understanding dive tables better

[frndrfoe]

Tables are my backup computer

 

[lowviz]

Tables kick in too late to be of any use as a backup to a DC. Just begin your ascent on a DC failure and breathe everything you have at 15 or 10 feet.

The DC will always be more accurate and liberal than tables. Due to that, if your DC goes paws up at the worst point in your dive, you are totally off any tables.

Tables, however, are the simplest way to "get" gas loading and unloading. Rectangular profile, no complications. Please entertain the OP's question in his quest for understanding.

 

[frndrfoe]

If I lost my computer I would sit out the rest of the day, not ruin a trip over it. I didn't mean continue diving the same day and try to figure it out. Just saying a computer is not required to dive so the tables are not without value.

 

[Diver0001]

Hi, so ive been diving for a little over a year and have quite a few dives under my belt now. however I am starting to do some deeper dives (apx. 100ft) and I would like to have a better understanding of the dive tables. when looking at the SSI tables and just for an example if I dive to 100ft for 20 minutes I would be f group designation. so being an f group with apx. 1 hour surface interval should mean I do my next dive as an e group. now this is where I get a little confused. so for my second dive being an e group if I was to dive to 80 ft does this mean I am limited to a 7 minute dive? any clarification on this matter would be greatly appreciated. I ask this because I am doing 2 wreck dives tomorrow and I want to make sure I'm doing this right.

I don't know how SSI teaches the tables but the concept you seem to be missing is the concept of Residual Nitrogen Time. You can sort of see it as "penalty time" to compensate for the residual nitrogen in your body after the first dive.

The group you have after the surface interval should allow you to find your "penalty time" on the table and this time is subtracted from the NDL for a "clean" dive at this depth in order to get the adjusted NDL for the second dive.

I'm not familiar with SSI tables or I would show you where to look but I'm hoping now that I explained what I think it is that you missed, the coin will drop.

Having said all that, I agree with the guys who said that you should consider getting a computer but probably not for the same reasons they said it. In my opinion tables are quickly losing relevance to recreational diving. Having a computer does not release you from the need to plan a dive, of course, but it is a MUCH better tool than tables for tracking your NDL on the fly (which tables can't do at all). A computer gives you more flexibility during a given dive but also helps you optimize your bottom time so you're getting the most out of your diving. Tables have their uses but they are limited in ways that the computer is not.

R..

 

[KenGordon]

If his first dive is actually not a square profile but multilevel the computer will give him a more realistic NDL for his second dive. The computer and tables are both tools. The tables are just an obsolete one. It seems a little ironic to me that a "rebreather pilot" can preach not relying on technology.

So he SHOULD just get in with a computer and swim about until it tells him his time is up? Without considering the dive beforehand?

Here is someone who is doing the rare thing of thinking about a dive before getting in the water. That should be applauded. Telling him he should follow a computer is the wrong direction. It seems in SB land only "tec" divers plan dives. Then you all wonder why DMs treat you like children.

 

[drrich2]

so being an f group with apx. 1 hour surface interval should mean I do my next dive as an e group. now this is where I get a little confused. so for my second dive being an e group if I was to dive to 80 ft does this mean I am limited to a 7 minute dive?

One use of tables is the ability to quickly assess how residual nitrogen assumed to be in your system can factor into a short surface interval repeat deep (or very long) dive, assuming square profile diving (which wreck diving often approximates). I use a dive computer, and I still keep Air, EAN32 & EAN 36 tables around so I can get a quick look if I wish (seldom do, granted).

That said, as you terminate your dive & ascend shallower, the computer can give you some credit for part of the dive spent shallower (albeit perhaps at the expense of conservatism), and nitrox is really useful for extending dive times.

DTMDanHalen, are you 'square profile' diving (e.g.: wrecks), or mainly multi-level reef diving? Are you a computer diver using tables to get a feel for how depth/time/surface interval interact to give you a shortened 2nd dive time, or are you diving without a computer & relying on tables only?

Richard.

 

[boat sju]

So he SHOULD just get in with a computer and swim about until it tells him his time is up? Without considering the dive beforehand?

Never said anything like that. Even my inexpensive XR-1 tells you your NDL at given depths (dive plan mode) so you can plan your next dive. And the NDL you're given is based on your residual nitrogen load from your previous multi-level dive. Everyone should be taught the concept of nitrogen loading and residual nitrogen load. It's not rocket science and what the computer tells you isn't magic either.

And for what it's worth, in the 200+ dives I've done, I believe four have been lead by a DM.

 

[DTMDanHalen]

Wow alot happened while i was gone. I only wanted to know for knowledge sake. I would always prefer to know excess rather than not enough. I was asking in regards to wreck diving and i do use a computer i use the oceanic oci.

 

[frndrfoe]

Wow alot happened while i was gone. I only wanted to know for knowledge sake. I would always prefer to know excess rather than not enough. I was asking in regards to wreck diving and i do use a computer i use the oceanic oci.

LOL! It's the nature of forums. You say one sentence and people think they know your levels of knowledge, recklessness, and ability better than your mother, usually all are sold short.

 

[TMHeimer]

drrich2 spells everything out nicely, and mentions perhaps less conservatism with computer on square profiles. Computers tell you the "exact" theoretical bottom time you have left. Tables use the max depth for the whole dive, thus are impractical for a real multi-level dive. But even on a square profile, if your max depth reads 80', you weren't there the whole time unless your nose was in the sand every second. Your "average" max depth was probably 77-78'.