Past NDL. And then this???

[DevonDiver]

I'm saying the other people he was with did not exceed their NDL's and were not bent. He dove the same dives and had over 1/2 hr decompression. The problem was in the computer not in the dives. Get a different computer.

As I mentioned a page ago...

With such an aggressive and extended diving history, there would be so much variance between divers that a comparison of algorithm performance would be near-irrelevant.

I dont care if you object to that statement.

You can't assume anything about the OP based on any other diver, or what their computer/s determined.

The OP didn't state that every diver did every dive together as a group. Even if they did, it isn't feasible to assume that every diver matched each other second-by-second, foot-by-foot, exact descent and ascent speeds, exact stop times excetra excetra....

On the dive in question, the OP did nearly 35 minutes of stop...who can say that he wouldn't have gotten bent if he hadn't?

Joe and Bill didn't get a hit, so it's possible to guarantee Bob wouldn't have gotten a hit either?

DCS is famous for typically presenting in batches and groups is it??

If you read everything the OP has written and have concluded that it's a computer error issue... then you really need to evaluate your most basic concept of diving safety and proper practice.

 

[runsongas]

stepfen, were you diving air or nitrox? if you were diving air, the easiest solution is to dive nitrox next time.

posting your subsurface logs of the trip might also be informative to analyze the discrepancy between subsurface and the cressi

 

[elmo]

Is your computer (or any?) capable of downloading NDL data to Subsurface? Or is is always recalculated?

There is a fine line between displaying enough information as is helpful, and displaying so much that it's unhelpful. The NDL (and deco) information displayed in the profile is calculated by Subsurface, but Subsurface does actually download (through libdivecomputer) and store NDL information from many computers. If you are really interested you can open the Subsurface XML file (probably at C:\Users\<username>\AppData\Roaming\Subsurface\<username>.xml) in a text editor you can find the dive and sample data. If you have a lot of dives, you file will be large, but search for the site name, or date in yyyy-mm-dd format, and you should it soon enough. E.g. from my last dive

<dive number='307' rating='4' visibility='5' divesiteid='1fa607c0' date='2017-05-27' time='10:29:24' duration='52:00 min'>
...
<divecomputer model='Shearwater Petrel 2' deviceid='dc701c49' diveid='8d82b167' date='2017-05-28' time='01:29:24'>
...
<sample time='14:00 min' depth='24.5 m' />
<sample time='14:10 min' depth='24.4 m' ndl='14:00 min' />
<sample time='14:20 min' depth='24.3 m' />
<sample time='14:30 min' depth='24.6 m' ndl='13:00 min' />
<sample time='14:40 min' depth='25.1 m' ndl='12:00 min' />
<sample time='14:50 min' depth='25.1 m' />
<sample time='15:00 min' depth='25.2 m' />
<sample time='15:10 min' depth='25.6 m' ndl='11:00 min' />
<sample time='15:20 min' depth='25.0 m' />
<sample time='15:30 min' depth='24.2 m' ndl='12:00 min' />​

Now just adding my 2c to the topic.
1) Congratulations for sharing your experience and being willing to learn from it. Many divers who exceed their NDLs will pretend it didn't happen, or decide that because they got away with it, it must be ok (normalization of deviance), which is an accident waiting to happen.
2) When diving with a dive computer as a means to stay within your NDL, you must pay attention when you near your NDL, and signal to your team that it's time to ascend. It doesn't matter what you think your buddies' priorities are, that's the rules of no-decompression diving. Someone has to be the one to call the dive, so if you're near or at (or beyond!) your NDL, that person should be you.
3) If you'd rather treat your dive computer as a gauge only, you'd better be following your tables and planning your dive ahead of time. With repetitive multi-level diving, this is a huge pain: you have a computer, you might as well use it.
4) If you are ready and wanting to progress to decompression diving, do it properly. Be prepared with the right training, mindset, planning and redundancy.

 

[Centrals]

Why would nitrox or liberal computer resolve the issue?
Nitrox also has ndl and so is liberal computer.
The main issues:
Why did the OP let his deco obligation gone banana?
Did the OP understand what his computer was telling him? He did not.
He got away with 30 mins of deco because he had gas. But If he was using nitrox or liberal computer, he might run out of GAS because he would had stayed down a lot longer!!!!

OP is not alone on NOT fully understanding what his computer was telling him! I can still remember one case when the DM would not let one of the diver in our group to surface. I only found it out later that the diver had accumulated 13mins of deco in her Sunnto Mosquito computer. So the DM stayed with her all the time during the decompression. We had about 8 divers in the group so I believe the DM would had really hard time to track the ndl of everyone of us.

 

[runsongas]

if he was diving air, nitrox would give an increased ndl so that he would possibly either have not picked up a deco obligation from repetitive dives or decreased an incurred obigation such that he wouldn't have run out of gas at 15 ft trying to clear an extended deco obligation

he clearly understood he had a deco obligation according to his computer, but did not have the gas to clear it.

 

[Centrals]

It has nothing to do with breathing gas or computer in this case!
It was how he reacted when he found himself to have couple of min of deco obligation in the beginning. He understood his deco obligation OK but did not know how to execute it properly. It took him 20min to swim from 20m+ to 4m and by that time the deco had shoot up to 20mins+! WHY? He was talking about his group, but what had they got to do with his deco? If he had kept his eyes on the screen he would had noticed that the deco time was adding up fast.

 

[stepfen]

Wow, lot's of new info to look at.

I'd like to comment in particular about this:

From the OP:
This was a multi-day, aggressive, repetitive dive schedule. Dives were routinely pushed to NDL.

Thanks DevonDiver a lot for your time and effort to transfer your knowledge. The above statement though is plainly wrong. You don't know our diving style you just assume and this particular assumption is plainly wrong. You don't need to "routinely push NDL" to know that your computer is more conservative than others. And we didn't. Here is how I fount out:

As the newest diver of the group (it was my first ever dive vacation - the fellow divers were much more experienced) I was a bit too cautious. Hence, I did notice that while I was getting near NDL everybody else was relaxed, especially during the last dive of the day. Based on that I did discuss it with them and compared times. You don't need to be near or on the NDL to do that. At any moment of the last dive I'd see how much time they had compared to mine. And my computer was giving me consistently lower times. This happened more or less in all of the previous days on the last dive of each day. Based on that, and mainly to stick with the group, I mistakenly decided from the fourth day onward to relax a bit and stay near the NDL a bit more with the known results.

 

[stepfen]

Nitrox is btw the class I am taking next, probably this coming week. Of course I'll discuss all these with my instructor. But in this particular case I don't think that breathing Nitrox would have much to do since I did past NDL.

 

[KenGordon]

To those suggesting a more liberal computer or Nitrox as a solution: this is just papering over the the cracks and would probably make the situation worse, next time he passed an NDL and behaved this way he'd have been down longer and have less gas to get away with the mistake.

OP. Read a good explanation of deco theory. You can download Deco for Divers as an ebook. Understand that computers and tables come with a set of assumptions. In particular an ascent rate. If the computer initially shows a time to surface of 7 minutes that is likely to be 3 minutes of ascent, 1 minute deco stop and 3 minutes safety stop. The 1 minute deco stop is calculated with the assumption that you actually take 3 minutes to reach the stop. If you ascend very slowly to begin with it will be more, on the other hand if you ascend at a reasonable speed to a medium depth and then slow down it MIGHT be less.

Personally I think all divers ought to be trained to handle this kind of deco, that way they have to understand this stuff to pass the course and will experience a deco or simulated deco ascent in controlled circumstances. I don't consider that 'technical' just diving. A PADI entry level Tec (40?) course or TDI Decompression Procedures course will teach this. Or BSAC Sports Diver, or probably some CMAS type course.

 

[Centrals]

Why would you let other computers interfere with your own one? It is none of their business.
I haven't been diving long, ~21 yrs, never ever compare the ndl of my computer with others. What is the point? It had given you false sense of security in this case!!!!

As I had said in my previous post, gas had nothing to do with the whole episode.
1. Dive conservatively and bow to no peer pressure.
2. Learn how deal with deco on dry land.

Happy diving.