Dive Computer Algorithms?

[BackrollO]

I have had 3 different computers and have really started studying the difference in computer algorithms. I recently purchased a new Oceanic VT4 and a BUD for myself and a VT4 for my wife. I was exposed to different NDLs on a trip to Cozumel in January. One diver went into deco with a Suunto Zoop whereas my Scubapro Luna did not but was showing much less bottom time remaining than our other buddies Oceanic. So I have been testing my BUD set in DSAT and my VT4 running the more conservative Pelagic+. Wow, what a difference in NDL especially in the 40-90' depth! How is one to know what algorithm is most applicable? I know the obvious answer is most conservative is most safe, but is most conservative too safe and shortens your remaining bottom time prematurely?

Recent Dive in Mahahual Mexico data between my two Oceanics:
46' depth pelagic+ :69 dsat :93
67' depth pelagic+ :32 dsat :69
90' depth pelagic+ :15 dsat :20
Why is there not a standard algorithm based on recreational dive limits?

I will do some more tests using my ScubaPro Luna and these two Oceanics on my next local dive trip and see if I can make heads or tails out of this data? Anybody have input on why such a wide latitude of NDL and remaining bottom time in different manufacturers?

 

[Scubanoobi]

You'll find that between the two oceanic algorithms, DSAT may be more liberal as long as you don't go into deco, once you enter deco with DSAT, the deco time will quickly pile on. Where Z+ will hit deco limits sooner but will have a more predictable deco schedule.

This is based on what I've researched - I've never been in deco. Between the two it probably doesn't matter much if you're staying within NDL's. It just depends what you prefer.

 

[Diver0001]

The short story is that they are all applicable within the context of a given algorithm.

Think of it like different maps of the same area but on different scales and resolutions. You might be able to get them to line up in one spot but they will never line up everywhere. Nevertheless, when trying to navigate that area, all of the maps are valid when taken alone.

It's a rough analogy but it's like that with computer algorithms too. In main lines there are two major philosophies and many variants on those. All of them will get you to the surface safely* if you follow it but when diving with another diver using a different algorithm it's possible that they'll need/want to make stops that your computer might not prescribe. Moreover, most, if not all, modern computers have different settings for conservatism so depending on the settings you're using even the SAME computer can give very different results.

For example, I have a Suunto computer that usually varies on deco dives about 2 minutes from my buddy's computer. One one dive I put my computer on it's most conservative setting. It went into deco earlier and accumulated deco faster than normal. When my buddy's computer was showing 10 minutes of deco, mine had (IIRC) about 40 minutes on the same dive. So yeah, settings can make a difference.

R..

(* "safely" means having a statistically negligible chance of having symptoms of decompression sickness. Nothing we know of yet can reduce that chance to zero).

 

[TSandM]

The determination of no-deco limits depends on a lot of assumptions. Different models use different numbers of compartments, with different half-lives; they may vary in the M-values they set (acceptable level of saturation in any given compartment). They vary in the acceptable level of DCS risk (which is never zero). Buhlmann algorithms can set different thresholds for first stop, or how close to the M-values you will be when you surface. Bubble models either do complicated calculations of bubble formation and expansion, or kludge the Buhlmann algorithm to approximate the output of a true bubble calculation.

DCS in recreational divers is so rare, that determining which model is the "best" is really not possible. The number of divers and dives that would be required to do that would be unmanageable. What we know is that 1) All algorithms on the market result in a low incidence of DCS, and 2) individual susceptibility factors probably account for as much DCS risk as the difference between algorithms.

How to choose? Honestly, I'd choose a computer for the other features that appeal to you. Display, functions, logging, size, service reputation -- all those things would outweigh algorithm for me, for a recreational computer. On the other hand, if you have been bent, or if you have major risk factors, like a known PFO or morbid obesity, I'd pick one with a very conservative algorithm.

 

[Lorenzoid]

Why is there not a standard algorithm based on recreational dive limits?

You mean somehow force or at least strongly encourage different manufacturers to use the same algorithm? How? Enact a law? Kick them out of the club if they don't comply?

At least some manufacturers use their algorithm as a marketing point to differentiate their product from others. Suunto, for example, makes a big deal out of its proprietary RGBM model, with literature explaining in vague/general terms how it works, which could be interpreted as implying that it provides some benefit that others do not.

 

[BackrollO]

I follow your thoughts as they make sense. We are both very healthy people with really low risk factors that could trigger DCS. It is just seems to be rather confusing for the average diver/consumer and having a bit of a engineering problem solving mind, I started studying the differences. I was with some divers off Molokai diving deep for hammerheads last year, and had two Brazilian girls go into deco, one locking up her computer and the other having to do a 20min safety stop. I watched and tried to get them to stop bounce diving up and down continually for 2 deep dives. Then when the Coz trip with 3 different computers giving such different profiles was intriguing. So I would like to use the Oceanic DSAT algorithm as it is more liberal but I don't want to put myself, my wife or others to undue risk. But if the Oceanic liberal setting is still well within the NDL limits for recreational diving I am leaning towards using that algorithm. Thank you for the help and insight.

 

[cbm32]

The determination of no-deco limits depends on a lot of assumptions. Different models use different numbers of compartments, with different half-lives; they may vary in the M-values they set (acceptable level of saturation in any given compartment). They vary in the acceptable level of DCS risk (which is never zero). Buhlmann algorithms can set different thresholds for first stop, or how close to the M-values you will be when you surface. Bubble models either do complicated calculations of bubble formation and expansion, or kludge the Buhlmann algorithm to approximate the output of a true bubble calculation.

DCS in recreational divers is so rare, that determining which model is the "best" is really not possible. The number of divers and dives that would be required to do that would be unmanageable. What we know is that 1) All algorithms on the market result in a low incidence of DCS, and 2) individual susceptibility factors probably account for as much DCS risk as the difference between algorithms.

How to choose? Honestly, I'd choose a computer for the other features that appeal to you. Display, functions, logging, size, service reputation -- all those things would outweigh algorithm for me, for a recreational computer. On the other hand, if you have been bent, or if you have major risk factors, like a known PFO or morbid obesity, I'd pick one with a very conservative algorithm.

I agree with most of this. I guess I would add that, if I had a dive buddy that I dove with very frequently I may make my first priority to choose a computer that very closely matches the one my frequent dive buddy has (probably the exact same dive computer actually).

 

[Diver0001]

I agree with most of this. I guess I would add that, if I had a dive buddy that I dove with very frequently I may make my first priority to choose a computer that very closely matches the one my frequent dive buddy has (probably the exact same dive computer actually).

I dive with two buddies on a regular basis. One of them has a computer that matches mine within a couple of minutes. The other has a computer that prescribes a deeper and longer ascent than mine would. We obviously know this so when he signs that he has (yet another) required deep stop, we just wait for him, even if my computer is telling me that I could theoretically ascend to 6 metres.

On the total dive time, it usually makes a difference of a few minutes, but if you're in such a big hurry to get out of the water.... why did you take up diving?

To me, the whole "conservatism" thing can be boiled down to staying together as a team and respecting any diver in the team that wants to make a stop during the ascent. It's not like it was when we used to dive tables and we needed to stick to plan like autistic mathematicians. If my computer is saying that I can ascend to 6 meters and I put in another 2 minutes at 20 metres then it will adjust.

Unless someone on your team is diving tables, this is really a non issue.

R..

 

[Basking Ridge Diver]

(* "safely" means having a statistically negligible chance of having symptoms of decompression sickness. Nothing we know of yet can reduce that chance to zero).

Well.... There is one thing that will guarantee that chance to zero.

 

[Diver0001]

Well.... There is one thing that will guarantee that chance to zero.

one really dull thing... yeah

R..