Deep safety stop question

[Charlie99]

V-Planner is and always has been VPM. (of slightly different flavors)

If it is not giving any deep stops it is because it is not a deco profile.V-Planner is a great tool for doing sanity checks on recreational dives IF you know what you are doing with it,but dont expect it to give deep stops (or any stops at all!) on NDL dives.

Just like dive computers --- when you are in the NDL region, no stops are required by VPM, but you can always add them in.

One can artificially force VPM into decompression on many profiles simply by cranking up the conservatism to the highest level. What this is showing you is how adding deep stops to an otherwise NDL dive is adding conservatism/safety factor to your dive.

Going overboard and starting deep stops super early doesn't add conservatism or safety ---- it simply adds to your loading. The VPM reports "offgassing starts at xxx feet". Doing a deep stop before then adds to your loading while not having any benefit. Ascending another 10 to 20' beyond that before the 1st stop is reasonable. Even without a deco program it is very easy to show that stopping at anything deeper than 70% of absolute depth makes no sense ---- when breathing EAN32 your inspired ppN2 is only 68% of the absolute depth (because you are breathing 32% O2, 68% N2). In practice, for recreational depths, a 1st stop of 1/2 of bottom depth is a pretty good approximation.

Running nominally NDL profiles through VPM with very conservative settings will also show you that the optimum deco profile spends more time shallow than deep.

Putting all of the above together has led me to an ascent profile that centers around:
1min @ 40'
2min @ 25'
3min gradual ascent from 20' to 10'.

If I'm very heavily loaded, I'll add some additional time so that my total ascent time is greater than the normal 6 or 7 minutes, but keeping roughly the same shape.

If I'm starting the ascent from greater than 80', then the 40' and 25' stops become 50' and 35'.
I base the ascent on where I have been for the last 10 or 15 minutes. If it's been a multilevel dive where the last 20 minutes have been shallower than 70', as far as the ascent is concerned, the fact that earlier in the dive I was at 130' is not relevant.

 

[TSandM]

Thanks for the references, Steve -- I clearly misspoke and had V-planner mixed up with something else (GAP, perhaps?)

I had read the deco myths article before, but wonder if it's been revised -- I don't recall it addressing Ratio Deco so directly, but maybe I read it before I knew what Ratio Deco was . . .

With regards to Jim's profile/deco, it got me thinking. The purpose of carrying deco gas is to reduce total deco, right? If you're carrying plenty of gas, and conditions are pleasant (eg. warm water and something to look at), why not do backgas deco and make your life simpler? Is the answer that the combination of depth and time and necessary reserves very quickly adds up to either very short bottom times or more gas than you can carry on your back? (I would imagine that's the issue.)

 

[Charlie99]

Thanks for the references, Steve -- I clearly misspoke and had V-planner mixed up with something else (GAP, perhaps?)

Or maybe Z-planner, also by Ross Hemingway, which is/was a Buhlmann program.

Ross's comments on the 80% deep stop of 300' dives being erroneously carried over to recreational depths are interesting. He didn't mention yet another factor that tends to make the right depth for 1st deep stop on a recreational dive a lower percentage of bottom ata. On a 300' dive you will be using a low FO2 gas. The higher FO2 gas used on a recreational dive means that your inpired ppN2 on the bottom will be a lower percentage of ATA, which of course moves the optimum deep stop to a shallower %ata.

The output from deco programs show you this, but it's also nice to understand why the program is saying what it does.

 

[ianr33]

With regards to Jim's profile/deco, it got me thinking. The purpose of carrying deco gas is to reduce total deco, right? If you're carrying plenty of gas, and conditions are pleasant (eg. warm water and something to look at), why not do backgas deco and make your life simpler? Is the answer that the combination of depth and time and necessary reserves very quickly adds up to either very short bottom times or more gas than you can carry on your back? (I would imagine that's the issue.)

I asked about just doing deco on backgas a while ago on TheDecoStop
http://thedecostop.com/forums/showthread.php?t=15064

Bottom line from that thread is that it is better to do some deco on a high O2 mix.

Empirically I find that if I do a dive that needs a lot of backgas deco (more than,say,30 minutes) I tend to feel tired afterwards. I dont think I am alone in that

My rough rule of thumb is that if the backgas deco time exceeds the bottom time then I would much prefer a deco gas.

 

[limeyx]

I asked about just doing deco on backgas a while ago on TheDecoStop
http://thedecostop.com/forums/showthread.php?t=15064

Bottom line from that thread is that it is better to do some deco on a high O2 mix.

Empirically I find that if I do a dive that needs a lot of backgas deco (more than,say,30 minutes) I tend to feel tired afterwards. I dont think I am alone in that

My rough rule of thumb is that if the backgas deco time exceeds the bottom time then I would much prefer a deco gas.

If I am doing a dive with Helium in the mix and have mandatory deco to do, I generally would bring a deco bottle (50% or 100%) to reduce the gradient on the Helium during deco.

Also, the reserve gas you need gets pretty big pretty quickly if you dont have a deco bottle.

For a 150 dive with a deco bottle, I already have to reserve 64cft, which is 1100psi in my 80's or 100's. If I dont take a deco bottle, then I have to reserve a lot more gas.

At 150 feet, the backgas deco (for the gases I dive) always exceeds the bottom time -- by a factor of 2:1 (i.e. a 30 min dive would be 60 mins BG deco but just 30 mins of deco on 50%)

If I am doing a "MDL" dive on 30/30 or whatever, then I generally wont take a deco bottle.

 

[limeyx]

Or maybe Z-planner, also by Ross Hemingway, which is/was a Buhlmann program.

Ross's comments on the 80% deep stop of 300' dives being erroneously carried over to recreational depths are interesting. He didn't mention yet another factor that tends to make the right depth for 1st deep stop on a recreational dive a lower percentage of bottom ata. On a 300' dive you will be using a low FO2 gas. The higher FO2 gas used on a recreational dive means that your inpired ppN2 on the bottom will be a lower percentage of ATA, which of course moves the optimum deep stop to a shallower %ata.

The output from deco programs show you this, but it's also nice to understand why the program is saying what it does.

I dont think anyone is actually saying to do 1 min stops at 80% of depth for recreational dives.

Pauses can be done (not "30 second" pauses, but literally just coming to a halt and then get moving again) which
1) Helps keep the team together
2) Does not really add any bottom time
3) is good practice if you want to move on to deeper diving
4) slows the ascent rate a little bit

besides, 80%, 65% etc. are all just rules of thumb to keep things reasonably simple while still getting most of the benefits. Better to have one (good enough) approximate rule that you can remember than a bunch of different rules that are more "accurate" easy to forget.

Especially where you are dealing with decompression which is still not really very well understood.

 

[Steve R]

With regards to Jim's profile/deco, it got me thinking. The purpose of carrying deco gas is to reduce total deco, right? If you're carrying plenty of gas, and conditions are pleasant (eg. warm water and something to look at), why not do backgas deco and make your life simpler? Is the answer that the combination of depth and time and necessary reserves very quickly adds up to either very short bottom times or more gas than you can carry on your back? (I would imagine that's the issue.)

You're speaking about higher 02 content gases in which case a lot of times I totally agree with. Like 100ft dives and not a pile of deco over you.

Now try and bring that forward to lower O2 content/deco diving 150-200ft and beyond type stuff and see just how long that dive turns into.

Backgas deco, well for 'semi-real' bottom times anyway, is not workable and is certainly NOT easier.

So yes, you're right on all fronts, it's a combo of depth, (Bottom) time and reserves, along with environmental factors (IE cold-arse diving, weather/sea change etc).

Personally I would think Jim would agree that adding any deco gas of his choice (50, 80, 100) would have given him longer on the wreck for the same TRT.

Or keeping it to his 15min BT, his deco would have been not much more than 1 minutes stops with a slightly longer SS at 15-20ft than your typical recreational dive. (I don't argue a few minutes here or there, it's pointless).

Also if by chance you have to blow some deco off, you stand a better chance having been cleaned-up better.

But again, deco is for he-men, us weenies like to avoid it when possible.

I'm just trying to ascertain the reasoning behind employing the most inefficient deco possible for this particular dive. (B/Gas deco), and where this stuff is coming from so I know to avoid it.

 

[nereas]

This has become a nicely technical and complicated theoretical discussion, even with no sign of Dr Deco anywhere in sight so far!

Deep stops for NDL scuba seem to be simply an optional, additional margin of safety. I proved this for myself by running more and more conservative V-Planner schedules for extended NDL dive plans. As to where you do your deep stops, SteveR's post and references on myths is a great summary.

The deep stop cannot be too deep, or else you are only doing a multi-level dive. Steve's citation points this out.

1/2 MOD works great for me, which is to say, I feel great after my dives, even on days with 4 and 5 dives. We each live in an anecdotal world ourselves, and our own experience, though statistically insignificant, normally carries the most weight with each of us.

Another safety feature that I use, in addition, is one-hour surface intervals. And yet a third is a notion of nitrox as the best mix when blended for the depth I plan to dive, for NDL depths.

So, for NDL diving, if you combine the concepts of Best Mix (nitrox blended for your planned MOD) + deep stops (for a total of 5+ mins on various safety stops) + 1 hour surface intervals, you have maximized your margin of safety for your repetitive diving.

This has become a fascinating thread, with lots of theory on V-Planner presented by several distinguished contributors, which I have enjoyed.

 

[nereas]

... The purpose of carrying deco gas is to reduce total deco, right?

If you're carrying plenty of gas, and conditions are pleasant (eg. warm water and something to look at), why not do backgas deco and make your life simpler?

Is the answer that the combination of depth and time and necessary reserves very quickly adds up to either very short bottom times or more gas than you can carry on your back? (I would imagine that's the issue.)

OK, I cannot resist the deco part of the question, as well.

Yes, TSM, my dear friend, and my favorite MOD, you are right, the purpose of deco gas is to reduce deco time.

But the real issue here is one of torture. Time spend in mid-column underwater is torture. Both mental torture, and if the water is cold, as it usually is, physical torture as well.

And in addition, some tech divers also see it as a safety issue. In other words, the longer you are on deco, the more time there is for something to go wrong.

But for me it is mostly about torture.

So I always bring 2 deco bottles with me, and 3 deco plans for those 2 bottles, in case one of them fails. Some people also calculate what they call back-gas deco, in case their sole bottle fails. But I always have 2 not just 1.

I do not sanction nor support backgas deco, neither as a primary nor as a secondary plan. But to each his/her own. We all need to play together nicely, and be polite and respectful, at all times.

 

[Cacia]

I read it all and then do my best to do what Charlie 99 says, because it is usually understandable.

Nice to see the "torture factor" factored in. Pleasure:effort ratio, I call it.