Hello all, I have a question for you all which I have not been able to find answered yet. For technical dives, I would like some thoughts on using decompression software and gas planning vs. redundant computers. I have done both: For my OW decompression dives I have been able to estimate a max depth and use that as the bottom depth for the entire dive(which is admittedly quite inefficient) and used a 1.5x gas safety margin in allowed gas usage, and for caves I have used the method of redundant computers, where you turn on 1/3rds, have 02 staged and are well in safety margins(for non-trimix diving anyways), where the 1/3rd left over is equivalent to the 1.5 safety margin. My question is, I am currently trying to plan a relatively shallow deco dive in a local quarry, and the profile is a little weird. You descend to 90 feet, then drop down gradually to 105-110 at a shelf, then it slopes down to 120ish +. If you used 120 feet as your bottom depth for the entire bottom time, you get a wildly inefficient result with lots of extra decompression and lots of bottom gas left over that could've been used, so I am talking with my dive buddy about having a couple of worst case scenario dive plans and a planned profile with 120 feet as a single bottom depth on a wrist slate, and then actually follow our redundant Shearwater computers by ascending when we have consumed 2/3 of our total bottom gas supply. I would easily model this as a multi-level profile if the profile was realistically a bit more square, but it is not sadly, so I am not as sure what to do. Any insights would be most helpful!
Decompression plan vs. redundant computers
- Thread starter nradkins
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Call it an unknown profile with a max depth of less than 40m/130ft.
The main constraint is the ascent; how much decompression you need to perform. Thus if you limit the maximum time to surface (TTS), you can calculate how much gas reserves you need.
Then the turnpoint is either the amount of gas remaining OR the maximum TTS
Computers continually calculate the TTS and will give you the ascent profile for decompression. If you're relying upon your computer, you need a backup. Your turn pressure for that TTS will be calculated in advance, i.e. for 30 minutes of TTS (including decompression), you'd use x litres/cubic feet of gas.
Gas usage is shown in MultiDeco (in bailout mode anyway) -- for OC you may want to fiddle with changing to another gas to explicitly calculate what's used. You would set the depth to 40m/130ft.
The above technique is used for bailout scenarios when diving a rebreather, but is perfectly applicable to OC diving.
The main constraint is the ascent; how much decompression you need to perform. Thus if you limit the maximum time to surface (TTS), you can calculate how much gas reserves you need.
Then the turnpoint is either the amount of gas remaining OR the maximum TTS
Computers continually calculate the TTS and will give you the ascent profile for decompression. If you're relying upon your computer, you need a backup. Your turn pressure for that TTS will be calculated in advance, i.e. for 30 minutes of TTS (including decompression), you'd use x litres/cubic feet of gas.
Gas usage is shown in MultiDeco (in bailout mode anyway) -- for OC you may want to fiddle with changing to another gas to explicitly calculate what's used. You would set the depth to 40m/130ft.
The above technique is used for bailout scenarios when diving a rebreather, but is perfectly applicable to OC diving.
ginti
Contributor
I would plan it as a multilevel dive. First part with an average depth of (110-90)/2=100ft, second part with a depth of 120ft (or whatever depth you plan to go). Estimate properly the time spent at each depth.
And having two computers isn't necessarily a bad idea, go for it if you can.
But plan the dive, always
And having two computers isn't necessarily a bad idea, go for it if you can.
But plan the dive, always
PfcAJ
Contributor
Average depth for tables planning.
lermontov
Contributor
lermontov
Contributor
you can do this on multideco by putting in the relevant legs and its pretty realistic -
depending on BT youll do most of the deco at 6m, work out your consumption rate at that depth and set that volume aside ( plus some) so when you get to your turn time you can cross check with your TTS
depending on BT youll do most of the deco at 6m, work out your consumption rate at that depth and set that volume aside ( plus some) so when you get to your turn time you can cross check with your TTS
mr_v
Contributor
Create two plans: average depth and multi-level. Then tell us which one you like better and why.
Spoiler: multi-level plans are more realistic and conservative because the leg where you spend the most time has the most influence on decompression schedule, and the decompression schedule commands your gas needs.
Spoiler: multi-level plans are more realistic and conservative because the leg where you spend the most time has the most influence on decompression schedule, and the decompression schedule commands your gas needs.
My first tech instructor planned everything in the predecessor to Multi deco. He then printed and laminated it to carry on the dive. Always the dive plan, a +5m scenario, a +5min scenario, and a lost gas scenario. He had his computer in gauge mode and just followed the plan.
For me if you get down there and the vis is crap, you don't want the full penalty. Same if the wreck is mediocre, do a lap, when the TTS or the @+5 is exceeding the enjoyment its time to ascend. I'd ascend on my primary knowing I have a redundant computer, the plan is now the worst case scenario.
Your turn pressure is likely higher than 1/3 remaining, its probably a little over half remaining but it does depend on what you're carrying for deco gas and how risk averse you are for lost gas decompression schedule. Do you have enough back gas and deco gas to both get out, based on the worst-case plausible scenario. Be careful with the argument about gas that could have been used. The worst case ascent gas requirements include sharing air with a buddy, that is your turn pressure.
Providing you have the gas covered, riding the TTS and/or @+5 to decide the final dive profile is fine by me.
For me if you get down there and the vis is crap, you don't want the full penalty. Same if the wreck is mediocre, do a lap, when the TTS or the @+5 is exceeding the enjoyment its time to ascend. I'd ascend on my primary knowing I have a redundant computer, the plan is now the worst case scenario.
Your turn pressure is likely higher than 1/3 remaining, its probably a little over half remaining but it does depend on what you're carrying for deco gas and how risk averse you are for lost gas decompression schedule. Do you have enough back gas and deco gas to both get out, based on the worst-case plausible scenario. Be careful with the argument about gas that could have been used. The worst case ascent gas requirements include sharing air with a buddy, that is your turn pressure.
Providing you have the gas covered, riding the TTS and/or @+5 to decide the final dive profile is fine by me.
PfcAJ
Contributor
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