AaronR103:
For whatever reason NAUI only taught me square-profile.
That's because the NAUI table is a slightly modified version of the old Navy table and there hasn't been any multilevel work done with it.
OTOH, PADI/DSAT developed a multilevel dive planning tool called the "Wheel". It has a bunch of extra rules for its use above and beyond the closely related square profile table, the PADI RDP. Of particular importance are the slightly reduced no-deco limits of the wheel, also known as the ML limits.
One can use the PADI RDP as described by a post above, but that method can generate dives that are beyond NDL. The PG (which is based upon 60 minute halftime compartment only) will be correct, but some of the faster compartments may exceed the model limits. In other words, in some instances it can generate decompression dives.
On a practical basis, I just go ahead and use the RDP "zero SI" method to approximately plan multilevel dives, knowing that if I approach the NDLs of the RDP closely, I will in fact be going a short ways into deco.
Luckily, one can essentially "unfold" the PADI Wheel and turn it into a table. One version of that is in my photo gallery with the title "Flat Wheel". A simpler version with just 10' depth increments, plus some gas planning aids is attached as an Excel file. One can also just use the PADI RDP with the following restrictions:
1. 3 multilevel levels maximum.
2. Each new level must be shallower than the next, per these rules ----
.. after 120 or 130' level, the next level must be 80' or shallower.
.. after 100/110' level, next level must be 70' or shallower
.. after 80-90' level, next level must be 60' or shallower
.. after 70', must be 50' or shallower
after 50' or 60' level, next level must be 40' or shallower.
3. Any time between levels gets counted as being at the deeper level.
4. Don't exceed the reduced NDL limits, aka "ML" limits
80' 28 min
70' 35 min
60' 46 min
50' 70 min
40' 128 min
35' 190 minutes.
5. If you get within 3 pressure groups of the ML at any level, then 3 minute safety stop becomes a mandatory decompression stop.
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The attached excel file has both an RDP-like / Wheel-like table that is generated using the same deco model upon which the PADI RDP is based. The table entries are +/- 1 minute from the PADI RDP due to differences in how things are rounded up or down.
The SI table is replaced by just one row of numbers, much as on the wheel the SI calculations is just one set of numbers on the back. Take the SI number in your pressure group, add the number of minutes of your upcoming SI to get your "SI from PG Z" and then move to the right as long as your SI from PG Z is greater than the number in SI row. The last PG with SI number lower than your adjusted number is your new PG. (Just like when finding PG from dive depth and time, always round UP on the PG)
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Using those rules and the PADI RDP, one can easily plan the decompression/NDL for a multilevel dive, but that still leaves out one important factor ---- Will you have enough air to do the dive ????
The air consumption table has the ata for each depth, and the ata-minutes or surface equivalent minutes for each depth and for time periods of 10-30 minutes in 5 minute increments. Simply total up the ata-minutes for each level of your multilevel dive to get total ata-minutes for the dive. Then multiply by your SAC to get cubic feet, or even simpler, if you are using an AL80, just use the table that shows you how many ata-minutes you'll get out of an AL80 at various typical SAC rate.
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Caveats ---- the attached excel file is not protected, so if you change formulaes or values, you will get garbage. While I believe the table accurately reflects the DSAT model, errors might still be lurking. The intended use of this is for planning a dive that will then be executed using a computer. If you want the real non-computer planning tool, then go buy the PADI Wheel.
Charlie Allen