Why no accurate computers?

[tursiops]

so now they give more NDL time?

At that depth.
Attached is the current Navy table from Rev7 of the Navy Dive Manual.

 

[scubadada]

I always thought the USN gave 30 minutes at 90 feet?

http://people.uncw.edu/bullockw/index_files/ppts/tables_draft_1.0.pdf

Old tables.

so now they give more NDL time?

2016, sorry, I see @tursiops beat me to it , not uncommon

 

[johndiver999]

Wow thanks.. I see they took away time at 70 and 80 and added time to 60 and 90...

Makes you realize that when someone says people have been diving "the tables" for such a long time with an acceptable safety history... that concept is somewhat flawed if they keep changing the tables themselves.

 

[scubadada]

Hi @Happy Spearo

You've had a lot to digest from this thread. Personally, I would suggest bagging the tables and diving your computer. A computer will nearly always give you more time than tables. Of course, make sure your computer is running correctly, as I posted above.

As has been alluded to, repetitive dives are very important and the way computers deal with them can be tricky. My primary computer, for more than 18 years, has run DSAT. I have been diving Buhlmann, in addition, for more than 4 years, about 800 dives. My DSAT computer is almost always more liberal on the first dive, especially when deeper. Buhlmann becomes the more liberal on repetitive dives, most marked when shallower. I would have no idea about this had I not dived them both, side by side, under a variety of circumstances.

First dive NDLs give you a clue to how a decompression algorithm works, it is easy to get this information. Repetitive dive data is quite rare and hard to come by.

 

[boulderjohn]

If your multi level diving your absorbing less nitrogen so you are technically already being more conservative with your diving and the need for a computer to be conservative on top of that seems to be to limiting of my dive time.

I am not sure you understand multi-level diving. What you said (if I understood it correctly) is true if you do a multi-level dive and stay within the time limit for the maximum depth of that dive. If your table says you have a maximum time of 25 minutes at 90 feet and you do a multi-level dive above that depth for 25 minutes, then, yes, it is more conservative. You are not, however, limited to 25 minutes ona multi-level dive because the computer is constantly recalculating your limits. Many a diver will go over 80 minutes on a multi-level dive like that. With the table, the maximum would still be 25 minutes.

I lose 8 minutes on the 90ft dive. SSI says 25 minutes, computer states 17

As some have pointed out, you are only looking at the first dive, and your numbers are inaccurate--it is 21 minutes, not 17. If the SSI tables are based on the US Navy tables (as they say they are), then I assume you will have a much longer surface interval requirement for your next dive, so if you go in the water when everyone else does, you will have much less allowed bottom time.

 

[ThatDogDontHunt]

I vaguely recall a thread several years ago from someone searching for a "more liberal" computer due to (I think) the OP feeling that they were being cheated of bottom time.

This raises the question: what problem is the OP trying to solve?

There seems to be complaints about their computer as a solution, but only vague handwaving about the real problem.

An easy solution would be: ditch the computer. But that makes NO sense in the real world.

 

[lowwall]

If your table says you have a maximum time of 25 minutes at 90 feet and you do a multi-level dive above that depth for 25 minutes, then, yes, it is more conservative. You are not, however, limited to 25 minutes ona multi-level dive because the computer is constantly recalculating your limits. Many a diver will go over 80 minutes on a multi-level dive like that. With the table, the maximum would still be 25 minutes.

This is the key point and a primary reason computers became so popular. Every second you are above your deepest point is figured into the calculation which then gets reflected into an increase in the NDL for that dive.

From what I've seen, nearly the entire vacation dive industry plans their deeper dives around this. You spend something well under the table limits at max depth and then move up around an atmosphere for another chunk of time and then maybe do it again if the topography allows. That's how we get the typical 50 minutes, 100'/30m vacation dive profile.

 

[boulderjohn]

From what I've seen, nearly the entire vacation dive industry plans their deeper dives around this. You spend something well under the table limits at max depth and then move up around an atmosphere for another chunk of time and then maybe do it again if the topography allows. That's how we get the typical 50 minutes, 100'/30m vacation dive profile.

I think almost all people do this when the topography allows it.

In a place like Cozumel where the coral rises up like massive towers, the DMs go to a maximum depth and slowly work their way up to the top of the reef. When people visit wrecks, they go to the deepest part of the wreck first and then finish near the top--on some wrecks that can be 50 feet or more. In most of my experience in recreational diving, the exception is a flat reef, were there is no option but to swim along at that depth for the entire dive.

 

[lowwall]

I've only been diving for 20 years, so even though we were taught tables in OW, everyone was already using computers and doing extended multilevel dives.

How was this handled in the pre-computer era? Did people dive more square profiles because they didn't get a time benefit for going shallower? I know there was a multilevel dive planner, but we didn't cover it in OW class for whatever reason.

 

[scubadada]

@scubadada, thank you, and I genuinely mean that.

You have just demonstrated a point made earlier in the thread. All tables have rules, they are not necessarily the same table to table. Applying a 'rule' from one table to another, may make the new table you are using invalid.

BSAC tables use max descent rate = 30m/min. Ascent rate 15m/min to the first stop or the ascent check depth.

If you ascend slower than specified, you need to leave the bottom earlier. OR, you are now on the NEXT line of the table. Which if you where on the edge of the no-stop dive (18m, 37min), you now have a decompression stop. Or if it was a decompression dive, you would have additional decompression stops to complete.

Thank you @Gareth J

I descend quite quickly, swimming much of the way, 30 m/min, nearly 100 ft/min is very fast, I usually calculate about 60 ft/min, that is not slow at all. Your ascent rate of nearly 50 ft/min also seems very fast. I rarely exceed my alarm rate of 30 ft/min, and that is quick. It is important to know the parameters of your dive tables. I would suggest learning the parameters of your dive computer and complying with them. I can't imagine any advantage of diving tables over a computer, under any situation.