No Decompression Limit question.

[fisherdvm]

Somehow, 13 years ago, I got through OW thinking that as long as you are in your NDL, you have no risk of getting DCS or the bends.

Now, it seems like there is more to NDL than just shooting up to the surface as fast as you want from any depth.

Apparently, from what I seem to understand, the NDL was created with the assumption that divers will ascend at no faster than 60 ft per minute. And that more rapid ascent will null the NDL calculations. Apparently it has to do with how rapid the bubbles are formed, and how rapid you are able to exhale and decrease the partial pressure of nitrogen in your blood stream.... Kinda like letting the air out of a shook up soda pop bottle slowly, instead of rapidly.

And since some of us are not under the bell curve, the additional safety margin is obtained by the slower than 30 fps ascent rate, the 15 ft safety stop, and some even advocates a extra safety stop at 1/2 of your deepest depth.

Am I correct??

 

[daniel f aleman]

Uhhhh... what was the question?

 

[fisherdvm]

Am I right or wrong in my understanding of the NDL? I think my understanding 13 years ago was wrong - I once thought that ascent rate had no effect on you if you were within the NDL.

Now, I think that you have to ascend at a safe rate to have the NDL accurate.

 

[ianr33]

Now, it seems like there is more to NDL than just shooting up to the surface as fast as you want from any depth.

Am I correct??

Yes.

........................

 

[daniel f aleman]

Why did you ever think that ascent rate had nothing to do with NDL? Generally, IF you use the tables correctly and stay well within NDL, and ascend no faster than 60' per second, you're fine - right there in PADI OW. Now, there are questions of residual nitrogen accumulation over several days of dives, but...

For the record, I ascend at 30' per minute and I do a 15' safety stop as a matter of habit...

 

[Gary D.]

Another thing to remember is that the tables were created around Navy Divers. When you figure in that not everyone is at an ideal weight, in excellent physical condition and young the results from the tables change.

Many people can stay well within the tables or their computers showing NDL and still get bent. Diet is another contributing factor.

It isn’t a cut and dried issue.

Gary D.

 

[Charlie99]

Another way of looking at it is that, for a given ascent rate and stops, that DCS risk increases with depth and bottom time. It very definitely is NOT a clear cut line where you are safe on one side and going to get bent on the other side.

One indication of the fuzziness is to compare the old USN table NDLs with more modern table and computer NDLs. For example USN NDL at 80' is 40 minutes. PADI/DSAT NDL is 30 minutes. Buehlmann model NDL is also about 30 minutes at 80'.

Another indication of the fuzzyness can be found in Bruce Weinke's book, Technical Diving in Depth. Specifically, the table "Nonstop Time Limits for 1% and 5% DCI Probability". (Table 2, Part 8, page 178)

The entries for the 80' row are: 5% DCI probability at 60 minutes bottom time, 1 percent at 15 minutes, USN table 40 minutes. (Ignore for the moment that the actual risk is about 100 to 1000 times lower, the point is that analysis of thousands of dives indicates that going from 15 minute to 60 minute bottom time at 80' increases the DCI risk by only a factor of 5).

Very, very, very fuzzy soft line between NDL and mandatory deco. Much fuzzier than most divers realize.

Going to 80', staying there for 25 minutes and then ascending is only marginally safer than going to 80', staying there 35 minutes (5 minutes past NDL and 4 or 5 minutes of required decompression at 15' per most computers and models). Both 80' 25 minutes and 80' 35' minutes are "very heavy loading" and a prudent diver will do a nice slow ascent and stops in both cases. The flip side of the slow increase in risk is that, in an emergency situation, I'd have no hesitation going directly to the surface at 30fpm or even 60fpm if that was what was needed to assist someone, even if my computer is saying that I have 5 minutes of "mandatory" deco.

======================

Just for completeness, here are the entries for some more depths. Each line is
DEPTH / 5% bend probablity / 1% bends probability / USN Table NDL limit
Depth 5% 1% USN
40' / 170 / 100 / 200 minutes
50' / 120 / 70 / 100 minutes
60' / 80 / 40 / 60 minutes
70' / 80 / 25 / 50 minutes
80' / 60 / 15 / 40 minutes
100' / 50 / 8 / 25 minutes
120' / 40 / 5 / 15 minutes

These are all for a straight 60fpm ascent, no safety stop. It is my personal guess that the very wide variance in 1% vs 5% times at 80' and deeper would be less if the table was showing data for a slower ascent profile, or one that included a 3 minute safety stop. But in any case, even at the shallower depths, you need to extend the bottom time by about a factor of 2 to get 5 times higher DCI risk. Very fuzzy indeed.

Charlie Allen

 

[TSandM]

Lovely post, Charlie.

 

[Matt S.]

I thought the current PADI tables were NOT the Navy tables. In OW class they talked about how they gathered data from a load of sport diver volunteers to create new, more modern tables suited to recreational diving.

> Many people can stay well within the tables or their computers showing NDL and still get bent.

That must be an exaggeration, right? "Many" implies that this is commonplace, which would make the tables useless. I don't read about a lot of people diving within NDL getting bent. That's not proof of anything, but it sure doesn't seem common.

(Lynne, aren't you supposed to be in Mexico right now?)

 

[muddiver]

PADI tables and Navy tables are based on Haldane theory and are empirically derived (i.e. probability calculated from real world data). Only the HUGI tables are non-Navy based. They were done using Doppler ultrasound technology to research silent bubbles in the blood stream after exposure to a hyperbarric environment.