Which tables do I use?

[DaleC]

Pardon me if this is a simpleton question but which tables does DIR advocate (and which depth planning system should I begin looking at for non deco dives)?
I currently use the PADI air and EAN tables but find them frustratingly conservative and hard to gain an accurate assessment from when working certain profiles (like this).
Today's dive:
Shore access Wreck. Direct descent to 100ft. 15min's at depth with a gradual ascent up a sloping seabed for a total of 38min's dive time. Avg. depth 49ft.

I'm not looking to learn deco off the net btw, just wondering which direction I should be looking to next if I want to improve my dive planning/evaluating skills.

 

[jjones9527]

I'm running out the door at the moment, but this should get you started

DIR-diver.com - Using the min deco table

 

[TSandM]

I was not given a table in Fundies. Discussions I have seen on this topic have sometimes said that we were to use the tables we already had. Jjones has given you a link to one table which is sometimes used. In another class, I was given a modified "120 rule" table:

40 - 120
50 - 60
60 - 50
70 - 35
80 - 30
90 - 25
100 - 20
110 - 15
120 - 10
130 - 5

Adapting tables to terrain-based diving is not easy. This is what computers really do best. You can do a degree of depth averaging, if the main bottom time portion of the dive can be defined. You need to understand the implications of the shape of the bottom time profile, and its implications on which compartments are most likely to be loaded (and the impact of prior dives).

Running without a computer on square profiles is pretty easy. Doing it on severely multi-leveled dives requires a fair bit of experience with a variety of profiles.

 

[Rick Inman]

TSandM, just for clarification for the OP, the table you listed assumes 32%? And also assumes a dir ascent profile?

 

[b1gcountry]

Like TS&M said, these are the sorts of dives that computers do really well. You can always take one along and see what its telling you at different points in the dive. Compare that to the profile you are trying to dive. Some let you download your profile, and you can use that to plug in a rough multi-level profile into a program like deco planner or the like.

I think even PADI teaches how to do a multi-level dive with its RDP. I'm sure it teaches it with the wheel. There's not that many profiles you can do for 100' or less. You can probably run off 2-3 multi-level tables that you can use for 90% of your dives. Table 1 might be Xmin at 100', Ymin at 70', then Zmin at 40' or something. You could use this for a number of different dives if you make it your own.

Tom

 

[TSandM]

TSandM, just for clarification for the OP, the table you listed assumes 32%? And also assumes a dir ascent profile?

Thank you, Rick. I should have clarified (the problems with posting in the middle of the night). The table is an air table, and assumes that you will do a minimum deco ascent (i.e. one minute stops from half maximum depth). For 32%, you take a 20% credit -- in other words, 100 feet on 32% is the 80 foot line on the table.

 

[DaleC]

Thanks for the replies all, sorry I didn't respond sooner.

Tom, that's a good point. The problem isn't so much violating the NDL outright but being able to calculate a more accurate profile for planning a repetitive dive etc... I was at max depth for 12 minutes and spent the rest gradually ascending up slope but, when I cut a square profile the PADI way it puts me way beyond the NDL. I do have a profile function on my computer so I could cut a multi level profile (somewhat). Kind of a funny problem if one is trying not to rely on a computer so much.

So, if I get it correctly Lynne, the 120 tables would look like this? (what goes in 40', 50' fields btw):

Air (21%)......Nitrox I (32%)

40 - 120.......40 - ?
50 - 60.........50 - ?
60 - 50.........60 - 120
70 - 35.........70 - 60
80 - 30.........80 - 50
90 - 25.........90 - 35
100 - 20.......100 - 30
110 - 15.......110 - 25
120 - 10.......120 -20
130 - 5........130 -15

 

[TSandM]

Well, you can get rid of the lines for 110 through 130, because a) DIR recommends a working ppO2 of 1.2, which makes the MOD of 32% 4 ATAs, and b) DIR recommends limiting your END to 100 fsw.

For the very shallow depths on 32%, the NDL limit exceeds the time you can spend with the gas on your back, so it's irrelevant to the vast majority of recreational dives. (However, going into deco at very shallow depths is a big deal, because you're saturating slow compartments, and this requires more deco than you would think).

 

[RTodd]

In most locations, for open water, non-mix dives you really only have a handful of different dive profiles and slightly more variations with multiple dives. What I did when learning this is printed out some really agressive 2 or 3 level multi level dives using decoplan with effectively no deco showing in the software if you factor in what you will do for minimum deco anyway. Then, use some really short surface intervals and run your alternative second, third, fourth and even fifth dives. You will find that in most real world open water situations, gas supply will be a bigger limiting factor than no-deco limits on 32% as long as you come off the bottom at reasonable times (and my profiles include 40 minutes @ 90' so reasonable has a wide range.) Also, once you add a 2-3 hour break mid day you can really start over with your dive one and two combos with just slight padding. The important part of this exercise is learning how the buhlman rules work, not the tables. You can use the tables and real world dive profiles to start working out your own rules and where to add safety padding. Many advocate using a computer during this period but personally I think that is a crutch that greatly slows a real understanding of what the models are telling you.

 

[rjack321]

Thanks for the replies all, sorry I didn't respond sooner.

Tom, that's a good point. The problem isn't so much violating the NDL outright but being able to calculate a more accurate profile for planning a repetitive dive etc...

How many repetitive dives are you doing? If its a 2 dive day just take a 90-120min SI and you can basically start over. If its a 5 dive day with 50min SIs you might want to pad some.

In general you are giving yourself quite a bit of EAD credit here. I would back off until you have the experience to make a personal assessment of how well these 32% times work for you.

Air (21%)......Nitrox I (32%)

40 - 120.......40 - beyond reasonable gas supplies
50 - 60.........50 - 120
60 - 50.........60 - [-]120[/-] 70 a fudged number but also not really realistic given normal tanks/comsumption
70 - 35.........70 - [-]60[/-] is pretty aggressive IMO, 50
80 - 30.........80 - [-]50[/-] is pretty aggressive IMO, 40
90 - 25.........90 - 35
100 - 20.......100 - 30
110 - 15.......110 - 25
120 - 10.......120 -20
130 - 5........130 -15