Planning multi-level deco dive?

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stuartv

Seeking the Light
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This question is for those of you who do deco dives using a dive computer as your primary in-water tool for planning your ascent. I.e. if you use a written plan, it's your backup to your computer.

Short version:

You plan your dive ahead of time using Multi-Deco or similar desktop software. You plan it as a square profile and determine your bottom time to be 30 minutes. Then you go do the dive and the reality is that you descend to your planned max depth, spend a little time there, then start working your way up (because the dive site has a lot of vertical relief). The reality of the dive is that you could stay on the site for a lot longer than the originally planned 30 minutes without exceeding your target deco parameters. If you started your ascent after 30 minutes, you would end up getting out with a LOT more back gas and deco gas left over than what your plan called for (not counting the planned reserves). I.e. your plan was to exit with 1/3 of your back gas and 1/2 your deco gas left, but if you begin your ascent after 30 minutes, you're going to end up with closer to 2/3 back gas and 3/4 deco gas left.

The question is: How do you deal with this? Do you begin your ascent at 30 minutes, no matter what? Or do you extend and, if so, how do you determine how long you can extend? Do you base it on the TTS shown on your computer?

Or, do you plan your dive differently in the first place? If so, what software do you use and how do you build the plan?

If it makes a difference to your response, assume you're diving with 2 Shearwater Perdix computers.

Long version (as if that wasn't long enough!):

I'm tentatively planning to dive the Oriskany at Thanksgiving. So, I started playing around with potential dive plans, just to get some idea of what I might look forward to. The flight deck is at 145'. The top of the island is at 85'. So, was thinking of a plan to drop to the flight deck, go over the side, and have a look into the hangar deck, then back up to the flight deck, over to the island, and gradually work my up.

I ran a plan for 160' for 30 minutes with 21/20 for back gas and 100% for deco. That gives me a plan that, with my settings for SAC, GF, and my tank sizes, looks very comfortable in terms of gas reserves and run time.

But, in reality, I'm not going to spend 30 minutes at 160. It might be 5 or 10 minutes and then, basically, a long, slow ascent from there up to 85', exploring the island. Then my actual ascent from 85' to the surface. For that, I think I ought to be able to get much more bottom time than 30 minutes and still not exceed the limits of the gas I can carry in my tanks (back mount double 120s plus a single AL40 for deco).

I have been using Multi-Deco for dive planning. AFAIK, it does not give you any simple way to plan a long, slowly ascending leg of your dive. I.e. there's no way to tell it "ascend from 145 to 85 over a period of 20 minutes."

OTOH, Subsurface does let me put a leg like that. If the previous leg was at 145' and I put in a leg of 85' for 20 minutes, it actually plans it out as a 20 minute ascent from 145 to 85. But, I'm not aware of very many people using Subsurface for deco dive planning, so I really would like to know how most people plan this kind of dive. Do you put in a bunch of short, gradually shallower legs to simulate a long, slow ascent?

Do you just plan it as 160' for 30 minutes and then begin your ascent after 30 minutes, period? If you say "I begin my ascent, but I make my ascent slower, so I can check out the island on the way up," then my question is okay, but in that case, the slower ascent means you're using more gas than you planned for (maybe) and increasing your deco obligation (maybe) by being deeper than planned for during that period from the 30 minute mark to the time you get to your first deco stop, right?. So, how do you know you won't end up, for example, needing more deco gas than you have? And by that, I mean, say, you have an AL40 of 100%. Your planned called for you to use 20 ft of that. That leaves the other half for your buddy in case he loses his deco gas. So, if you extend your ascent to check out the island, you could end up needing, say 22 ft of deco gas. Well, what if you do that and then your buddy loses his deco gas and now you're having to share?

I'm not saying it couldn't be dealt with. I'm just wondering how to plan it so that, one, you don't end up getting out with a wasteful amount of gas left over (beyond your planned reserves), and, two, you ensure that if everything goes according to plan but you do have a contingency emergency (that you planned for) that you know you'll still be okay?

Thanks for any insight!
 
An easy solution would be to simply plug two levels in for "bottom time." One level would be 160ft for 15 minutes (or whatever) and the other would be, in this example ~35 minutes at 100ft. I'd keep that second level lower as it'd allow a decent average depth. You could then print out your plan for 160ft/30min, that two-level plan, and then a deeper/longer scenario plus lost gas.

The other thing is you could just pick the longer deco schedule, find the total deco time, note it down, and just follow the longer deco schedule if a computer died. That way you have one plan that's flexible. If your computer ever reads a first stop deeper than your first planned stop or a TTS longer than your planned deco schedule, that's your cue to thumb it. At any point where your computer is giving you the same or shorter TTS and first stop at the same or shallower depth as your deco schedule you'll be safe to ascend following your planned schedule. If your computer is giving you a longer TTS or deeper first stop, you should have already ascended to make that not so. Add into that a MinGas calculation and it's kinda "flying" the computer, but with a fairly strict turn criteria.
 
so I would plan to ride my computer, but my contingency would be based on the theoretical gas reserves. I.e. if I plan for a third of backgas at the bottom, then how many minutes does that equate to at 160 and set that. Call it 15'@160, then I'm comfortable going another third at slightly deeper than 85, so say 35'@100. That would give me my deco schedule and I'd make sure I had enough deco for that. If I didn't, I would start lopping time off and go to 10/160 and 30/100 and see if that works. Once it does, it's good enough. What we have to remember is that deco is still measure with micrometer, mark with chalk, and cut with an axe, so everything is just "close enough". You have a worst case deco scenario with the contingency plan, and then ride the computer based on what you are comfortable with as long as it doesn't exceed the limits you set on your written plan
 
I would like to add that I always planned my OC dives with lost deco gas. Downside, always came back with a lot more gas then I needed, upside, always had more gas then I needed. If my buddy lost deco gas, we would just have a longer deco.
 
What Victor said. You can easily plan a multi-level dive using the Multi-Deco. Just put in the different depths you want to dive, in whatever order you want to dive them. It would be pretty simple to plan two scenarios; one as a square profile, the other as a multi-level and put both of those plans on your slate.
 
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another possibility would be to plan for rock bottom gas at various depths then turn at the pressure / computer / tables whichever is most limiting.

Your deco gas stops after this would all be similar, so would need to calculate only for backgas. A cheat sheet of your SAC vs depth to get a psi/min rate for various depths should give you a good idea of what gas reserves you need.

The Perdix has a 5 min look-ahead TTS which is also pretty sweet
 
Yeah, I'd do pretty much want the others said, but would plan for some time at (say) three levels (160, 145, 100), then stay within those constraints while using the computer to tell me where I'm really at. You want to be able to fall back to the tables if necessary and know you have at least that much gas available. By the way, the O is a good dive to use a 40 of 50% and another 40 of 100%. Play with a two-deco-gas scenario in your planning....
 
I understand what you guys are saying, but I don't "get" the answer to my question.

If I'm diving with an AL40 of 100% for deco, then I take 20 cu-ft of deco gas as my "hard" limit on consumption, for planning purposes. The rest is reserved for my buddy. So, assume I do whatever plan comes out to using exactly 20 cu-ft during deco.

If I start flying my computer, then I could get into a scenario where the computer says my TTS is some number that looks okay, but it is now based on a shorter amount of time to get to 20' (say, because I'm now at a shallower depth than where my "planned" ascent started, and then a longer time at 20'. So, I'm now using more deco gas and not leaving enough for my buddy (if he loses his deco). Or it might not be as simple as that. My actual profile may result in tissue compartments being loaded differently than "as planned" and result in an ascent time with a different distribution (because of different controlling compartments) that results in less time to get to 20' and more time at 20' than I planned for - which means using an unacceptable amount of deco gas (i.e. cutting into my buddy's gas).

Because the popular computers (e.g. Shearwaters) don't know how much gas you're carrying and what your SAC is, they can't tell if you will be able to do your ascent with the gas you are carrying. So, flying the computer means the diver can't really know for sure that they have enough. The actual profile that is calculated on the fly could be a different shape than what was calculated on the desktop ahead of time. It could be shaped to give more or longer stops deeper, requiring more back gas. Or it could be shaped to have more time shallower, so you have excess back gas but not enough deco gas. The diver is essentially just making a somewhat educated guess that the currently calculated ascent profile "shape" is close enough to the planned profile that the gas requirements will still be within acceptable margins.

That statement is based on planning a multi-level dive and where the levels are not used as the max depth for each leg and adhered to by the diver. IOW, if you plan, say, 160' for x minutes, then 100' for y minutes, and you dive so that you don't exceed 160' OR x minutes for the first leg, and then you don't exceed 100' OR y minutes for the second leg, then you could be confident that the planned gas will be enough.

But, it appears to be to be the case that if you plan a multi-level, the first leg would typically be a hard bottom limit and time limit. But, after that, if the subsequent legs are treated as sort of "average" depths that you stay around as you ascend, then your ups and downs (particularly being on the deeper side of the average target in the later part of the leg) could easily result in a longer deco than planned or a different "shape" to the required ascent.

Regardless, I gather there is no well-defined procedure for how to do this. You plan as close to your expected "reality" as you can, leave yourself a good safety margin, and then make sure you don't exceed the planned parameters, fuzzy as they may be, and apply a healthy dose of conservative judgment as you "fly" the computer.

Thanks for the replies!
 
I have been using Multi-Deco for dive planning. AFAIK, it does not give you any simple way to plan a long, slowly ascending leg of your dive. I.e. there's no way to tell it "ascend from 145 to 85 over a period of 20 minutes."

Yes there is, it's 115ft for 45mins.
 
And by that, I mean, say, you have an AL40 of 100%. Your planned called for you to use 20 ft of that. That leaves the other half for your buddy in case he loses his deco gas. So, if you extend your ascent to check out the island, you could end up needing, say 22 ft of deco gas. Well, what if you do that and then your buddy loses his deco gas and now you're having to share?

For deco gas planning, I don't sweat a couple of ft3 difference. I plan to have roughly enough deco gas for both myself and my buddy. But if my planning shows a few ft3 short, I just make sure that I could also have enough back gas to switch to and finish out our decompression. Of course this is not ideal, but if your buddy had an OOG situation and you both finish off your 40, then your plan should put you fairly close to the end of your deco. If you had to switch to your back gas, you should have enough to spend another 5-10 minutes on that to finish out your deco with a little safety margin. If you're pushing up to the edge of not having enough deco gas and also not having some back gas in reserve, then you just need to dial back the plan a bit.
 
https://www.shearwater.com/products/swift/
http://cavediveflorida.com/Rum_House.htm

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