What makes us think we can trust any of them

[Potapko]

After reading this article: http://www.scubadiving.com/gear/dive_computers/crunching_the_numbers/0/

I begin to understand why the DIR folks keep telling people not to dive according to their computer. The difference in remaining ndl time between the tested computers on a given profile was as great as 66 minutes. That's nuts!

I need some training on how to calculate multilevel profiles and change my computer to Guage mode.

I'm getting closer, someone pass me the coolaid! :14:

 

[halocline]

There are lots of threads about the differences between various computer algorithms; I did some research about it before I bought my last computer. You might search.

The oversimplified bottom line is, to the best of my knowledge, fear not! They're all safe for single tank recreational depth diving. You're much more likely to get DCS from dehydration, poor ascent control, gas management problems (forcing a fast ascent) exertion immediately after diving, or any other contributing factor other than computer algorithm. If you get into doubles and/or deep diving, all bets are off and I can totally understand why those guys want to plan their dives much more closely.

NDL differences are a little confusing; for example, if you have a haldanean (like oceanic) computer, it will really add on lots of NDL towards the end of your dive as you ascend. But, if you look at the nitrogen loading bar, it might still be in the yellow. While there is an apparent conflict between these two, remember this is the end of a single tank dive and you'd never have the gas for that 66 min NDL anyway.

One thing I recently read in another thread is that the suunto computers will penalize you for faster ascents, I'm not talking about the ascent alarm, but if you take less time than it likes to get up, even under the 30fpm limit, it will cost you some NDL on your next dive, depending on surface time of course. I don't know if the oceanic computers do that. I just keep in mind that all the algorithms are just theories and use common sense when diving; no reverse profiles, nice slow ascents, added time at safety stops when there's enough gas (almost never an issue) and very slow ascent from the safety stop. Sure hope that approach works!

 

[drbill]

I think it is wise to never depend entirely on your computer. However, by selecting a reasonably conservative computer and diving IT conservatively even on repetitive dives, I think you have a good safety margin built in. I do not worry about diving my Uwatec Aladin Pro, although I tend to add a little to my required deco stops, etc., just to add extra safety. No DCS incidents in 45 years of diving... so far.

 

[TSandM]

One of the things that I was surprised to learn when I started diving was how completely theoretical the models are for gas loading and offgassing. For example, most algorithms assume a linear progression of gas from the fast compartments into the slow ones. One algorithm (that I know of) assumes all compartments are loading simultaneously. Physiology says that something between the two is probably true -- gas has to load into the bloodstream, but then is distributed fairly simultaneously to all the tissues of the body.

But remember the tables are based on the same algorithms the computers are using . . . and even if you're planning dives with decoplanner, you're using an algorithm. The diver has to make a choice of what kind of model he is going to depend upon, which is why it's good to read about decompression theory and where the different models came from, what empirical testing has been done to validate them, what endpoints were used to determine their validity, and so on.

 

[Web Monkey]

Tables will give you the same NDLs as a computer that uses the same algorithms and data as the table was derived from.

Complaining about your computer's NDL is like complaining that you don't like your income tax done using a computer and would rather use tax tables and a pencil. If you do it the same, the answer will be the same.

Just as there are different dive computers, there are also different tables, and you would still need to choose which one you're going to use and how conservative you want to be.

The difference isn't computers vs. tables, but different methods of calculating nitrohgen loading and O2 tox. Whether a computer is on your wrist or not, they both rely on math.

Terry

After reading this article: http://www.scubadiving.com/gear/dive_computers/crunching_the_numbers/0/

I begin to understand why the DIR folks keep telling people not to dive according to their computer. The difference in remaining ndl time between the tested computers on a given profile was as great as 66 minutes. That's nuts!

I need some training on how to calculate multilevel profiles and change my computer to Guage mode.

I'm getting closer, someone pass me the coolaid! :14:

 

[Snowbear]

The oversimplified bottom line is, to the best of my knowledge, fear not! They're all safe for single tank recreational depth diving.
...
If you get into doubles and/or deep diving, all bets are off and I can totally understand why those guys want to plan their dives much more closely.

Although she was very fit, diving nitrox (32%), compulsive about staying hydrated and about staying within all of her conservative computer's NDL and ascent rate, there was a woman on this trip who got a case of mammary bends. She was (rightly!) very concerned about this and as she knew I was a paramedic with some dive medicine training, she asked me about it and asked what she could possibly be doing wrong.

I asked a few questions about her dive profiles. She said she never got within 5 minutes of using up her NDL and never let her ascent alarm (>30fpm) sound. She always did her 3 minute safety stop and was reasonable able to hold her depth while doing it.

I suggested a couple of things she could try on the order of running her own dive rather than having the computer run it for her. She did those and both she and her buddy (hubby) were amazed at how much better they felt after the dives and especially at the end of a long day of diving... aka no more fatigue, "niggles" or rashes

For the rest of the trip they kept asking a lot more questions about how I dive and why I do some of the things I do (there were a group of 4 other DIR trained divers on the trip as well, but in the words of this couple, with the exception of the LDS owner who is working on becoming a DIRf instructor, they would get a bit "preachy"). They said another difference was that I looked like I was truly having fun in the water where some of the others looked like they were mostly concerned with following procedure. To me, that was the greatest compliment they could have given me about my diving

 

[drbill]

Mammary bends? Am I in danger of getting them?

Seriously, does this really occur and what are the symptoms? I've never heard of it and dive almost exclusively with female buddies if I'm not solo.

 

[halocline]

Oh man, I know this is a serious topic, but the opportunity for tasteless jokes here is tough to resist.

Okay, okay, certainly people get DCS in unexplainable situations, (about 1 in 23,000 dives, I think DAN stats show) but my point is that in rec diving it's unlikely that one computer algorithm vs another will result in greater probability of a hit. At least there are no stats to show this, despite the intuitive reaction that more conservative algorithms must be safer.

I'm willing to believe that understanding DCS and controlling your profile accordingly, rather than simply following the computer, will minimize DCS probability, but again, I bet there's no proof of this either.

 

[Darnold9999]

I am curious re what the couple of things you told her were? Always looking for ways to take the stress of the old body when doing repetative dives

 

[fdog]

Back to the OP (original post): what makes me think that I can trust my recreational computer is about 300 dives, every variety of profile, on the same device.

Which means nothing for the next guy, of course. It just means, that computer works for my physiology.

Non-recreational, what makes me trust V-Planner? The same thing, plus the results of the community.

All the best, James