Bent in Cozumel

[rab]

Foo,
The highly oversimplified answer is that a multi-level dive is like a series of dives with a 0:00 surface interval. Look at your table and see what a 1min@100' + 0:00 SI gives as your pressure group and then what the NDL time for a dive to, say, 70' would be. (OK, even without a table in front of me that's not going to allow another 54:00, but you hopefully see the point.)
-Rob

 

[gj62]

Foo,

Another thing to consider is that the dive profile may have included (in fact, almost certainly) included a very short time at 100'. The rest of the dive was spent at various depths above 100' for various times. This is where a dive computer helps to track a multi-level dive and take into account these variations. For the average rec diver, this will help you extend your times, but does allow you to "ride the NDL" much closer than you would using square profiles and tables.

 

[Foo]

So, it's like a proportion thing? Individualized for each minute that a person is in the water? For example: 35 ft/20 seconds; 36 ft/15 seconds; 37 ft/17 seconds; 36 ft/5 seconds; 37 ft/3 seconds, etc? So that it's supposed to monitor an adjustment of the maximum time allowable? Evidently I didn't really know what a dive computer's function is, but it makes a bit more sense now. But are the computers more "liberal" than the square tables? Because, my point was that even if the person stayed at exactly 60 ft the entire time, 55 min is the maximum no decompression limit, according to this table- and that's for the first dive, much less the multiple ones. Hope this is not considered off-track for this thread, but I'm sure I'm probably not the only newbie out there that's a bit confused about dive times/SI, etc., and that does seem to relate directly to this thread- since we all wish to avoid DCI.
Foo

 

[DoUDive2]

I'm going to Coz at the end of June, we'll have several certified teens with us (my daughter included) and I don't want to have a similar story to report when we return home. Or is this just a more advanced type dive that I know nothing about? Thanks for any help you can give!
Foo

Hi Foo,
For a square profile, tables and computers should give the same results. Therefore computers are not "more liberal" than tables. However, most rec dives are multi-level (not square). The table method will use the max depth, while the computer will use the depth profile. Therefore, in a multi-level dive, the table results will be more conservative than the computer.

While this thread has, at times, made scuba diving sound like a high tech version of Russian Roulette, I don't believe that that is the intention and I certainly don't believe that such a view is accurate. The importance of this thread can be summarized into a few good points: 1) the symptoms of decompression sickness (DCS) may, at first, seem benign, but can escalate and become serious over time. 2) We should seek help promptly when we notice medical effects that are not normal or beyond our experiance. 3) There is a tendency to ignore DCS symptoms when they occur -- especially when we don't have adequate insurance to cover the high cost of treatment. And 4) we have learned that when we are willing to describe the problems that we have had, everyone can learn from our experiences.

I see nothing wrong with the way that you are applying the tables. If you apply your certified Open Water (OW) training (your knowledge of the tables, staying hydrated, the hours needed between diving to flying, etc), the chances of being hit with DCS are extremely remote. Go to Coz. Have a great time.

Chris

 

[Foo]

Thanks, Chris- I can hardly wait! And to Tim, I am so glad that you are recovering from your accident. And I thank you for this thread, and your efforts to help others. I am so glad I found this board. It has already made diving so much more pleasurable.
Foo

 

[divematt]

Foo,

Some of the dive had to be above 60 feet to get down to that depth, and back from down there, to surface slowly, as well as a safety stop at or around 15-20 feet for several minutes. That was also calculated into the computer.

 

[Foo]

Quote: Some of the dive had to be above 60 feet to get down to that depth, and back from down there, to surface slowly, as well as a safety stop at or around 15-20 feet for several minutes. That was also calculated into the computer.

Yes, divematt- thanks. That makes sense, and I can see how the proportion of time/depth works, better, now. The computer takes into account the time spent at 1 ft; 2 ft; 3 ft; etc., whereas the table figures are bottom time and don't make allowances for descent/ascent. Now I understand what the phrase "ride the NDL" means. Being a math teacher, I took the tables to heart, and I really didn't understand how a person could dive that profile and still stay safe. It just didn't add up. Obviously, the other people in the group didn't have problems. I do think that I will still consult my dive tables, especially if a plan doesn't feel right.

The chances for DCI seem to have a lot of variables, some of which are within a person's control, and a some that are not. I'm sure that Tim will always wonder exactly why it happened to him- and he has tried to factor in the variables to understand better. Retrospect. I do appreciate him for sharing his story, as I have learned from it and from you all. Thanks, you rock!
Foo

 

[gj62]

Foo - DAN statistics show that a diver will experience some form of DCS 1 in 28,000 dives while staying within table or computer limits. So yes, it is something you should be aware of. That said, be safe and cautious and you likely will not be part of that statistic. You've taken a bigger step than many divers do, simply by becoming more aware of this. Plus, as you start to dive more, you'll realize that for most rec divers, a square profile doesn't exist for most of your dives. If you look at the graph on the PC in this pic, you can see what many profiles look like. Depth is on the vertical axis, time is on the right ("Distance" is not graphed). This diver descended to the deepest depth, didn't stay very long, then slowly dove along a ridge (going up and down some) to get to where a final direct ascent was made at the end of a dive. I realize it is difficult to see, but I couldn't find a clearer image... http://www.sea-quest.com/products/pc_interface.html

 

[Otter]

Ok thats it, I am doing 27,999 dives and no more!

 

[Texas Diver]

Ok thats it, I am doing 27,999 dives and no more!

Otter:

I like the way you think. But I am adding just a tad more cushion by only doing 27,998 dives before I retire. Safety first!