Place of dive tables in modern diving (Split from the basic thread)

[Eric Sedletzky]

If you are going with the PADI RDP exclusively, it's simple. Your maximum depth is 100 feet. The maximum time for that is 20 minutes. That time includes the descent, and it ends when you begin your direct ascent to the surface with a safety stop). If you are hanging at 70 feet, it is part of your dive to 100 feet. When you left 100 feet, you were at 15 minutes, so you have 5 minutes from the time you left 100 feet until you begin your ascent to your safety stop. If it took 2 minutes to get to 70 feet, you have 3 minutes left.

That was too easy.
Next time let it soak for a few a little while to get all sorts of crazy answrs

 

[lowviz]

Quite, honestly, Eric:

-I find doing thirds and sixths (sidemount) much harder to do in my head.

 

[TMHeimer]

If you are going with the PADI RDP exclusively, it's simple. Your maximum depth is 100 feet. The maximum time for that is 20 minutes. That time includes the descent, and it ends when you begin your direct ascent to the surface with a safety stop). If you are hanging at 70 feet, it is part of your dive to 100 feet. When you left 100 feet, you were at 15 minutes, so you have 5 minutes from the time you left 100 feet until you begin your ascent to your safety stop. If it took 2 minutes to get to 70 feet, you have 3 minutes left.

Good example of use of RDP for a square profile, even though it wasn't square. And an example of why a DC works better for such a dive. If those times and depths were exact, that would be the only reason I would plan a dive with the eRDPml. But you'd have to know the site, and be pretty darn close to exact depths and times.

 

[lowviz]

That was too easy. ...//...

The best examples begin with those that are too easy.

Is that example something that every OW graduate would get today without thinking? Do they care? Does it matter?

 

[Eric Sedletzky]

This is something that some people who try to outsmart a table would do:

Many wouldn’t consider the descent.
Or if they did they wouldn’t credit the 100’ depth with all the time because it wasn’t all at 100’. Next, after they left 100’ they would try to figure out the 90’ and 80’ percentages and add that to the 10 minutes at 100’ and try to subtract the three of those from the 70’ portion, then use up whatever NDL number is left at 70’ (the actual number on the RDP)
This is basically what computers do
and why they scale the algorithms back. This would be a pretty aggressive dive plan right up to the edge and way off the table, even though it may seem like it makes sense to credit the shallower part of the dive.
I think RD is something like this but then they figure for several stops which somehow makes it all OK.

 

[lowviz]

One doesn't outsmart tables. (you know that) You either choose to follow them or just make stuff up.

Tables, if dived precisely, are very aggressive. But if you play by the rules in real life diving, they rapidly become very conservative as you don't get any credit for downtime spent above your deepest depth. The entire dive is one depth, period. Knowing tables makes this obvious. Worthless knowledge to some, useful for me.

Approximations are fun too. The 120 rule breaks right at the depth you picked. At 110' feet, it is massively conservative and you can't even go to 120' using that rule.

80 cuft tanks are the salvation of clueless divers. You just don't have enough gas to seriously bend yourself on a single dive unless you have a medical condition such as a PFO.

 

[Kevrumbo]

A table question for the experts:

This is for the first dive of the day, using the PADI RDP dive table.

You start your dive and it takes you 5 minutes to descend to 100 fsw. You stay at 100’ for 10 minutes, then you decide to go up to 70’. It takes you two minutes to go from 100’ up to 70’. How long can you stay at 70’ before you hit your NDL and have to go up?

• PADI Wheel RDP indicates 7min NDL remaining at 70fsw for Air breathing gas.

Solution:
100fsw for conservative 17min BT, yields Pressure Group M;
Wheel shows 70fsw or shallower allowed for next multi-level depth;
With Pressure Group M, the NDL for 70fsw is 7min;
Ending Pressure Group of multi-level dive is R;
Total Bottom Time for multi-level 100fsw to 70fsw: 17 + 7 = 24min.

--------

• Conservative Weighted Average of 100fsw and 70fsw -->translates to 90fsw for UTD Minimum Deco Table, which indicates 25min NDL for Air.

--------

The PADI RDP I just found online says the NDL for 100fsw is 20 minutes.

In which case, you arrive at 70' after 17 minutes total dive time and can stay for 3 minutes before you must begin your ascent at 30fpm to your safety stop.

Disclaimer: I've never taken a PADI class, but I believe for NDL diving, PADI, SDI, NOAA, etc. tables all work the same way. Max depth determines NDL and all your dive time counts towards when you have to start your ascent, including the time you spent on initial descent.

If you are going with the PADI RDP exclusively, it's simple. Your maximum depth is 100 feet. The maximum time for that is 20 minutes. That time includes the descent, and it ends when you begin your direct ascent to the surface with a safety stop). If you are hanging at 70 feet, it is part of your dive to 100 feet. When you left 100 feet, you were at 15 minutes, so you have 5 minutes from the time you left 100 feet until you begin your ascent to your safety stop. If it took 2 minutes to get to 70 feet, you have 3 minutes left.

This is something that some people who try to outsmart a table would do:

Many wouldn’t consider the descent.
Or if they did they wouldn’t credit the 100’ depth with all the time because it wasn’t all at 100’. Next, after they left 100’ they would try to figure out the 90’ and 80’ percentages and add that to the 10 minutes at 100’ and try to subtract the three of those from the 70’ portion, then use up whatever NDL number is left at 70’ (the actual number on the RDP)
This is basically what computers do
and why they scale the algorithms back. This would be a pretty aggressive dive plan right up to the edge and way off the table, even though it may seem like it makes sense to credit the shallower part of the dive.
I think RD is something like this but then they figure for several stops which somehow makes it all OK.

 

[lowviz]

@Kevrumbo:

Approximations tend to converge as the max depth becomes shallower. I have no problem with that calculation at OW recreational depths, but its proponents may wish to take such thinking to much bigger dives or on highly repetitive dives.

I do have unanswered concerns there. And they are all beyond the scope of this thread.

 

[Kevrumbo]

@Kevrumbo:

Approximations tend to converge as the max depth becomes shallower. I have no problem with that calculation at OW recreational depths, but its proponents may wish to take such thinking to much bigger dives or on highly repetitive dives.

I do have unanswered concerns there. And they are all beyond the scope of this thread.

Just a FYI, a verbatim advisory note in the old PADI Wheel user's manual p.11:

• Since little is presently known about the physiological effects of multiple dives over multiple days, divers are wise to make fewer dives and limit their exposure toward the end of a multi day dive series (1993, Diving Science & Technology).

 

[AfterDark]

This works on day 1 and/or if you're using DSAT tables and DSAT PDC. Otherwise by day 3 or 4 your PDC should be giving you less BT than the table did -- what's your reaction gonna be? That the table has failed?

All my years diving I've haven't had that experience but; like when Mary-Jo told Ted she might be pregnant he replied we'll cross that bridge when we get to it.