DC algorithm comparison. Which is "best"?

[mocca]

Hi ,

My girlfriend wants to buy her first DC and we tried to get some information on different models and different manufacturers and found that the main difference seems to be, that every manufacturer has his own algorithm.

Now we tried to get some info on these algorithms but couldn't find anything conclusive.:shakehead Some said that the Mares RGBM used e.g. in the Nemo Wide was no good because of the Deep Stop theory. Some say it is better than say Suunto. Some seem to prefer Uwateq.


Now I was hoping to hear from you guys which algorithms there are and what their pros and cons are and what you would recommend.

Thanks,

Alex

 

[TSandM]

How are you going to define "best"?

The fact is that DCS is pleasantly rare among recreational divers, no matter what decompression algorithm you are using, so long as you do controlled ascents. It's not zero, no matter which model you follow, so it's hard to say one model is vastly safer than another. They do vary in bottom times. Divers make choices -- If you're risk-averse, you go for a conservative algorithm (which will limit your bottom times more than a more aggressive model).

 

[mocca]

I guess "best" is subjective, hence the " " .

I was interested in learning about the pros and cons of each algorithm or what destincts them from the others.
e.g. Mares has a deep stop theory which seems to add extra dive time if you remain at one depth for longer periods, whether it is 10m or 30m.

So really I would like to know how they each work and what YOU feel is valid and what isn't and which you would recommend for whom and why.

My girlfriend is 26, as am I, and is relativly fit. I would say I am even more so as I do sports 3 times a week. But neither of us run marathons, swim for miles or do heavy training. I would say we are healthy and fit. More so than most divers we have met but less so than some sports fanatics.

 

[halocline]

I think you're asking a valid question, although there is no correct answer. In fact, there are no statistics, none, that show any corrolation between incidence of DCS in rec diving and specific computer algorithms. Maybe someday there will be, and when that happens, I feel sorry for the company making the computer that loses.

My guess, however, is that factors other than computer algorithm are far more influential in determining DCS risk, like ascent rate, dehydration, obesity, exercise immediately after diving, and overall control of your dive profiles. What you might do is study the differences in the algorithms (there's an old scuba online magazine article that shows some interesting comparisons) as well as learn what you can about DCS incidence and suspected causes. Then you might decide, as I did, that it doesn't really matter which computer you use for rec diving; what's far more important is that you dive in a way that minimizes the risks. For me this involves extending time at 15 ft, and a VERY slow ascent from 15 ft to the surface, often taking 2 minutes to do get there. It also means being very cautious about reverse and "bounce" profiles.

 

[TSandM]

Quite a few of the algorithms are proprietary, so the actual math isn't readily available for scrutiny. What the algorithm spits out in terms of NDLs can be compared, and it's pretty well recognized, for example, that Suuntos tend to be conservative in comparison with Cochrans. Some, like my Vytech, allow the insertion of deep stops.

Since the incidence of DCS is so low, it's hard to say one is "better" than another. It's more that the individual diver has to read and learn and make decisions about how he wants to conduct his dive. Do you want to be extra careful (because you have recognized risk factors, perhaps?) or are you convinced of the utility of deep stops? These questions will determine which computer appeals most to you.

FINDING the information about which is conservative and which is not, or which permits deep stops and which doesn't, requires some detective work.

 

[Charlie99]

Something that a lot of people seem to miss is that the DIVER sets the profile in an NDL dive, not the computer.

The profile -- ie. the depth vs time plot, including the time spent in the ascent and what stops are done, ends up making more difference than whether your computer says you have another 5 minutes of NDL or 30 minutes of NDL at a specific depth.

Both the Suunto and the Mares dive computer algorithms are neo-Haldanian dissolved gas models, just like virtually every other computer out there. How they differ from the others is that they reduce the limits on repetitive dives in response to certain types of behavior on previous dives --- sawtooth profiles, reverse profiles, multi-day diving, fast ascents, skipping safety stops. In this sense, the Suunto and Mares RBGM computers are better for divers that aren't paying attention to their dive profile.

OTOH, I personally prefer computers that have tissue loading bargraphs that show the relative N2 loading. If you have relatively high N2 loading levels, even when you go shallow, this type of computer (Oceanic and Aeris computers and several other brands) continues to show that you have high loading levels until you have spent enough time at shallow stops that you have offgassed. In other words, it provides clear feedback on overall N2 loading level, much like a pressure group or repetitive group on a table. Other computers, however, such as (all?) Suuntos use the bargraph to simply display NDL time, and when you go shallow and NDL goes to a high number, the bargraph immediately goes to the green --- even though you still have relatively high N2 levels.

Charlie Allen

 

[Charlie99]

FINDING the information about which is conservative and which is not, or which permits deep stops and which doesn't, requires some detective work.

I'm sure you know this, but ALL computers will permit deep stops. I've never had a computer reach over and put air in my BCD so I can't make a stop. :banana:

And even on the question of "how much credit for deep stops", there isn't any significant difference in the algorithms in the recreational realm -- even when comparing neo-Haldanian models to true bubble models like VPM. Or to put it another way, at recreational depths and within NDL or a few minutes of deco obligation, there is essentially no difference between the behavior of the different models (RGBM, Buhlmann, VPM, etc) in how the respond to a diver making deep stops.

 

[UP4AIR]

All points here are valid and good responses. One point I would like to make is that decompression is often spoken of as THEORY. With that being said I think it is reasonable to say that any legitimate computer made for diving today has a working algorithm. No one, is NECESSARILY better than the other by rights of which algorithm it applies. It is up to the individual diver to dive a safe and reasonable profile based on their condition and the prevailing diving conditions of the day. I think one could find an individual using each of the available computers on the market today and getting an "undeserved" hit. So is it the algorithm of the computer or the individual?

I would say look at all the computers out there and choose one based on the rep of the manufacturer and one that will fit your style of diving and budget.

Am I making any sense here or just rambling?

 

[halocline]

OTOH, I personally prefer computers that have tissue loading bargraphs that show the relative N2 loading. If you have relatively high N2 loading levels, even when you go shallow, this type of computer (Oceanic and Aeris computers and several other brands) continues to show that you have high loading levels until you have spent enough time at shallow stops that you have offgassed. In other words, it provides clear feedback on overall N2 loading level, much like a pressure group or repetitive group on a table. Other computers, however, such as (all?) Suuntos use the bargraph to simply display NDL time, and when you go shallow and NDL goes to a high number, the bargraph immediately goes to the green --- even though you still have relatively high N2 levels.

Charlie Allen

I agree with this statement; on my aeris computer, towards the end of a typical dive, I routinely have far more NDL than I could ever use on a single tank. However, the N2 bar can be in the yellow at the same time, and this is a much more important bit of information. People tend to make a big deal out of the relative NDL calculations of various computers, when the more important info is dive time, depth, ascent rate, which is all actual data rather than calculation. The N2 loading bar is a helpful cautionary calculation, at the time in the dive (near the end) when it's especially important to have a sense of your N2 load.

Researching the various computer algorithms can be a useful process if it give you some insight into dive behavior that is considered risky by some manufacturers and inspires you to avoid that behavior. Even more useful is gaining understanding of the factors associated with DCS that no computer considers; dehydration, exercise, etc...

 

[bperrybap]

Here is a link that compares several computers NDL
times/algorithms: Dive Computer Comparison

While I disagree with many of the conclusions made by the
author of this paper especially when he recommends against
certain models, the actual comparison data is very interesting.
It is a good comprehensive comparison of the algorithms
used by different manufactures.
So even though it may not include the specific computer
your are looking at, manufactures tend to use the same
algorithms across their products.

Also, try not to be too influenced by his recommendations
for or against certain models. i.e. plenty of recreational divers
use Oceanic/Aeris/Pelagic computers and don't have any
problems.

--- bill