NDL

[Uncle Pug]

In another thread a problem arose when the diver ran his computer to the NDL and then ended up on the wrong side of the NDL thereby incurring a deco obligation… according to his computer.

In reality there is no NDL… there is no finely drawn line on one side of which you have no need for decompression while on the other side you have a absolute need for decompression.

Every dive is a decompression dive in that you start on-loading inert gas as soon as you descend and start off-loading inert gas when you ascend to a ambient pressure that is less than the pressure of the inert gas in your tissues.

Up to a point this off-gassing can be tolerated without symptomatic DCS. However the point at which this off-gassing becomes problematic is ill defined and many variables come into play beyond just the inert gas load itself. And many of those variables can be and should be managed before, during and after the dive but that is another topic.

To treat the so called NDL as a line which you can approach with impunity as long as you do not cross it is unwise. To use a computer to do so is doubly unwise.

It would be better IMO to gain an understanding of what the NDL represents and manage your dive accordingly. Plan on spending time shallow to offset time spent deeper…. but not just because your computer told you to do a safety stop. Know what you are doing and why.

 

[saying]

I see alot of people knock dive computers as brain numbing. I understand the criticism of people who will only adjust their depth according to what the little digital read-out says without understanding the theory at all. That seems, to me, just ignant and reckless. But to totally discount computers?

I myself am a mathmatical moron. I understand the tables and have decided to work out pressure groups and such after all my dives untill I know better (or indefinitely, if accumulated knowledge so dictates... I dunno) but I know that without a computer there is no way I would trust my own mathmatical ability enough to try a multilevel dive. Not without a waterproof calculator to double and triple check myself, anyway.

I'd much sooner trust a computer to run the equations every two seconds... accounting for exact depth, exact time, even exact temperature and air consumption rate.

Am I dumb? I just worry sometimes that my mathmatic disability puts me in danger... especially when it's time to balance the checkbook; I really wish Suunto would address this next.

 

[wreckchick]

Uncle Pug,

Although I respect you and your experience I take issue with the idea that using computers to push NDL's is more unsafe than to tap the limits of your NDL without one. Both methods are stupid.

A computer is a tool and for a new diver a very valuable one. Since I'm a new diver there are lots of things I don't know intuitively like more experienced divers do. I can't automatically know when I've exceeded ascent rates; I don't have the tables fixed in my brain; and I haven't been taught how to properly plan multi-level dives. This might very well be a fault in the training by most if not all agencies, but this is the world we live in and we need to adjust to that world until a better alternative is presented.

I think my diving is safer for me when I know how fast I'm ascending, and how long I have at any given depth. If I use my computer as a tool and use it conservatively I believe it is safer than trying to make up a dive as I go along and hope I stay within limits.

JMHO

Rachel

 

[Epinephelus]

biscuit7 once bubbled...
I take issue with the idea that using computers to push NDL's is more unsafe than to tap the limits of your NDL without one. Both methods are stupid.

Now let's think about this a bit...
On an absolutely "square" profile, tables and computers are so close to each other that going to the NDL on either is pretty much equivalent, so relative safety isn't an issue. But most dives aren't absolutely "square" profiles, and "riding the zero" on a dive computer will allow considerably more bottom time than the table NDL. Now, which is more unsafe?
Your bottom line conclusion, however - "Both methods are stupid." - is perfectly correct.
E. itajara

 

[Bob3]

I've been preaching about the "hazards" of dive computers for a long time, it's the one piece of gear that can become hazardous when used as intended.
The critters allow a person to load up right to the ragged edge of the NDLs, allowing them to think they're "safe".
The problem becomes more pronounced when on a dive vacation with multiple dives per day.
The most common comment heard at one hyperbaric facility as the chamber door slams shut is "I don't understand it, I was still in the green".

 

[bwerb]

the problem is that far too many people do not really understand what the computer is actually doing. Just reading the numbers without understanding what they mean can cause the user to misinterpret or incorrectly use the data presented and get themselves into real trouble (as in the post UP refers to).

Computers are great IF you really understand the data and the methods being used to generate the data. Too many of us don't take the time to really understand the theory, therefore when we are presented a situation where we are forced back to making decisions based on theory independent of the computer, we fall apart.

Question for everyone...what is your recommended reading for really understanding decompression theory. Are there better or easier to digest resources than others? I realize that I do not know enough myself, I want to do the work and would like your help getting there.

Thanks!

Brian

 

[wreckchick]

I was more referring to the computer's usefulness in giving real time multi-level profile information than a square profile. Of course the computer and the table are going to be roughly identical on a square profile, but a planned multi-level dive can be thrown off by leaps and bounds by seemingly minor variation in the dive plan. This is where the computer can save your butt. If you overstay your deepest depth on a multi-level dive by a couple minutes, you have effectively killed the rest of your dive if you don't recalculate on the fly. Since I don't have the intuition of an Uncle Pug to determine how to adjust the rest of the dive, I would be uncomfortable with trying to do that under the water. With a computer I can get back on track and understand how to finish the dive safely.

R

 

[Raven C]

about crunching the numbers lately due to the referenced post that started people doing the same. Currently, I do not own a computer. I have always worked my dive plan based on tables and concepts taught in a highly debated fundamentals course here on the board that helps you easily keep track of allowable dive limits. After diving with a computer a few times I wanted to learn more about the true benefits of a dive computer.

I decided that for me, I will buy a computer to help document my diving in order to allow me to learn more. I decided on a Sunnto Mosquito for a few reasons, one is the capabilites or features it offers, two the mixes that it keeps track of and three, it is smaller than some others and like many divers, I look for gear I think is best for me but unlike many divers I have to look for "minature" versions of stuff. It's like looking for grown up toys in the Kaybees sizes. Anyway. I use my watch to time my dive and plan according to my tables. However, I needed to come up with some tricks to help me. Because I am dyslexic, yes it sucks, I can't just look at a table. I have to learn and pattern a system that helps. I am not the best person to educate here because I am a new diver only having close to 100 dives over the past year and I only have a few certification trainings under my belt but I have read a lot of material and I would say that learning a system that helps you remember your limits and appling that system is best. Also, I don't trust someone else to completely develop the dive plan. It is important to communicate with your buddy what you feel is the best plan and discuss what your buddy thinks. Never count on your buddy's equipment either. And, remember that something may happen that causes your buddy to deviate from the plan. Stop at that point and make sure that you both agree or don't continue the dive. Your buddy may have gotten confused or narked or something may have caused your buddy to not realize the change or that the change is not the right one for both of you and your dive. I have been known to bring my buddy to the surface because the dive plan is not working in order to straighting out the problem and then continue the dive. I am still learning methods to improve my system and I am still learning about all the important information that goes into improving my dive planning skills. However, I hope that I can learn from the after dive information that my computer can provide me. R

 

[MikeS]

Bob3 once bubbled...
I've been preaching about the "hazards" of dive computers for a long time, it's the one piece of gear that can become hazardous when used as intended.
The critters allow a person to load up right to the ragged edge of the NDLs, allowing them to think they're "safe".
The problem becomes more pronounced when on a dive vacation with multiple dives per day.
The most common comment heard at one hyperbaric facility as the chamber door slams shut is "I don't understand it, I was still in the green".

IMO that is unmitigated BS. A computer doesn’t allow you to do anything! It’s just a tool.

For the most part computers are based on the same algorithms as the tables. , which tool you use to push the limits is a moot pint. The problem that I see with computers is that many divers don’t take the time to learn how they work. If you take the time to understand how your computer works and the theory behind it, computers are inherently safer as the reduce the possibility of human error and allow for adjustment to unforeseen circumstances on the fly.

Mike

 

[MASS-Diver]

UP, in reality there is an NDL, there's just no way to know exactly where it is, right?

I mean at some exact point in my dive I will accumulate enough N2 that if I go straight up I will get bent. From a physisolgical view point the line does exist. But, since we all dive on a bunch of theoretical equations there's no way to know exactly when we have accumualted enough N2/He to give us subclinical bends or an all out hit. So, the line, in reality, is fuzzy for a number of reasons. Persoanlly, I won't ascend directly from any dive deeper than 30' even if I haven't stayed too long.

BTW, any thoughts on Netdocs post about Naui's new saftely stop (rule of halves) - seems alot of people have been doing that for a while and it's nice to see a major agencey adopt it.