Dive Computers and Deco

[Bubbletrubble]

Thanks Everyone! A couple of very nice computers mentioned. Being early in my dive career, I've been looking at the Element II, Zen, react pro, or Archimedes and such. I'm glad to find that they might be overly conservative on a deco dive, but still useable. And continued value if I move to more advanced diving. Any other thoughts?

For recreational dives, pretty much any computer on the market will be good enough for your use. All dive computers will continue to operate once NDLs are exceeded. At that point, the computer will instruct the diver to do a deco stop (or series of stops). As others have already mentioned, so long as you fulfill those stop obligations, the computer will continue to function.

Look for a computer with gauge mode and user-replaceable battery. There are certainly other options to consider (nitrox-capability, ease of use, backlighting, legibility, wristmount vs. console, PC downloadable, conservative vs. liberal deco algorithm, manufacturer's warranty, etc.). Your budget will also play an important factor.

 

[Sephiro444]

I hate to resurrect old topics, but the post just previous to mine sets up the exact question I have regarding how dive computers handle deco.

Using computers designed for more recreational diving (specifically for me, Oceanic's OC1), I understand that if you go past the NDL you'll have to do a computer-prescribed deco stop(s), and that doing those results in no ultimate lock-out. Not that I'd plan to do it, but what if the computer told you to stay at 30 ft. for 15 minutes, and you stayed for 10, then ascended (presumably with the PDC screaming at you the entire way)? Would you be "locked out" of diving for 24 hrs? Your nice, pretty dive computer becomes a liability you want to think about leaving behind for a while because the only alternative is losing a day of your dive trip?

Again, I'm not condoning entering deco on rec diving in the first place, much less a rapid ascent if you do... I'm just curious about what the computer will do, since hopefully I'll never have to find out for myself.

 

[Cleavitt]

Sephiro444,

If it called for 15 minutes of deco and you only did 10 then you would be in violation and locked out. You can't really blame the computer though. You have ignored the computers recommendations and put yourself in an unpredictable situation. As far as the computers algorithm is concerned, you are probably bent.

Something to consider though...

If you are relying on your computer to calculate deco and it's telling you 15 minutes, why would you decide that 10 is good enough and surface early? It would be similar to looking at the dive tables and then deciding to dive 10 minutes past NDL without deco stops.

While it could happen in the real world for good reason (it's happened to me a couple times) it's unlikely in normal recreational diving situations. The most likely situation is a diver that accumulates an unplanned deco obligation and doesn't have enough gas to complete the deco time. In that situation, the lack of planning is a serious issue and the omitted deco means the diver really should stay out of the water.

BTW: 15 minutes @ 30' is some pretty serious deco on most computers. Most computers start at 10' and add deeper stops at 10' intervals as the deco obligation grows. 15 minutes at 30' probably means about 45 minutes at 10' and some amount of time at 20' as well (this is a guess, no tables in front of me). I know you were just making up an example, but I thought you might be interested.

 

[DiveNav]

Why do many computers lock out for 24 hours once they enter deco? .....

We have analyzed / reversed engineered a lot of dive computers and so found we have not found ANY of them locking out because diver went into deco.
They lock out when diver does not properly follow the dive computer deco procedures (as an example ascending above the deco stop ceiling while performing a deco stop).

Alberto (aka eDiver)

 

[Sephiro444]

Cleavitt, you are absolutely correct about safe dive planning and following the PCD's instructions. That said, I just made up the example deco stop time and depth. My point was that dive computers are there to simplify calculations and take into account the full range of depths into determining surface intervals, gas loads, etc. It seems counter to those aims to lock the user out for an arbitrary amount of time because he ignored the instructions related to five minutes of a deco stop.

Also, IS the period on most PCD's for ignoring the deco procedures a 24-hour lockout? I've never had to experience it (and hope not to), hence my curiosity.

 

[ianr33]

It seems counter to those aims to lock the user out for an arbitrary amount of time because he ignored the instructions related to five minutes of a deco stop.

When you ignore the stops the computer is asking for you are outside of the computers algorithm. It no longer has any idea where you are as regards nitrogen loading and bubbles. So it gives up,plays safe and locks you out once you have surfaced.

More sophisticated and expensive computers (Shearwater ,Liquivision) will give a best guess and not lock you out.

If you are not going to follow a computers deco instructions why use it? If you surface early due to some emergency a prudent diver (assuming he's not bent) would take at least the rest of the day off anyway.

This stuff can kill you!

 

[Sephiro444]

Absolutely, ianr33. My persistent questioning here is specifically so that I can satisfy my curiosity here as opposed to a real situation.

When faced with something like a new PCD, I'd rather know how it's going to function BEFORE I'm actually in the water. That's almost entirely what accounts for the success of something like DiveComputerNavigation.com, which btw I cannot praise enough! I just finished the OC1 course and quizzes, and seeing very gradual, step-by-step tutorials on how the computer will behave in the water before I get in the water is invaluable to me.

 

[Slamfire]

(My husband's Suunto -- set on the proper mix -- gave him 20 minutes of shallow deco on a dive when none of the other five divers had any deco at all. He did it, because he wanted the computer to continue to work.

According to the manual of my Mares RGBM (same algorithm as Suunto RGBM?), it will do certain repetitive dives automatically setting the M-values more conservatively. In my computer you cannot disable or tweak that feature. Also, if it tells you to do 20 mins at 10', it better be at 10' otherwise 20 mins easily turns to 30 mins if you do that stop at 15' or 20'.

...It no longer has any idea where you are as regards nitrogen loading and bubbles. So it gives up,plays safe and locks you out once you have surfaced.

More sophisticated and expensive computers (Shearwater ,Liquivision) will give a best guess and not lock you out.

IMHO, it's more about liability issues than not having an idea. Technically, I don't see what would be so difficult in a computer still tracking theoretical tissue loadings even after M-values have been exceeded. Phisiologically, when the M-values are exceeded you are at high risk of getting bent, so it maybe less risky legally, to have a computer lock out, rather than keep on functioning giving instructions to go back down, or even tracking your nitrogen loading for a new repetitive dive. I think it was GI3 that said that getting bent is just another excuse to go diving right away.

Along those same thoughts, I wonder if computers that keep on working after a violation -- eg Shearwater, Liquidvision -- could be useful for in-water-recompression.

When faced with something like a new PCD, I'd rather know how it's going to function BEFORE I'm actually in the water.

Read the manuals that came with the computer. They're usually pretty good at telling you what will happen to the computer when you violate a deco stop.

FWIW, my computer is a fairly cheap Mares Puck with "deep stops". It has nitrox capabilities to EAN50%. Single gas. Has a bottom timer mode. AND it will not lock you out completely. It WILL lock you out to bottom timer mode for 24 hours. Which is very nice when you do multi-gas staged decompression dives, since you can use it with the regular algorithm on as another lost deco gas scenario. And after that first dive you do have a bottom timer, not a useless paper weight.

 

[fnfalman]

When faced with something like a new PCD, I'd rather know how it's going to function BEFORE I'm actually in the water.

At the risk of sounding like a smart ass, but you know that by reading the operator's manual. The manual will tell you when the computer will lock you out.

 

[garyd54220]

I agree with the comments on the Nitek Trio. If all you're going to dive is Nitrox then the Trio or the other Seiko computers (Tusa IQ-950 Zen Air, Cressi Edy II or Zeagle N2ition3) will work fine for you for a long time and you can use them as a back up BT when you move up to a trimix computer. In most cases I don't even think you'd need 3 mixes so could save money and buy the 2 gas versions; diving just one back gas and one deco gas (most likely 100%). I have never personally owned a Seiko as I started out with very basic rec computers and then moved up to a Vr3 and now a Shearwater. My impression is that they appear to be well made and have a reasonable algorithm.


Gary