Soon to be newbie diver with algorithm questions?

[AquaExplorer]

So my wife and I are going to be getting certified next month.

We will rent most of our gear for awhile until we figure out what "works" for us, but one item we both would like is a dive computer.

I have done some research and see the Suunto, Dive Rite, Cressi, etc. are known to have "conservative" algorithms and that Oceanic is more liberal.

I know what that means in the broad sense, i.e. bottom time, but what does it mean in real, practical sense? If the wife and I are on a 5 day, 2 dives-per-day trip, can we get screwed if the computer says we have not had enough SI, even if a hard-card calculation shows we have?

I apologize if this specific question has been brought up but my search-foo did not grok (ie. find) anything regarding this issue.

Thanks!

 

[giles45shop]

You will probably find that almost any computer will be more "aggresive" than a tables calculation if your dives are not perfectly square profiles (descend directly to depth, stay there X minutes & surface). If you you did a dive where you went to 60' for 1 minute and then acended to 30' and spent the next 30 minutes there before surfacing, you would have to treat the dive as a 60' dive for 31 minutes, even though your average depth and time is significantly different.

With a computer, it will do real time monitoring of your depth and times, resulting a much better approximation of what your depth/time really is.

Diffrent computers handle excessive bottom time in different ways, but in general they do tend to lock you out in one way or another if you exceed their limits. You probably would not have an issue w/ 2 dives per day with any computer, unless they are really deep or long.

The issue is more likely to happen with 4-5 dives/day, multiple day trip.

Hope that helps,

John

 

[astrofunk]

I'm curious about what other have to say on this topic, but my feeling is that you won't experience a material difference in the total amount of diving you could do with one computer or another.

I dive an Oceanic; my dad dives a Suunto. We spent a week diving pretty aggressively in Cozumel, and we never ran into a situation where our computers mandated I could dive longer or more often than him. This was partly due to the fact that our dives were typically limited by air consumption, not by approaching our no-deco limits. However, as much diving as we were doing, the difference in algorithms didn't express itself.

Once after a couple of deep dives in Kona, the dive master signaled that his dive time remaining on his Suunto was running out, and that he wanted to ascend. I still had a few minutes on my Oceanic. That was a direct result of the difference in algorithms, but it was only a matter of a few minutes.

 

[TSandM]

Well, what it comes down to is that decompression is not an exact science. Various people have modeled gas loading and unloading from the body in different ways. Models vary in the number of compartments, the half times of the compartments, the M-value (a measure of how much nitrogen tension a compartment can have before it begins to bubble), and how much bubble mechanics are taken into consideration. No decompression time at a given depth can vary as much as 12 minutes between models. Some models will drastically shorten the no-deco time if you do reverse profiles, and some will not.

There is no one model which is "right". A "liberal" algorithm (one which usually permits more no-deco time) may suit your diving style, but run a very slightly higher risk of DCS. Obviously, to be conservative, one could simply keep shortening dive time, but the only way to avoid any risk of DCS is not to dive at all, which is not an option for most of us

What's important to realize is that, just because your computer is happy with you, doesn't mean you are safe. People get bends symptoms when their profiles have met every requirement of their computers. It is a very low incidence, for sure, but it does happen. So if you choose a computer with a very liberal algorithm, and max out your time several dives a day over a multiple-day trip, you may be pushing your body over its limits while your computer is completely satisfied.

I think it is highly unlikely that any computer will be unhappy with you, if your dives, calculated by tables, are within limits. Because of the intrinsically conservative nature of tables, and the iterative calculations of computers, a computer will almost always give you more "legal" bottom time than tables will.

 

[AquaExplorer]

Thanks everyone. I do want to be safe but I also want to be a part of the decision process in risk management (I am a pilot so I am very familiar with risk management) and not abdicate all of that decision to a computer.

TSandM....thanks a bunch, very insightful.

 

[TSandM]

If you are interested in knowing more about all of this, I can recommend two books: Lippmann & Mitchell's Deeper into Diving, and Mark Powell's Deco for Divers. The former has a long section comparing models, with tables showing what the no-deco time for various depths is for each, and the latter is probably the most readable book on decompression theory out there.

 

[Hatul]

If you are interested in knowing more about all of this, I can recommend two books: Lippmann & Mitchell's Deeper into Diving, and Mark Powell's Deco for Divers. The former has a long section comparing models, with tables showing what the no-deco time for various depths is for each, and the latter is probably the most readable book on decompression theory out there.

There is also a nice comparison chart on scubadiving.com

http://www.scubadiving.com/upload/images/pdf/200611_scubalab.pdf

In fact the website has some nice comparisons of the various computers. Personally, I would avoid the most liberal of the computers to maximize safety.

Adam

 

[scuba.dude]

tdager: I'm not familar w/ the books mentioned above, but if they don't fit the bill, I am told Diving Physics with Bubble Mechanics and Decompression Theory in Depth by Bruce R Wienke covers all basic dissolved gas algorithms. I'll let you know after my copy arrives.

-s.d

 

[eponym]

So my wife and I are going to be getting certified next month. We will rent most of our gear for awhile until we figure out what "works" for us, but one item we both would like is a dive computer. I have done some research and see the Suunto, Dive Rite, Cressi, etc. are known to have "conservative" algorithms and that Oceanic is more liberal. I know what that means in the broad sense, i.e. bottom time, but what does it mean in real, practical sense? If the wife and I are on a 5 day, 2 dives-per-day trip, can we get screwed if the computer says we have not had enough SI, even if a hard-card calculation shows we have? Thanks!

Tdager,

I and lots of other people make five dives a day on a typical recreational liveaboard wearing a dive computer, without running up against non-deco time limits. Two in the morning, two after lunch, one night dive. The deepest dive of the day comes first, followed by shallower dives. The boat keeps the surface intervals long enough to make each dive doable. Two dives a day is not a problem unless they're both deep and close together.

Your gas supply will most likely be the limiting factor, rather than the nitrogen loading. As Giles said, no dive tables will ever give you the time underwater (and minimum SIs) of a dive computer unless you're diving truly square profiles.

Welcome to diving!
Bryan

 

[AquaExplorer]

Thanks everyone. I will definitely look at the books everyone has recommended.

Based on a lot of research I am leaning towards the Cressi ARCHIMEDES 2 and some version of SUUNTO.