How much gas in case of accidental deco.

[freeclimbmtb]

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 try to stay in the habbit of using my air in 3rds. So (for simplicity sake) I start with 3300psi, turn at 2200, with the intention of being out of the water with 1100. I want to stress that I have no intention of planning deco dives without formal training, But I have seen a few Rec divers lock out their computers because they clipped that envelope and didnt know it, or didnt know what to do about it. I am also putting togeher a stage kit to carry a sling bottle, but for redundancy, NOT for increased air supply. What I mean by this is if I have 100cf of back gas, and an 80cf sling bottle, I my intention would be to use 33cf, turn, and finish with 33cf of back gas plus 80cf in the sling. NOT breathe my 100 down to 40, and then switch to the sling when the back gas runs out.

 

[dumpsterDiver]

Wow you got a lot of gas. The answer is not much... If you are watching your computer and you cross over the line into deco... your penalty should be minimal.. say 5 minutes of deco or so.... normally you would do a 3 minute safety stop, so the requirment to do 2 minutes more is not gonna take a bunch more gas.

You should also be aware that if you cross over the line on the 3rd or 4th dive of the day, my computer at least, will spank you hard... it will give you more deco than if you did this on the first dive.

Of course your deco, depend on depth and time and residual nitrogen loading, but a SHORT, unitentional venture into deco at moderate depths.. say 100 feet and staying too long for 2-3 minutes.. is not going to give you a huge penalty.. if you are diving 165 feet and stay for an extra 4-5 minutes.. well you can end up with a lot of deco...

 

[Scott L]

In my opinion, you would be better served by carrying extra gas by selecting a larger cylinder such as a HP130. Much less drag. Also, avoiding violating NDL limits should be rather easy. If not, you may have some subconscious issues at play...

 

[Agility]

If you click on the Liquivision ad over the postings you can download a simulator to play with. It will show you the time to ascend and the deepest deco stop. If you know your SAC you can calculate the gas you need to reach the surface.
Of course this is for demonstration only, not for planning.

 

[freeclimbmtb]

In regard to me carying a lot of gas, my calculated SAC on my last dive was down to under 0.3 So its even more gas than you think!

As for the 130, I will sling 4 bottles and sacrafice drag 8 days a week in the name of redundancy. If you clip that envelope and have a 5 minute deco obligation, fine. But if your reg free flows before you clear that obligation, now whats your backup plan? If you free flow an 80 when you have an obligation to clear and you have another 80 on your hip. No sweat. 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 can hand off that sling, shut down the bad reg, and clear that obligation with greatly reduced risk. (ie not have two divers trying to clear an obligation off one tank...because in an out of air situation compounded with deco...I dont think either diver is going to be at a calm cool and collected 0.3 SAC rate.)

Again I want to stress that with my training and level of experience, I never get in the water with the intention of deco, but it seems irresponsable to me to have the mentality that "I will NEVER exceed my NDL" because lets face it. Plenty of divers look at air remaining over NDL remaining. Especially those with low experience, because when your new, you huff and puff your tank dry WELL before you run out of NDL (yes, not everyone does, I know.) But then 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.

Incidentally the first time I got into deco, I was on a 130, new in a drysuit, and underweighted. I blew my stop because I couldnt hold depth and really didnt understand what was going on, and locked out my computer. I had no symptonms of DCI, but it was a little bit of a reality check that I needed to think about these things.

 

[Colliam7]

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?

Interesting question, and a bit tough to answer precisely. But, three thoughts occur to me:

1. If you are diving a 100 cf tank, unless you have an incredibly low SAC rate, the chances of incurring a significant deco obligation are minimal. Yes, I can incur a few minutes of deco obligation if I dive to 100 feet and stay there with minimal finning effort (and, therefore, minimal gas expenditure) until I reach my usual turn pressure (~1000 psi for me). But, if I then ascend at a normal rate I will generally work through and out of that minimal deco obligation fairly early on the ascent. So, it will have easily cleared by the time I reach a safety stop at 15 ft. Therefore, my answer to the specific question would be, 'No more gas than you would otherwise have planned for the dive.' Having said that, as dumpsterdiver mentioned, it could present a bit more of a challenge if it occurred on the fourth dive of the day, and you already had signficant calculated nitrogen loading from previous U/W time. In that situation, the timer runs a bit faster and you go from no obligation to substantial more quickly.

2. The recommendation to go with a bigger tank, rather than a slung pony bottle, is an interesting one. You clearly indicated you were considering the addition of a pony bottle as a measure of redundancy, NOT to expand your gas supply. So, going with a bigger tank would not accomplish that goal. In fact, going with a bigger tank might actually change my answer to your question. In that situation, having more backgas might provide the opportunity to incur a bigger deco obligation. This is one (of many) reasons that diving doubles without proper training is not considered a good idea - too much extra gas without extra dive planning training can, for at least a few divers, present an opportunity for problems.

3. Kudos to you for considering using a slung pony bottle. Setting one up, and learning to use it, is a valuable dive skill. An 80cf pony is a big bottle to manage at first, but after a while you hardly notice that it is there. If I was starting out, I might choose a 30 or a 40 (which I did) as my first slung bottle. But, if I already had an 80, and an additional regulator, available and wanted to minimize the added cost, there would be nothing wrong with that approach.

 

[DaleC]

I'm trying to imagine someone diving thirds and slinging a 80cuft pony on a normal rec dive. It goes beyond conservative. There is another principle at play as well as redundancy (which I like) called KISS - keep it simple stupid. Take what you need in the simplest, most compact streamlined package possible. Complexity only leads to confusion.

Thirds in rec diving is a beginners strategy borrowed from a different type of diving that often falls short of appreciating the conditions one currently finds themselves in. It belongs in a world where a diver cannot surface, either due to overheads or deco obligations, and eliminates an important safety factor that most recreational diving is based upon - being able to directly ascend to the surface. It would be more advantageous to learn how to calculate rock bottom reserve values and choose a pony that holds that reserve volume as redundant gas. In rec diving that should never exceed 40cuft. which is a good size bottle if one wants to move into the tech arena later on (standard size small stage).

There are a few exceptions to direct surfacing in rec diving; boat traffic, kelp beds, surface currents... but those are specific situations that require specific plans.

As for going into deco; a much simpler old school plan that worked effectively would be to dive a SMALLER tank. Try a Steel 72. With less available gas one cannot stay down deep enough, long enough, to get into serious trouble (using common sense). It's one reason old school divers did not need the 3 minute safety stop either.

 

[ktomlinson]

It sounds like a gear solution to a training or behavior issue. I'm nearing 200 dives and have yet to accidentally enter deco. It's pretty easy to avoid. You can have all of the gear that money can buy and if you don't follow your training or are absent minded the gear won't save you. You've received the answer to your question already, most accidental deco will clear with ascent if you use a proper ascent rate. If you are diving your own tanks you may just add an h valve for redundancy...if you are going to sling a stage you may consider an al40 instead of an 80 since you shouldn't need 80cuft of gas for accidental deco.

 

[mathauck0814]

You should not "accidentally" go into deco. But if you do, remember that your stops will be at 10 or 20 feet where gas lasts a long time. You'd be better served to sling a 40 for redundancy as that's PLENTY of gas to ascend safely on most recreational profiles. Better yet, a set of small doubles will trim out better and give you stability and redundancy.

I don't believe you'd mentioned this in your original post, but is the purpose of your configuration to solo dive? If not, then your buddy is your spare gas and you're solving a training issue with equipment (as ktomlinson said above).

 

[ianr33]

Do the numbers.

Any "accidental" deco should only be a few minutes and will be at 10 feet. Lets say 10 minutes. (5 minutes mandatory deco and 5 minutes safety stop)

A stressed SAC rate might be 1.0. At 10 feet that's going to be around 1.3 Cu Ft /minute. 10 minutes gives you 13 Cu Ft. which is about 500 psi. with an Al 80.

So the standard "be back with 500 psi" should cover it, unless you do something monumentally stupid!

---------- Post Merged at 09:58 AM ---------- Previous Post was at 09:55 AM ----------

In regard to me carying a lot of gas, my calculated SAC on my last dive was down to under 0.3 So its even more gas than you think!

How exactly did you calculate that?

I'm not saying it's impossible but 0.3 is a really low SAC rate. Especially for a new(ish) diver.