Safety of some popular dive computer algorithms???

[Austin]

Hey, I'm a AOW diver who's training for my Rescue and Nitrox cert, and i'm looking into getting my first dive computer. I really want a hoseless model, and have been doing some research lately into the different algorithms the different computers use. I picked up a copy of August "Scuba Diving", in which scuba lab tested 11 top dive computers. They put the computers in a Hyperbaric chamber to simulate 3 repetitive dives.

Some of the computers, based on their algorithms, added a lot of no-deco time as they ascended from the max depth. The computers that added the most deco time, almost twice as much as the others, were using the Haldanean Algorithm: which groups the bodies thousands of different tissues into theoretical tissue compartments, each with a different rate for absorbing and releasing inert gas. It also states that gas was always released at the same rate at which it's absorbed. As I'm sure many studies have proven, and which Scuba Lab confirmed, it's a very liberal algorithm.

Scuba Lab believes its a very liberal algorithm when its compared against the Buhlmann Algorithm and the DCIEM Algorithm and especially the RGBM (Reduced Gradient Bubble Model). Scuba Lab says that this Algorithm was developed after Doppler bubble research showed that "silent bubbles" unknown to the Haldanean algorithm often form after a dive without causing DCS symptoms. On the theory that these bubbles would be slower than dissolved gas to leave your body, and could even interfere with the outflow of dissolved gas, the RGBM algorithm is very conservative.

I'm very intrested in the AERIS Atmos Elite and The OCEANIC VT PRO wrist mounted, hoseless dive computers, my concern is that they borth use the Haldanean Algorithm, which at some points on the last dive, gave 74 minutes of no-deco time when ascending, while the RGBM gave 30minutes of no-deco time.

In conclusion, i want a safe computer, but not a paranoid one. Based on these different algorithms, is it okay to add that much no-deco time when ascending? Which algorithm or theory is correct? I don't intend to pretend that i know a whole lot about this subject, i've just read the Scuba Lab's results and some other material. Thanks for your responses, i know this is kinda long...

 

[eponym]

Hey, I'm a AOW diver who's training for my Rescue and Nitrox cert, and i'm looking into getting my first dive computer. [snip] Which algorithm or theory is correct? I don't intend to pretend that i know a whole lot about this subject, I've just read the Scuba Lab results and some other material.

You'll doubtless get some good replies here, Austin, from more experienced divers than I. But I'll start you off with two general observations, recommendations for more study, and a comment on my own computer.

1. I wouldn't select a computer using Rodale's (Scuba Diving Magazine's) recommendations only. I don't think they're uninfluenced by advertisers.

2. The question, "which algorithm or theory is correct" has no answer. All of the theories, and all of the algorithms created to apply them, work from some combination of guesswork, historical data, and modeling of physiology. But there's a lot we don't know definitively (for instance, just how do those bubble seeds get started?). And since we can't observe the processes we're interested in as they occur in the body, we have to rely on inference and modeling.

3. I congratulate you for exploring the issue as you're doing, before purchasing a computer. Dive Training magazine ran a four-part series on deco theory this year--March through June, I think. It's a good introduction. Also good as an introduction is a slim volume from Best Publishing (they're on the web) called "Beating the Bends." Chapter two is titled, "If you want a guarantee, buy a washing machine."

4. I use a Vyper from Suunto. It's a recreational dive computer. While I get terrific dive times compared to diving tables, the Vyper is quite conservative on the question of oxygen toxicity when diving nitrox. The manufacturers say that because it's made for recreational use, they don't want to be pushing the limits. From what I've seen, a lot of divers who do serious decompression dives work out their schedules and gasses before the dive and don't rely on a computer during the dive.

Have fun, and let us know what you pick, willya?
Bryan

 

[Dr Deco]

Hi austin:

Algorithms

There are basically only two algorithms (= calculation methods) in dive tables and computers today. The “Two phase” algorithm (RGBM and VPM) allows for bubble nuclei and takes into account the Laplace pressure in the microbubbles from surface tension. The other is the “Haldane method” that tracks only dissolved tissue gas loads. For all of the algorithms, there are additional constants that control certain factors related to tissue gas loading and ascent.

The differences between the algorithms within the Haldane group are the constants, the ascent limits or “no decompression limits”[NDL] and supersaturation limits (M-values). Variations in these will allow for the construction of both a very liberal or very conservative table (and dive computer) and anything in between.

Computers

As I see it, it matters little which computer you use since all have constants that have been shown to be safe for recreational diving.

What is not said is that the constants are arrived at by testing divers in a laboratory where physical activity is minimized; the diver test subjects mostly just sit around in the lab. The constants in the table would be different if the divers were required to perform some strenuous activity before the chamber dive (to simulated hauling dive gear) and simulate strenuous active post chamber dive (to simulate climbing ladders, moving tanks, etc). No table would ever be test with the subjects playing beach volleyball.

If you believe that you will be hauling gear around post dive, I would suggest a meter allowing on conservative gas loads - and then maybe staying away from the limit. Deep diving with decompression would be better served with meters that have a dual phase model.

Dr Deco :doctor:

Readers, please note the next class in Decompression Physiology :1book:
http://wrigley.usc.edu/hyperbaric/advdeco.htm

 

[padiscubapro]

Hey, I'm a AOW diver who's training for my Rescue and Nitrox cert, and i'm looking into getting my first dive computer. I really want a hoseless model, and have been doing some research lately into the different algorithms the different computers use. I picked up a copy of August "Scuba Diving", in which scuba lab tested 11 top dive computers. They put the computers in a Hyperbaric chamber to simulate 3 repetitive dives.

Some of the computers, based on their algorithms, added a lot of no-deco time as they ascended from the max depth. The computers that added the most deco time, almost twice as much as the others, were using the Haldanean Algorithm: which groups the bodies thousands of different tissues into theoretical tissue compartments, each with a different rate for absorbing and releasing inert gas. It also states that gas was always released at the same rate at which it's absorbed. As I'm sure many studies have proven, and which Scuba Lab confirmed, it's a very liberal algorithm.

Scuba Lab believes its a very liberal algorithm when its compared against the Buhlmann Algorithm and the DCIEM Algorithm and especially the RGBM (Reduced Gradient Bubble Model). Scuba Lab says that this Algorithm was developed after Doppler bubble research showed that "silent bubbles" unknown to the Haldanean algorithm often form after a dive without causing DCS symptoms. On the theory that these bubbles would be slower than dissolved gas to leave your body, and could even interfere with the outflow of dissolved gas, the RGBM algorithm is very conservative.

I'm very intrested in the AERIS Atmos Elite and The OCEANIC VT PRO wrist mounted, hoseless dive computers, my concern is that they borth use the Haldanean Algorithm, which at some points on the last dive, gave 74 minutes of no-deco time when ascending, while the RGBM gave 30minutes of no-deco time.

In conclusion, i want a safe computer, but not a paranoid one. Based on these different algorithms, is it okay to add that much no-deco time when ascending? Which algorithm or theory is correct? I don't intend to pretend that i know a whole lot about this subject, i've just read the Scuba Lab's results and some other material. Thanks for your responses, i know this is kinda long...

I wouldn't ho by Rodales report, The models will vary depending on diving parameters.. RGBM penalizes the diver on repetitive profiles, while normal Haldane models do not..

BTW the computers you mentioned and ALL the ones tested by Rodales are Handane models.. The difference is that some have RGBM limiting factors folded over them.. If you want a TRUE RGBM cmputer buy the HS-Explorer (Its expensive)..

On simple NDL profiles times are pretty close, once you get into mandatory decompression stops Bubble models (VPM, FULL - RGBM) tend to get you out of the water quicker and in some case ALOT quicker..

I do dives with students decompression students all the time many carry Suunto dive computers, when we do the dives against true RGBM tables and verified by my explorer we generally will "bend" the Suunto computers as they want ALOT more deco than the tables predict..

 

[Austin]

Thanks for the responses. And i'm suprised about all of those computers having the Haldenean algorithm. So, is it alright for the Haldenean computers to give that much more no-deco time during ascent on where some of the other computers don't give any or actually take away time on the same ascent? Would getting the Oceanic or the Aeris ones and just staying away from the critical nitrogen levels on the computers be safe enough, even if your going to do some physical work afterwards, but try to keep it to a minimum. Also, in general(i know there are a lot of factors), how long must one wait to do a fair amount of physical excercise? 6 hours? a day?

 

[Dr Deco]

Not diving to the limits of the more liberal computers will definitely reduce the gas loads in your body. Then, refraining from strenuous activity will be the additional safe step.

Following recreational diving, I would suggest at least four hours befor strenuous activity - six is better.

 

[ianr33]

I have an Aeris Atmos Pro and as has been pointed out it uses a liberal algorithm.This is not a problem if you are aware of it.I always consider the yellow "caution" zone to be a "do not surface unless you want to maybe get hurt zone"

Personally I like to hang out at 15 feet or so until until it goes one blob into the green.

I also make a habit of doing deep stops for several minutes (say at 50 feet after a 100 foot dive) the computer will not give any (or very little) credit for that ,but your body will thank you for it.

Use the Aeris in an informed manner,ascend slowly.do deep stops and only surface when it is in the green and it is a fine computer. Push it to the limit on every dive,make a habit of surfacing in the yellow and you deserve to get hurt.Of course,if you are going to do deep stops,and long safety stops you need to know that you have enough gas,which is another topic completely.

I would MUCH rather have an aggressive computer that I do not dive to its limits than an overly conservative one that will lock me out of diving for a day for no good reason.

 

[Dr Deco]

"I would MUCH rather have an aggressive computer that I do not dive to its limits than an overly conservative one that will lock me out of diving for a day for no good reason."

That is a very interesting idea.

Dr D.

 

[DepartureDiver]

In general if the computer looks the same, it is the same (except for bells and whistles). Aeris, Oceanic, and others are made by Pelagic which is owned by Oceanic I believe. I too like the indicator bars. They indicate a straight percentage of the maximum load/M-value allowed. I confirmed this with Oceanic years ago. In general, depending on the number of bars, green keeps you at 75% of the max. Of course if there are 7 green bars out of 10, then it is 70%. FYI, if you keep the M-values at 75%, you are diving as though the no-stop times are about one-half of their maximum. So if 100% of the surfacing value/indicator lets you dive to 60 feet for 55 minutes, surfacing while in the green at 75% would be keeping the surfacing value the same as diving to 60 feet for about 30 minutes. This of course does not speak to any issues with repetitive diving or off-gassing rates. When I use an Oceanic computer as a guide for no-deco dives. I too keep it in the green with the deep safety stops. If you are well into the caution zone, your first safety stops should be deeper than 15 feet.

 

[chrisch]

Some good advice here. As an AOW you are not qualified to do decompression diving. If you chose to do so take a deco course and learn more about the subject.

Nearly all computers use Buhlmann ZHL algorithms. The model is based on the work of J S Haldane, but I don't think he had a PC on his desk!!!!

For no-stop diving there is little to choose. (No such thing as "no deco" - the ascent is the deco that's why there's an ascent meter built in!!)

The Suunto is a good model and the lip-service to RGBM is also worthwhile.

Whatever you buy get one Nitrox ready for when you have completed that course.

I had an Oceanic for years and I too like the little bars. It was a pain to reset for different EAN mixtures between dives.

The Suunto (wife has one) also has a gauge mode for using tables on accelerated deco and trimix. Its pretty future-proof in that respect.

Chris.