Faith diving and magic?

[MXGratefulDiver]

I don't rely on my computer for anything more than basic sanity checking ... I have a pretty good idea before I get in the water how much gas I'll need for the dive profile, and at what point I'll need to start heading shallower. Part of that is based on repetition ... you do certain profiles often enough you develop a basic idea of where you'll be at any given time. The other part is putting some thought into your dive before you get in the water ... the old "Plan your dive, dive your plan" adage.

In general, dive computers aren't really all that reliable ... because the NDL they're going to display is heavily dependent on the algorithm used to make the calculations. I used to own two dive computers ... a Suunto Vytec and an Oceanic Versa Pro. Once just for giggles I decided to dive them strapped side-by-side on my arm on a fairly aggressive dive to about 110 fsw. At one point in the dive, the Suunto was telling me I was skirting NDL while the Oceanic was telling me I had 26 minutes of NDL time remaining.

Which one am I supposed to believe?

The bottom line is this ... your no-decompression limit isn't a number, and it isn't based on your dive computer's algorithm ... it's based on your physiology, and on a number of factors that affect your physiology that change on a daily basis (hydration, sleep, stress levels, exertion, and a host of others). People who rely on their computers are generally OK because the computers have a lot of conservatism built into them ... some way more than others. But the other side of that coin is that people who rely on their computers can sometimes focus on the wrong thing ... never run yourself out of air because your computer's telling you that you "have" to stay down an extra few minutes.

Always remember that the chances of bending your computer and coming out OK are much higher than the chances of running out of air and coming out OK ... go with the odds. Better yet, don't push your limits to the point where you have to make that choice. If staying at depth an extra few minutes is really that important to you, then build it into your plan by taking a bigger tank. That way if you end up having to spend a few extra minutes shallow to keep your computer happy, it won't be a big deal ...

... Bob (Grateful Diver)

 

[scubadiver888]

In general, dive computers aren't really all that reliable ... because the NDL they're going to display is heavily dependent on the algorithm used to make the calculations. I used to own two dive computers ... a Suunto Vytec and an Oceanic Versa Pro. Once just for giggles I decided to dive them strapped side-by-side on my arm on a fairly aggressive dive to about 110 fsw. At one point in the dive, the Suunto was telling me I was skirting NDL while the Oceanic was telling me I had 26 minutes of NDL time remaining.

Which one am I supposed to believe?

... Bob (Grateful Diver)

I'm assuming the Versa Pro was telling you 26 minutes until NDL after you had come shallow. If you descended to 110 feet and then looked at your computer I would expect it to be some where under 20 minutes to NDL. The PADI tables have 110 feet at 16 minutes puts you at NDL.

I test software for a living. I tend to have a VERY negative view of computers. I'll trust my computer but only so far. If my computer told me I had 26 minutes at 110 feet I'd be a little leery of that computer. I might trust the algorithm if it was say 26 minutes after I went to 110 feet then came back up to 80 feet.

 

[Karibelle]

If you are still with me, then here is the reason for the post, how much faith do you put in your computer? (I risked our lives on ours).

That sounds less like a computer issue than a relationship issue to me... you knew you were close to your limit, you were uncomfortable with that, and rather than risk having your wife peeved with you, you went and took video of her. Don't blame your computer for that.

Glad to hear it ended well, and you've learned something from the incident. That's good.

-kari

 

[Bob DBF]

One factor that has not been brought up is the 80 -120' depth. As a recreational diver I know I don't do my best thinking in that area. That may account for the cavalier attitude and poor choices at the outset of the problem.

I dove the tables for most of my time diving, and if my computer allows me more time at depth than I thought reasonable I'd use my head rather than the computer. Conversely if the computer limits me I use the computer, due to the "new" dive tables, ascent rate, and safety stop (which I use as I see fit).

Bob
-----------------------------
I may be old, but I’m not dead yet.

 

[Dave1981]

I've run a computer into deco a few times on square profiles, accidentally leaving it on air and using ratio deco with 32% to govern my stops.

The thing with it is (lets say ascent from 100' for example), that if you have even 1min ndl left, as you start your ascent and do all the stops you'll have a pretty good credit by the time you get to the shallow stops. If you're 1min over NDL by the time you get to the surface you'll have picked up a good healthy deco obligation and it'll be in a right bad mood with you. They (well my gekko) penalise you heavily for not following the rules. Rightly enough I think, if you have no business going into deco best do a good bit extra to be safe, and the long wait is good incentive not to do it again.

Personally I just use a bottom timer now as when someone sees you come out the water with the computer screaming and heaps of deco left on it people can get a bit nervous. And it's annoying

 

[UCFKnightDiver]

How much do you count on your dive computer?

do you dive twice a day and it never really shows you near any limits so you feel safe?

Do you push every last second out of your computer and ride the thin line of NDL right to the boat?

Are you a thrill seeker and occasionally put your watch into Deco, but manage to clear it by the time you get on the boat?


Short story of a near miss, and maybe this will be moved there but it is in advanced diving now because it is not a technical or beginner discussion.

my wife and I were on our 4th dive of the day in Cozumel. Great diving and we had 3 computers between us. (stuff left out or this would be real long).

end of the 4th dive and my watch beeps so I look at it. 2 minutes to deco, oops we had been diving right up to the limits this trip, seems we did a lot of 80-120ft diving, the rays were about, and life is good. my wife and I usually can touch when we dive but the current and all the fun stuff to see and my interest in photography have kept us only nearby, and not hands length.

so 2 minutes, where is my wife. she has found one of her favorite sea creatures, a sting ray in the sand, so she is slowly swimming to it about 15ft down and 30 ft away heading down a sand slope. hmmm well my watch is bitching, so hers must be screaming..... I am the anal one, I am always pointing to my watch and she looks at hers sees that we have 5 minutes or whatever and gets mad at me for messing with her mojo. we have done this a lot so I know she is going to be pissed if i interrupt her time with her new friend to point to our watches. but no choice she is headed for trouble, and at that point I did not know how much.

I did some calculations, based on air, and predicted deco time, we would have enough air if we went into a small amount of deco at this point.***

So I swim after her, take one and a half minutes of video of her playing with the ray and then grab her arm and show her, her watch, and I get a look at it myself, 14 minutes of deco, OH ##$^^$*

we did an ascent to 20ft and started our stop, after a few minutes I realize that the computers are not giving us much credit on the obligation,** they wanted us at 10 ft, #$^#$ ok well we are too light to hang at 10ft, so clip off some of my weight to my wife and we go to 15ft, better but still not clearing her time fast enough, we are going to run out of air. I am working on slowing my breathing, better to get a head ache than to run out of air, she needed me because she does not usually think about this type of situation and I have.

Both of my watches clear (they did not show the same obligation despite being the same manufacturer). hers is at about 8 minutes*, I am thinking of the rouses and Ritchie Kohler, the seeker and other stories I have read, getting ready to call in the coast guard, (@%$# we are in Mexico) remembering first aid for DS. they have chambers in COZ so I was thinking about if we were within a 1/2 hour or not.

ok she is at about 5 minutes, i lost track but we are out of air time to face the music. we get out, I ask the DM for the O2 and put my wife on it for the trip back and apologize to the other divers for making them wait and scaring them.

No symptoms, no problems, bullet dodged.

*** My calculations were based on my computers not hers, mistake #1, #2 was not realizing the profile of the dives we had done, we did a lot of dives at 80-90ft with the average being about 60ft, we were at a limit that takes a long time to deco from not like single deep dives.

**With a shallow obligation you have to burn it shallow, ie at 10ft, we did not weight to be able to do a deco stop at 10ft, big mistake if you have a risk of needing it.

* we did a total of 21 minutes of deco, total dive time about an hour(58m) max depth about 80ft.

I will never do unplanned deco on the fly again, that was stupid and could have resulted in catastrophe.

If you are still with me, then here is the reason for the post, how much faith do you put in your computer? (I risked our lives on ours).
We do multi level diving these days, and as such we are a little off the reservation with the tables and the science and guesswork that went into them.

last story, I said I had 2 watches, one had the option for a different algorithm, and as a scientist of sorts, I put it on the other one for the first dive, it went to Deco when my original watch said I had 24 minutes. and on the second dive showed a deco obligation of 28 minutes at the safety stop when my other one showed nothing. so I had to switch it back to the same algorithm or else it would not be usable. what is that Alg. for? really old people with heart conditions? hypochondriacs? What science was used for that one that is so different from the other?

I hope this will stimulate some adult conversation and not a bunch of pointing out my mistakes.

And you're an instructor...??? I agree with JahJah in that you and your wife need to examine and learn proper gas planning before you seriously hurt yourselves or another!

As to relying on a computer, no I use a bottom timer.

 

[wve]

How much do you count on your dive computer?

Nicely told story. Thanks for sharing.

I realize that people have their preferences regarding air-integrated computers and such, but this story illustrates one reason that I do like them.

Although I occasionally approach a nitrogen or oxygen limit after several days of repetitive dives, the limiting factor is usually my gas supply.

I like my computer because it tells me how much time I have left at a specific depth, and it tells me whether it is the NDL, the O2 limit, or the gas that is in control of the time limit.

If it is the NDL, then it has also taken into consideration that I will ascend at a safe rate, that I will do a safety stop (or, heaven forbid, a deco stop), and that I will still surface with a predetermined amount gas left in my tank. Would it let me go into deco without enough gas to surface? It's conceivable that it might, but I'd have to be doing something unusual to get into that position.

Although it is easy to say "plan the dive, then dive the plan," it is not so easy to do when you are diving a different site every dive at a location where you have never been before. And when you are sight-seeing, even the guide may not be able to tell you exactly how long you will be at various levels along the way.

So, yeah, I use my computer quite a bit -- both to help me with gas management and with DCS prevention.

But I do not follow it blindly. Before I hit the water I have at least given some thought to what my NDL is at the maximum depth (ie, as if it were going to be a square profile), and from experience I have a reasonable estimate of how long I can stay down with the amount of air in my tank.

In other words, if I could make the dive with a table, then I'm almost certain that I can make it with a computer without violating any limits -- even if the computer has a more liberal algorithm.

 

[MXGratefulDiver]

Man, some of you guys are harsh.

Everybody makes mistakes. I know I've made a few I wouldn't want to talk about on a public forum ... some I could even look back on later, slap my head and say "what the hell was I thinking?"

Bet you have too ... so what say we try to focus on what can be learned from the incident, rather than flogging on the guy who was brave enough to bring it up ...

... Bob (Grateful Diver)

 

[Jax]

Good discussion -- where does complacency come in? I saw some complacency in SailNaked's dive, as well.

I'm not as experienced as most of you, but I have studied the algorithms and various models of computers.

If I were to be doing more than two dives a day, I will set my Uwatecs to the higher levels of conservatism . . . and I don't turn off the audio alerts. I may have a plan in my head, but I cannot calculate nearly as fast as my computers. There is no oops, I didn't know I was so close to NDL. Not following it blindly, but not complacent, either.

 

[AbyssalPlains]

Long story, simple lesson: Deciding to record a video clip when you're running out of NDL at depth and don't have enough gas to complete a deco stop is, well, uh, probably not the best choice a diver could make at that point? (Oh, yeah, I've done it ;-)

But I do think that having death flash before your eyes only because you may be missing five minutes of deco is a bit of an overreaction, don't you think?

To answer the original question, I trust my computer all the time. And why wouldn't I in the absence of anything else that I could trust? Since mine is probably the most conservative dive computer out there (Nitek Duo), I am ALWAYS the guy who is pushing his NDLs and sometimes going beyond, while everybody else is blissfully diving around below me, nowhere near their NDLs...