Decompression plan vs. redundant computers

[nradkins]

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!

 

[Wibble]

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.

 

[ginti]

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

 

[PfcAJ]

Average depth for tables planning.