As a rec diver, what to do if I breach my computer's NDL???

[TSandM]

One of the reasons why recreational computers can't be used for staged decompression diving is that they don't behave like real deco algorithms. I have read stories of dive computers racking up ridiculous amounts of deco when the diver didn't immediately march to the stop depth and stay there. Some of the computers do not give you any credit for offgassing below the mandatory (shallow) stop depth. Others will do as described, and clear as you ascend. Unless you are quite knowledgeable about the behavior of your own computer, it's unwise to dawdle when you are showing a ceiling. People without decompression training won't know if the 60 minutes of mandatory deco their computer is now showing is reasonable, or whether it's the product of a device that was never designed to do what you are doing with it.

My husband did a dive at the Catalina Dive Park years ago, using his Suunto computer in computer mode. It was the second dive of the day. There were six of us diving together. Of the six, only Peter incurred any deco (even though we had all dived very close together, and Peter and his buddy did EXACTLY the same profile). By the time we got to the stop depth, his computer was showing TWENTY MINUTES of decompression, and nobody else had any. Luckily, he had the gas to sit and wait it out, and he did, because he wanted to use the gauge for the next dive and so he had to make it happy. He knew darned well, as did we all, that he didn't really owe a twenty minute deco obligation. But if you don't have the training and this is what your computer is saying to you, and you realize you don't have the gas to do the time, you are in a world of hurt.

 

[rhwestfall]

I'm totally confused. You were at 135 ft on Nitrox?

28% would work.....

 

[boulderjohn]

I don't think anyone is disagreeing that a recreational diver should not intentionally go into deco. I don't think any is disagreeing that if you do go into deco, you have made a mistake. I am not sure it does a lot of good to simply say, "Don't do it!"

What happened to if you violate your puter, make a safe controlled ascent and you are done for the day?

I don't know what happened to that, because I never knew that notion even existed. Why would you do that instead of doing what the computer told you to do? Why are you done for the day if you have a minor deco violation that clears out during the safety stop?

 

[Tigerman]

28% would work.....

As would 32% if you adhere to the "old" standards of 1.6/1.8 instead of the newer standards of 1.4/1.6..

 

[boulderjohn]

My husband did a dive at the Catalina Dive Park years ago, using his Suunto computer in computer mode. It was the second dive of the day. There were six of us diving together. Of the six, only Peter incurred any deco (even though we had all dived very close together, and Peter and his buddy did EXACTLY the same profile). By the time we got to the stop depth, his computer was showing TWENTY MINUTES of decompression, and nobody else had any. Luckily, he had the gas to sit and wait it out, and he did, because he wanted to use the gauge for the next dive and so he had to make it happy. He knew darned well, as did we all, that he didn't really owe a twenty minute deco obligation. But if you don't have the training and this is what your computer is saying to you, and you realize you don't have the gas to do the time, you are in a world of hurt.

I know Peter has a Cobra, as did I. I had a similar problem once, and I discovered that somehow the physical conservancy setting had been set one step higher than normal. It made an unbelievable difference. (I think that if I had gone up one more setting, it would have said, "Stay in bed.") I had a similar problem with it last year. My regs were set for doubles, and I made a last minute decision to do a single tank dive. I put together a regulator set that included my Suunto Cobra intended to be used as a pressure gauge only. I was relying on a different computer on my wrist for the actual dive details. I was about halfway through a very benign, shallow drift dive in the ocean, nowhere near NDLs, when I realized that the last time I had used that computer was in Colorado. It does not automatically adjust for altitude, so it was still set for high altitude diving. Consequently, even though I knew I was well within limits, it wanted me to do a ridiculous amount of deco. I knew that if I surfaced with that obligation, it would go into error mode, meaning that for 48 hours it would only tell me the basics of the dive, including gas pressure. Well, that was all I wanted it for anyway, so I surfaced.

So the moral of the story is yes, follow your computer, but do have the sense to realize when it is wrong and what you can do about it when it is.

 

[victorzamora]

I don't know what happened to that, because I never knew that notion even existed. Why would you do that instead of doing what the computer told you to do? Why are you done for the day if you have a minor deco violation that clears out during the safety stop?

I think that one reason to "penalize" rec divers who push their NDLs too far by making them take a dry day (or 12 hours, or 6 hours, or whatever) is to further denounce their pushing of limits. If you were a rec-only diver, on a week-long liveaboard in paradise.....and you were pushing your NDL, but had plenty of gas left. If you had been taught to "you'll be fine if you follow your computer".....wouldn't you stay? What if you were taught, "follow your computer to the surface, and then follow it up with a 24-hour SI" instead? If that's what was taught, it might make a lot of divers think twice about breaching NDLs.

 

[scubadada]

Sorry, I don't get all the talk about penalizing recreational divers for purposefully exceeding NDL and going in decompression, see my post #26. So, for a relatively small minority of my dives, I do less than 10 minutes of deco. I make sure I have plenty of backgas to cover the obligation, have a redundant air supply, and, forgot to mention earlier, complete the routine safety stop after the deco stop. I should be penalized or chastised for what?

Good diving, Craig

 

[boulderjohn]

I think that one reason to "penalize" rec divers who push their NDLs too far by making them take a dry day (or 12 hours, or 6 hours, or whatever) is to further denounce their pushing of limits. If you were a rec-only diver, on a week-long liveaboard in paradise.....and you were pushing your NDL, but had plenty of gas left. If you had been taught to "you'll be fine if you follow your computer".....wouldn't you stay? What if you were taught, "follow your computer to the surface, and then follow it up with a 24-hour SI" instead? If that's what was taught, it might make a lot of divers think twice about breaching NDLs.

I think that was the thinking behind the movie Reefer Madness that we were forced to watch in high school to scare us away from the absolute terror of marijuana usage. Of course, when we learned the truth, it made us discount any later warnings about drugs as meaningless scare tactics.

Here is a story about one such scared diver.

I met him on a trip to the Galapagos, where as the only unmatched divers on the trip, we became roommates and dive buddies. One one dive, he suddenly, with no warning or signal, started a rapid ascent to the surface. Feeling something must be wrong, I went after him and caught him half way up. I gave a "What's wrong?" shrug, and he showed me the face of his computer, a quite familiar to me Suunto Viper. His eyes were wide with terror. I didn't see anything of note on the computer. I gave another signal of confusion, but he was on his way up again. I caught him at the safety stop area, and he again showed me his computer, along with that same look of terror. All I saw was a 3 minute safety stop countdown.

On the surface, he explained things. When we were at depth, he saw that he had only 2-3 minutes before going into deco, and he knew it would take him that long to get to the surface. He knew he was facing unimaginable horror if he went into deco, so he began that rapid ascent. He did not look at the computer again until after he showed it to me for the second time. He refused to believe me when I told him that he had something like 45 minutes of NDL when he showed it to me the first time. "Are you trying to tell me that I got more no deco time as I went up?" he asked, as if it showed I was an idiot. I assured him that was exactly what had happened. He dismissed me as a know nothing, and he told everyone else as they got into the panga about his narrow escape from decompression sickness.

Personally, I would prefer that people have a realistic understanding of decompression theory and procedures than have them so terrified of it that they do really stupid things.

 

[victorzamora]

I should be penalized or chastised for what?

You shouldn't be. You're CLEARLY not who any of this is aimed at. You know better. When you see your computer have 2 minutes of NDL left, you don't sprint to the surface. You know better. I think that "penalizing" rec divers is a good thing, because once they get to a point in their diving where they realize it's not necessary they'll be able to make the decision much more safely....as you're doing.

 

[markfm]

...
I usually recommend using DecoChek for students to plan dives on my recreational-level courses. DecoChek isn't free-ware, but the trial version is unlimited duration and has full function down to 40m/130ft - so it's perfect for recreational divers. There is also a free, limited function, mobile phone (java) version.

There's also a good article included in the dive planner documentation: Diving and Decompression - A primer by Mark Ellyatt (at appendix 1) ...

For Android, DecoChek apparently isn't available/working, though I may be missing something.

V-Planner is, however, explicitly available for Android, free (at least a base version). Would this be useable for a neophyte, or is the learning curve too steep unless someone has really gone into the tech side of things?

(Apologies if this was already answered. I had a telecon in the middle of writing this post.)