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!
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!