Cobra 2 100% vs 50% RGBM setting

[Scuba Duner]

Last week, my wife and I did 11 dives in Cozumel in 5 days. All of the dives were 2 tank morning dives with the usual hour between dives and one evening we did a night dive. The first dive of each day was usually to 100'+/- and the second was 70'+/-. As the week progressed, our bottom time was getting less and less compared to the divers with other brand computers (Oceanic was one). The last day the DM suggested that since we were using Suunto computers, we might want to consider Nitrox if we going to continue diving with them.

When we got home, I began doing more research and found an article in SCUBA magazine (2009) that compared a dozen computers and their algorithms over multiple dives and the Suuntos finished second from the bottom with the amount of available bottom time without deco.

I am assuming that the RGBM was set at 100%. I would like to know if anyone has used the 50% setting and how it may have compared to other computers or the 100% setting.

We will be diving wrecks in the Keys next month and I would like to dive with a computer that will allow me to maximize my bottom time.

I understand computers being conservative to provide safety margins, but I think Suunto may be to conservative for my type of diving. My health is excellent, I am not over weight, and have experienced no diving issues ever.
Maybe the 50% setting will get me closer to some of the more liberal computers.

Thoughts???

 

[iamafish]

What's the big deal about "deco time"? A diver is always either taking on more nitrogen into tissues or the reverse, on every dive. If your dive computer goes into a little "deco", next to a diver with a different computer with "no deco", that simply means Suunto is asking you to make one or more mandatory stops on the way up, and if the profile of the second half of your dive is multilevel, most small Suunto "deco obligations" will clear automagically on slow ascent. Suunto is more conservative because of their theory on micro-bubble formation; and this is a good theory. If you are diving with a diver who has been diving similar profiles over the same periods, and their non-Suunto computer shows no deco obligation, and yours shows a small deco obligation, you are much better off with yours, and follow Suunto and enjoy a short mandatory stop below a relatively shallow ceiling.

Did you know that a Suunto 5 minute ascent time on a 3 meter ceiling "deco obligation" contains both the ascent time (at 10/meter per minute) and the recommended three minute safety stop? This means, in a nutshell, that if you were at 20 meters with a 5 minute deco obligation on your computer (and 5 meter ceiling), and your buddy, who had been diving the same profiles but with a different computer, showed no deco obligation, your ascent would be the same, except your buddy would be doing a "recommended" safety stop, and yours would be "mandatory".

I was going to upload the excellent PDF by Suunto on their computer model, but it was a bit over SB file size limit for attachments; but you can Google it... it is freely available on the net.

Personally, I would not "change computers" for more "bottom time"; or use the 50% Suunto attenuation; instead, I would accept a bit of deco obligation on the Suunto and follow the directions on mandatory stops and ceilings (since you will only have a small deco obligation when diving with a buddy who is profiling within recreational limits with you), and your ascent profile with be safer and less likely for micro-bubble formation.

Don't be "fearful" of a small deco obligation on your Suunto... the stops on the way up (mostly at 5 meters or higher for a small deco obligation in recreational diving), are for your own good

 

[Scuba Duner]

I have no problem with deco time, since I have to enjoy it with by Cobra 2 often. The issue relates to having to cut my time short at depth while the Oceanics do not. The real problem begins on the dives after the first one. The Cobra 2 at 100% is at least half the NDL of the Oceanic and others for any dive 40' and deeper.

When the wall starts at 60', we either go into deco trying to dive the reef with the others or we stay up above to burn off the deco time and then continue the balance of the dive 20' above the reef.

Cozumel is different in that most of there diving is at 50' or deeper compared to the Keys where you need a shovel to get to 30' on the reefs.

I follow your thought process, but it really doesn't work with my style of diving unless I want to surface with a surplus of air or enjoy alot of deco time.

I believe the 50% setting may be the happy medium.

 

[Hatul]

I dive with two computers: on Oceanic and they other Suunto Vyper. On a deep dive the Suunto may put me into a mild deco, but as the poster above said, so what. It just means it requires you do surface more slowly, and buys you a safety margin. If I follow the Oceanic recommendation to surface with the nitrogen loading graph in the green zone it very similar to the Suunto. Aso mostly the deco clears on a multilevel dive as you come shallower.

Adam

 

[Kern]

The 50% RGBM setting will reduce the usual RGBM penalties a little. I won't hurt to use it. Many thousands of dives are made safely with algorithms that are far more liberal.

One reason not to enter deco, is when the coxswain, as part of the dive brief says, "no deco diving".

 

[airizzy]

I read a blurb from Suunto, I can't remember where, that the 50% setting doesn't change the base algorithm. It only reduces the penalties given when you mess up, like ascend rapidly. If this is true then 50% may not make much difference, if you are doing everything right.

 

[iamafish]

From Suunto's paper on RBGM:

The Suunto RGBM algorithm adapts its predictions of both the effect of microbubble build up and adverse dive profiles in the current series of dives. All Suunto dive computers are shipped with a default setting that provides the full protection of the Suunto RGBM algorithm.

However, some more experienced divers may prefer not to use the full Suunto RGBM model, and therefore, in the Suunto Vytec it is possible to adjust the algorithm to an Attenuated RGBM Model which reduces the effects of the Suunto RGBM model by 50%.

Honestly speaking, I recommend you follow the standard model and, for recreational diving with your buddies with other computers, use any potential minimal "deco obligation" to hover on your multi-level ascent, and further reduce micro-bubble formation. In other words, if you don't mind me saying so, realize that every dive is a "deco dive" of some form, and also realize that if you are recreational diving with a buddy with a different recreational dive computer, diving similar profiles. reduce micro bubble formation by looking at mild deco obligations as a nice way to remind you to slow your ascent, and clear your deco obligation on the way up with slow ascents. Don't let an "attenuated setting" make the decision for you. Use the normal setting and then think what is best for managing micro bubble formation.

OBTW, in hundreds of dives, I have never had a DM say "don't let your computer go in to deco mode"....... maybe others have. In reality, a 5 minute deco obligation from 20 meters at 3 or 5 meter ceiling is the same ascent profile as a non-deco diver following the recommended safety stop. Normally, these small deco obligations clear themselves during slow (safe) multi-level ascent.

 

[ajduplessis]

Changing the config to 50% will have little effect. By controlling your ascents better, adding deepstop to your profile and doing a safety stops(3min @ 15ft) or even extended (5min) stop will improve your multi dive NDL's. I would also strongly recommend Nitrox, or diving 6-10 feet above the rest of the divers as you see NDL approaching.

I have various Suunto DC and have very rarely surface before other make DC's within our dive groups. Start managing your dive profiles better.

Suunto's are sensitive to short SI, quick ascent rate and no completion of safety stops. Hope this helps.

 

[Scuba Duner]

Got an Oceanic Geo 2.0 to compare to my Cobra 2 while in the Keys in a couple of weeks. Should be an interesting comparison. Will probably dive the Cobra 2 profile with nitrox so it won't go into ERR mode. The decision will be made after that trip. It's no fun diving above other divers just because you have a conservative computer.

 

[ajduplessis]

I hear your pain, but diving deeper does not equal better diving. You will also not always know what gasses other divers are using, thus trying to say with them is not a good idea. Some divers (even thou not allowed for rec dives) will go into deco. Forget about what other divers do or what their bottom time are. Diving is about having fun and staying safe. The deeper you go and the longer you stay down the greater the possible risks!!!

As mentioned earlier, better control your ascent, do your safety stops (even extended) and your multidive NDL will improve. Suunto's will penalise you if you don't do things to the T.

Getting bent is not worth it!!! You want to get out of the water feeling refreshed, not tiered or feeling like an afternoon nap.