I went into deco, I'd better deploy my 40 and do my time," than have someone look at their computer, recognize they're in deco, have no idea what they need to do now, and panic -- it's happened, and people get hurt that way.

Kudos to the OP for asking the real question -- if you have the gas, you can do the time.

 

[DaleC]

Ah... the old school approach might be if you don't have the gas, you won't be lulled into thinking you should do the time

I too think it is a good question as it has stimulated a positive discussion regarding various approaches to rec diving and NDL limits. Also, just as agencies have various approaches, individuals will vary their approach with time and experience. I remember going through a tech/rec phase myself (which was good for adding to my knowledge base) and a total redundancy for every dive phase as well. I now have a modified outlook in which I tailor my approach to the dive I am doing. At less than about the 50' mark I now usually dive without a second air source though I limit those dives so I don't push either gas limits or the NDL's. The surface is my RAS. Below 50' I carry some form of redundancy (pony/doubles) as I no longer feel certain I could reach the surface in one breath and the risk of DCS also increases. I have not changed the belief that I should always dive self sufficiently regardless of which strategy I employ or whether I'm solo or diving with a buddy.

I'm something of an oddball in my region because I dive small tanks (72's). As singles, they allow me shallow dives for as long as I want without venturing into deco and doubled up they are a small light package that allow me to do deeper dives including some limited deco.

And, at the risk of repeating myself, just like the Nitrox course which taught me to see breathing gas as a combination of elements with various effects, getting turned on to Lamont's link to rock bottom calculations was probably the single most important paradigm shift I experienced in regards to gas management. Until then it was all guesswork.

 

[Scuba_Shane]

I don't understand all those against getting a pony bottle. It does provide redundancy, granted a bit excessive with respect to your buddy. On the other hand, it provides experience with handling extra bottles which might be useful down the line. Might as well get oriented with it while you're in the shallow end.

 

[DA Aquamaster]

Deco isn't death. Tech divers do deco all the time. The reason recreational divers are told to stay out of deco is not because deco is horrible, but because recreational divers don't have the planning tools to know that they can do the deco they incur (gas, thermal protection, redundancy) and they may very well not have the diving skills to actually DO the deco (hangs in midwater with no visual reference, or a stepped profile that requires precise buoyancy control and timing). It is my understanding that CMAS includes some backgas deco at their higher levels of recreational diving; GUE teaches a "deco stop" approach to recreational dives, to make the transitions from no-deco to mandatory deco a pretty minor change.

I don't think the OP should plan to exceed his no-deco limits. But he is asking the right question -- IF you end up with a deco obligation, how do you know how much gas you need to execute it? I'd much rather see a recreational diver look at his computer and say, "Oh, drat, I went into deco, I'd better deploy my 40 and do my time," than have someone look at their computer, recognize they're in deco, have no idea what they need to do now, and panic -- it's happened, and people get hurt that way.

Kudos to the OP for asking the real question -- if you have the gas, you can do the time.

As noted above, when I took PADI OW and AOW in 1985, they included the information for 10' stops on the tables and they covered the basic procedures, the need for neutral buoyancy, etc. The intent was exactly as you have suggested - to ensure that a diver who screwed up, had the knowledge to do something about it, and as noted above, if the diver arrived at the stop with 500 psi in an AL 80 (12.8 cu ft) or steel 72, (14.4 cu ft) he or she would have enough gas to handle 5-10 minutes of a deco stop.

At the same time, PADI required a mandatory safety stop on dives approaching the NDL (within 2 blocks) and aside from adding some safety factor to the tables, it required the students to have the skills needed to halt the descent and hold at 10' to 15' for 3 minutes. In that regard the difference between a mandatory safety stop and an "oh crap, I exceeded the NDL" decompression stop was a matter of semantics and the procedures were the same.

Somewhere in the late 80s to early 90s the discussion of deco procedures for a recreational diver went away to be replaced by the "never exceed the NDLs" approach. I suspect that was unfortunately part of the general simplification of OW and AOW instruction as the dive industry moved to a DM led destination dive vacation/traveling diver model. However, the decision may have also had something to do with the inclination of some people to let the line blur and start doing intentional decompression without what we'd regard as proper equipment today.

In short, while I see a great deal of advantage in a redundant gas source, and view that as essential for planned decompression, applying it to the recreational diver is assuming and planning for two failures - the failure in judgment or situational awareness in terms of exceeding the NDL, and a failure in planning and having an adequate reserve to deal with that contingency. I admit the two may go together adding a pony bottle in the mix creates the temptation to not view the pony as a thing never to be used except in an emergency back up, but rather to start relying on it as extra gas, cutting the back gas reserve to nothing, and eventually deciding to use that pony to get another few minutes of bottom time. Once that occurs, you've just made it a lot easier to exceed the NDLs at the same time the diver is now back to only a single source of gas, but with an even greater deco obligation.

 

[The Chairman]

An 80cf sling bottle is probably overkill and will affect the quality of your dive... especially exiting. I say that, but then I do dive sidemount where I sling two AL80s and sometimes two HP130s. I actually want a few AL60s so I'll have plenty of air, complete redundancy and a lighter rig for those ocean dives.

 

[Superlyte27]

If your SAC is really .3, you don't need an AL40. If you are diving an HP100 to thirds, with an H-Valve, you have redundancy. If you accidentally run into deco diving thirds (which is pretty unlikely) you have more than 1000 psi to deco on, which at .3 sac rate should last you a long time. In the event that you have a system failure with a reg, just turn off that reg. An H-Valve will cost you a lot less than an AL-40, and unless you have a very unlikely burst disk failure(which almost never happens underwater, but during the fill or setup on land) or a tank o-ring go (which I've never seen happen), you are all squared away. No issue. You're planning overkill.

In the unlikely even that you brush deco, and have a catastrophic gas loss, while diving thirds, with a .3 sac rate, you want to dive a pony bottle every time? Here's a thought, plan conservatively enough that you are never near deco. Then if you have a catastrophic gas loss, head to your buddy or the surface. Don't re-invent the wheel.

 

[DevonDiver]

Roll a dice and multiply the result by 2. The result in 'minutes duration' gas you need.

That's assuming you're a gambler.

If you're not a gambler, then stay conservative, dive prudently...and you won't go into deco.

 

[DaleC]

Interesting comments about ponies. I have used one on and off since very early on and have never unintentionally gone to it to extend a dive, though I know others who have. But I was thinking that ones situation and temperment probably has a lot to do with ones approach to these things. I dive locally, almost every week so I don't get the sense that I need to overstay any particular dive. If I see something noteworthy I just return with a fresh tank. A destination dive for me is doing a charter to a wreck and for those I'm always diving doubles of some sort.

I also began soloing quite early and subsequently approach most of my dives with a preconceived blueprint of what I'm going to do for how long. When I vary from the plan (which I do if I see something I did not anticipate) I have a little alarm sense that tells me I'm diving somewhat blind in that regard and I tend to watch my values more than usual. Being alone a lot in low vis water tends to remind me that I'm just a visitor there and to remember where home is and how to get there.

I imagine if one is a travel diver, who may never return to the same magical tropical place again, the urge to stay down longer might be more pressing.

I will say I have used a 40 or 80 pony/stage to extend a dive intentionally but those were planned events. In each case I calculated my needed rock bottom, started the dive on the pony till I got to that rock bottom value (in psi) and then switched to my back gas which I then dove to rock bottom psi, thus still leaving me with the necessary gas in each tank. I also do that quite often when diving independent doubles (BM or now SM) and want to swap out one tank for a second dive. The remaining tank still has my rock bottom for the second dive and I just dive the fresh one to that value before turning.

 

[tracydr]

I went into deco while on a a two week trip, not using nitrox ( won't dive air on those trips again!). It was on a deeper dive, many days into the trip and I was diving my AL 63. I was with my husband, an instructor ( who also went into deco at the same time, he had a suunto, I had a TUSA). I think we were around 100 feet at the time and we were given a rather long deco stop by both computers, 15-20 minutes, if I recall. We really only went a few minutes into deco, too, so don't be surprised if you incur a long stop, depending on previous days diving. I remeber this stop was long enough to get bored playing around in the shallows doing mask and safety drills. I'd have to dig out an old log book to see how long it was.
Luckily, I have a very good SAC as I would have never expected such a long stop from going just a few minutes into deco. I think a slow ascent caused us to incur more time, looking back on it.
We both had plenty of gas ending the dive but I'm now much more careful and will not dive on long trips without nitrox anymore. I also won't dive deeper without a bigger tank, even though I find a longer tank uncomfortable. ( I have some hp80s for when we can bring our own tanks).
Someday soon, I'd like to take a class on advanced nitrox and deco, plus trimix, as i hate diving over 100 feet, due to narcosis. I plan to start diving either back mount or side mount doubles this winter, once I'm really comfortable with my drysuit. Until then, I did find that my little experience with putting the computer into deco and my reading about decompression theory very enlightening. I do agree with TSandM, having some knowledge, rather than just knowing rules, is important. Rules with no reason can cause anxiety or even panic in some people if accidentally broken.

 

[TSandM]

I refuse to be judgmental about this. Several years ago, before I had any kind of technical cert, I did a deep recreational dive to 100 feet with my friend Bob. We were going to look for an octopus that had been reported in a specific place. At 100 feet, I had one of my cambands come uncammed, and the tank began slewing back and forth on my back. I didn't know what was wrong, but I knew something really was. I had a little trouble getting Bob to realize where the issue was (narcosis, anybody?) but he eventually figured it out, and he then had to sit on me and tighten my camband.

The end result of this little issue was that we seriously pushed our deco limits AND ran short on gas. This was the day I discovered that weighting yourself neutral at 10 feet with 500 psi doesn't work very well if you have to shoot a bag and try to keep it upright, and you actually run your tank down that far. I finished that dive with 400 psi.

Not all excursions into deco are due to inattention or carelessness. I now dive bigger tanks or doubles for deep dives like that; I learned my lesson. If the OP wants to carry a 40, that gives him personal redundancy and extra gas. I dive doubles for the same purpose, but they have their downsides, too.