Enriched Air RDP Formulae?

[Skippums]

I am wondering if the formulas are available that allow a person to compile his or her own RDP table for any enriched air mixture. Essentially, I would like to make my own RDP for EANx28, complete with what pressure group I will end up in for the various dive profiles. I know that I could just use the formula to calculate my equivalent air depths, then use the air table, but I should be able to come out a little better if I can calculate it directly without using the air table. Does anyone know of such a formula? Thanks,

-Jeff

 

[Mr Carcharodon]

A couple of good links to get you started:

A collection of Eric Bakers papers
Untitled Document

And

Stuart Morrison’s explanation of calculating deco profiles per Buhlmann.
DIY DECOMPRESSION
This sounds more like what you are after. The approach is to calculate the pressure of 16 (or 17) tissue compartments based on different time constants for each compartment, ambient pressure, gas mix and exposure time. On ascent the tissue gradients are kept below a maximum value (m-value). There are no letter groups per se. But you can calculate tissue pressure as function of time at the surface. And then treat repetitive dives as one long dive with outgassing considered for the surface interval. NDLs are rather easily handled. Do know that the PADI RDP is more conservative than Buhlman, but it has not been shown that it is more efficient or safe.

Of course there are number of ready to go programs out there like VPM and Decoplanner.

 

[Diver0001]

I am wondering if the formulas are available that allow a person to compile his or her own RDP table for any enriched air mixture. Essentially, I would like to make my own RDP for EANx28, complete with what pressure group I will end up in for the various dive profiles. I know that I could just use the formula to calculate my equivalent air depths, then use the air table, but I should be able to come out a little better if I can calculate it directly without using the air table. Does anyone know of such a formula? Thanks,

-Jeff

The forumulas are in your nitrox book. You can convert any percentage of Nitrox to work with the air table using the "equivalent air depth" calcuation.

R..

 

[DA Aquamaster]

Jeff acknowledged that but expressed that he wanted to actually generate his own table for a specific mix as opposed to just using equivalent air depths.

On the other hand, I entirely agree with you - you could do exactly that with equivalent air depths to generate the "new" table.

 

[Diver0001]

Yeah. He's a software engineer. It should be child's play for him to whip up a spreadsheet that uses the EAD to make any table he wants. I'm not a software engineer (or haven't been for many years) and I recall whipping up exactly that in a couple of hours once just to see how hard it would be. The worst part about it is copying all of the values from the RDP into excel by hand and double checking it. If he's any good with VBA he can probably do it faster than I did.

R..

p.s. just checked. I don't have it anymore. sorry.

 

[redacted]

While I recall a prohibition on using interpolation for odd depths with tables, I'm fairly sure that is probably because the function is not linear (and maybe we are deemed to stupid to do such advanced arithmetic). But I suspect one could do a little curve fitting to allow more precise interpolation and apply EAD to get reasonably accurate results for any O2 percentage in the recreational range.

 

[h90]

Yeah. He's a software engineer. It should be child's play for him to whip up a spreadsheet that uses the EAD to make any table he wants. I'm not a software engineer (or haven't been for many years) and I recall whipping up exactly that in a couple of hours once just to see how hard it would be. The worst part about it is copying all of the values from the RDP into excel by hand and double checking it. If he's any good with VBA he can probably do it faster than I did.

R..

p.s. just checked. I don't have it anymore. sorry.

but what is the point of it? Tables for Nitrox exist and can be downloaded for free from the internet. (If I am not wrong in some countries it is not allowed to copy the PADI tables but in others it is complete OK for educational or private use). So why develop it new if it is already available.

 

[Diver0001]

for the fun!

 

[Skippums]

"The forumulas are in your nitrox book. You can convert any percentage of Nitrox to work with the air table using the 'equivalent air depth' calcuation."

Thank you for the suggestion, but what I am really attempting to do is to eliminate the rounding error when using the EAD with the Air table. So, for example, if I get an EAD of 51 feet, the rule is to round to 60 feet. However, this only approximates the actual duration that would put a person in a given pressure group, since you are using values for a depth 9 feet below your actual. What I am attempting to do is to take out the rounding when going to the lookup table.

 

[Skippums]

Yeah. He's a software engineer. It should be child's play for him to whip up a spreadsheet that uses the EAD to make any table he wants. I'm not a software engineer (or haven't been for many years) and I recall whipping up exactly that in a couple of hours once just to see how hard it would be. The worst part about it is copying all of the values from the RDP into excel by hand and double checking it. If he's any good with VBA he can probably do it faster than I did.

R..

p.s. just checked. I don't have it anymore. sorry.

Yeah, I was really excited with the data from the first response... a link that includes a C implementation of the formula? Jackpot! I know what I am doing next weekend [sigh].