During my recent Advanced Open Water certification, we did a multi-level dive, and charted it using all 3 devices: RDP, eRDPml, and dive computer.
- According to the RDP, I should have been dead.
- The eRDPml permitted the dive, made a lot of sense to me, giving me dive groups at each stage.
- The dive computer permitted the dive, but gave no dive groups.
So I liked the eRDPml best, but I can see that it's not as accurate as the dive computer in figuring out your precise no-decompression limit.
How do you log your dive if you used a dive computer? Do you just leave off the dive groups from the log book?
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Matt Beckwith: Although the discussion has gone off on several tangents, I'll try to address some of your initial questions...
All of the modern dive computers which I've encountered automatically track nitrogen loading in the post-dive interval. As the surface interval increases, the diver should note that the NDLs for subsequent dives will also increase...up to a certain level.
Entering the "dive plan" mode during a surface interval will reveal allowed NDLs for a given depth on the next dive. At most dive destinations, the DM/boat captain will plan for roughly a one-hour surface interval. Before splashing in, divers will take note of the NDLs for the planned max depth of the upcoming dive. During the dive, people will watch their dive computers and make sure that, whatever profile they adopt, NDLs do not go past zero (i.e., enter "deco" mode). In practice, this is the extent of dive planning among the vast majority of recreational divers.
Lots of newbies ask how to log the pressure group for a dive conducted while using a dive computer. This question comes up because the PADI dive log is geared toward paper-based logging of dives conducted with dive tables. There's a special spot on each dive log page to enter the "pressure group." When making the transition to a dive computer, the novice encounters a head-scratching moment since most computers don't spit out a PADI-style pressure group after a dive.
As others have mentioned, the purpose of the pressure group is to track nitrogen loading. Since the dive computer is already automatically tracking your nitrogen loading, you'll be "OK" so long as: (1) you use the same computer on subsequent dives, (2) the computer continues to function properly, and (3) you do not violate the NDLs on subsequent dives.
If you feel so inclined, you can certainly "back out" table-equivalent pressure groups from your dive computer. No expensive software programs or mental gymnastics are needed.
Simply enter "dive plan" mode
immediately after surfacing from a given dive.
You will see NDLs represented for various depths on subsequent dives. Take note of these numbers.
Cross-reference these numbers with the numbers on a dive table to come up with a pressure group.
For example, let's say you just conducted a computer-based, multi-level dive to a max-depth of 70 fsw with a run time of 60 minutes.
(N.B.: The standard PADI RDP tables wouldn't allow such a dive since a square profile is assumed.)
Upon exiting the water, you enter "dive plan" mode and see that you are given the following NDLs/depths: 67 min at 40 fsw, 28 min at 50 fsw, 14 min at 60 fsw, and 6 min at 70 fsw.
If you reference the back side of your PADI tables, you'll see that this series of NDLs/depths is roughly equivalent to pressure group Q.
Theoretically, you could use this pressure group to approximate the nitrogen loading "credit" you would earn during the surface interval. You would note that after a one hour surface interval, your pressure group would be F.