Deco dives and "normal" computers

[partridge]

I would just like to ask about the decompression capabilities of "regular" computers like the one I use, Suunto Stinger. I have been reading quite a bit on this board but don't think I have come by a thread about this.

Anybody ever compared the deco obligations on a RGBM versus a dive planning software? How safe is it to follow the minimum deco obligations on the Stinger? or any other computer?

I know some will say that I should get proper training etc etc. Lets assume I know this and just want to know because Im curious. I mean the computer is capable of deco dives right? and they would not put it on there if it were not so, because of liability etc?

 

[Snowbear]

May I suggest perusing these threads?...

http://www.scubaboard.com/t47813.html
http://www.scubaboard.com/t49358.html
http://www.scubaboard.com/t36984.html

 

[partridge]

Yeah, I read some of those threads. But anytime it seemed like I was going to get an answer to my question above, everybody just said to get deco training. I will someday, but in the bottom of the Philippines, it's not easy to get an instructor certified to teach it.

The last thread really helped though. Thanks. I guess the answer is NO.

 

[Jai Bar]

One of the problems of using a regular computer for deco diving, is the ability to plan the dive. As you may know, it is rather unwise to go into deco without planning in advance how much gas you will need to make all the deco stops (including cases such as you are sharing air with your buddy).

Therefore, most deco divers will plan their dive in advance. In most cases, planning in advance will be a square profile, or several square profiles added together (with softwares like v-planner, abyss etc. you can plan several stages). As you may know 'Plan your dive, dive your plan' can not be allways fulfilled. Many times you'll find yourself not doing an exact square profile as you planned. The computer takes it into account, your pre-prepared dive plan does not. Therefore, a diver may stick to its plan and do all the deco stops as planned beforehand, even if the computer 'knows' that it is already safe to go out. You need to get proper training for that, as people suggested.

However, say you were doing a 'regular' dive (without planning to pass NDL limits), and from whatsoever reasons you got into deco, a computer with deco capabilities (like the Stinger you own) will do the job- it will help you get out of the water safely, assuming you follow ceiling instructions and you have enough air to do the stops.

About your question- I have tried to compare the profiles from a Vyper, especially in cases where I did 'classical' square profiles and software like V-planner. The problem is that the computer remembers previous dives, which are difficult to implement in software. I even build a simulation (with Matlab) based on several Haldanian like Buhlman models (In the future I'll try to implement RGBM), that can read/import the Vyper profiles and make calculations according to the actual dive profile. I found that in most cases the computer 'allowed' to get out of the water before the software/table-based planning. I was surprised to see this, and I am not sure of the reason- perhaps the sampling time in the computer log (10, 20 or even 30 seconds) may bias the calculations.

Another thing- Vyper/Stinger and co. are not true RGBM models. You will find some info about these computer's algorithms written in this very forum, by Bruce Wienke. Look for it.

 

[vjongene]

I would just like to ask about the decompression capabilities of "regular" computers like the one I use, Suunto Stinger. I have been reading quite a bit on this board but don't think I have come by a thread about this.

Anybody ever compared the deco obligations on a RGBM versus a dive planning software? How safe is it to follow the minimum deco obligations on the Stinger? or any other computer?

Deco diving with a computer and deco diving with standard tables can be quite different, even though the underlying formulas are similar if not identical. Deco diving with pre-computed profiles (as done with dive planning software) is somewhere in between. BTW, Suunto computers as well as some dive planning programs use an RGBM mathematical model, so the difference is not in the method but in the implementation.

A dive computer is capable of calculating on- and off-gassing of different "tissues" in real time. This information can be used either to generate a deco schedule (stop lengths and depths) adapted to the current dive profile, or to set safe ceilings relative to dive history so far. The Aladdin computers take the first option, i.e. they calculate deco stops using the Buhlmann algorithm. Suunto computers take the second, i.e. they tell you how far you can ascend safely (your current ceiling), and warn you if this is not at the surface. In addition, they tell you how long it will take you to reach the surface safely. None of the Suuntos (at least to my knowledge) will tell you what your deco schedule should be. In fact, it often happens that after having given you a "ceiling" warning (i.e. telling you that you are outside NDL), the computer will not make you do any formal deco stops, IF your ascent rate is slow enough. Recent Suuntos all include a safety stop of 3 min between 6 and 3 m, though, and the safety stop is included in the calculation of "safe time to surface".

The fact that Suuntos will not give you "hard" data about deco stops is often frustrating, because it is customary to ask your buddy what his/her current deco obligations are. All you get on a Suunto is time to surface and ceiling. So you have to do the mandatory deco calculations in your head. For example, if you are down at 30 m and the computer tells you that it will take 7 min to reach the surface, your mandatory deco is in fact less than 2 min, because you have to deduct a little over 2 min to ascend from 30 m to 3 m, and 3 min for the safety stop.

I have attached a profile from a dive I did yesterday to illustrate the point. I was using an Eon, an older model that does not include safety stops. You will see a ceiling at 2.4 m, and ascent time of 5 min. By the time I reached 3 m, the ceiling was gone, i.e. I had no mandatory deco left, even though I had not done a formal stop.

If I had been diving tables, I would have had to lookup deco time on the basis of having spent about 6 min at 48 m (the maximum depth), and I would have had to spend some time at 3 m and possibly 6 m (sorry, don't have my tables handy), before the safety stop.

HTH,

Victor J:

 

[cornfed]

I know some will say that I should get proper training etc etc. Lets assume I know this and just want to know because Im curious. I mean the computer is capable of deco dives right? and they would not put it on there if it were not so, because of liability etc?

Suunto has the manuals for all their computer on their website. If you look at them you'll notice that they tell you not to use their computers for planned decompression dives and that the deco calculations they make are for "oops, I overshot the ndl time" situations.

 

[partridge]

Thanks for all the info. I am just trying to fully understand my computer so I know how far I will trust it. I do sometimes put it into deco but the maximum obligation I have ever had was 2 minutes at 10 feet, not including the safety stop. What I do is I fulfill this obigation deeper than the ceiling. Although the time does not count down in real time, (i.e. it will take me about 4 minutes to fulfill at 25 feet) I feel more safe. I also extend my safety stop as long as possible during these times.

The reason for having these deco obigations are for another thread. Sometimes, my fault, sometimes not. Regardless, I am trying my best to "professionalize" my diving style.

This is really interesting to me and I wish I could take the class. I think the only way is to go on a vacation and take the course. We do not even have nitrox here, not that the bottomless wall we dive is really good for nitrox.

Thanks again.

 

[Outlaw14]

Thanks for all the info. I am just trying to fully understand my computer so I know how far I will trust it.
Thanks again.

Not sure if this is the right thread, so please direct me onward if not. Yesterday while diving in our local quarry, my vyper (with fully charged battery) gave me a deco ceiling warning as I was ascending to surface from a relatively shallow (avg 31 feet) dive. This was not something I expected to see in my normal rec diving days. It concerned me because a) either I am doing some weird profile or b) the computer is somehow flawed. I have about 20 dives on this computer, many to much deeper, and have never had a deco ceiling pop up before (no audible alarms though). It had recommended the normal 3 minute safety stop but the instructor I was with wanted to surface, chat with the group quickly and then get underway again. So the SS was not made. I was able to descend quickly enough that this whole event still counted as one dive. Even descending the computer briefly still showed the ceiling warning, but then soon disappered (right as I was trying to show it to the instructor). Nevertheless I am trying to figure out what could have made the computer issue that kind of warning. I can't remember exactly what it wanted, but seems like it was 8 ft for a couple of minutes.

The dive was relatively shallow, no deeper than 53 feet or so and then only there for a couple of minutes as it was rather chilly. This dive was dive 3 of the day and was after similar other dives. Had a 1hr 23min SI before this dive too.

Suggestions or education? Thanks in advance.

Lauren

 

[MechDiver]

The dive was relatively shallow, no deeper than 53 feet or so and then only there for a couple of minutes as it was rather chilly. This dive was dive 3 of the day and was after similar other dives. Had a 1hr 23min SI before this dive too.

Suggestions or education? Thanks in advance.

Lauren

Dive 3 to mid50 ft depths with short SIs could put you into deco with a Suunto, as they are very conservative. Without specifics its hard to do anything other than guess. Suunto has some dive manager software, I believe, that lets you plan repeditive dives. Run your days dives through it and see what it says
And, if you did go into deco, I would suggest paying more attention to what the computer is telling you while diving.

MD

 

[Outlaw14]

Dive 3 to mid50 ft depths with short SIs could put you into deco with a Suunto, as they are very conservative. Without specifics its hard to do anything other than guess. Suunto has some dive manager software, I believe, that lets you plan repeditive dives. Run your days dives through it and see what it says
And, if you did go into deco, I would suggest paying more attention to what the computer is telling you while diving.

MD

Thanks for the reply. I have already loaded the dives into Dive Manager and reviewed the output, maybe I missed it, but I didn't see any gotchas at first glance. I will go back and look again. Like I mentioned we were only at 50 feet maybe 2-3 minutes tops, the rest of the dive was rather shallow (out of a 45 minute or so dive). I know the vyper is conservative, that was one of the reasons I got it; however, I still can't fathom why it would have put me into deco. It's because I was monitoring my computer that I even noticed it. However, it all happened so unexpectedly and disappeared to quickly as well that there wasn't even time to follow its guidance. What can I post here that might give those of you more knowledgeable than I some clues about why this might have happened? Thanks for the help.