Are dive computers making bad divers?

[Landstander]

However I encountered a 'subtle' failure mode on my Suunto Mosquito dive computer early this year. The pressure transducer failed in such a way that the computer was under reading the depth by 3m, based on my observations on the anchor line on the ascent. I can only assume the NDL calculations were off in accordance with the incorrect depth reading. This was a particularly insidious failure as the computer still appeared to be working correctly and on a deeper/longer dive I could easily have unintentionally omitted deco because of it.

This seems like something which could just as easily occur with a depth gauge, however.

 

[oya]

I feel like this thread should be merged to the Deceptively Easy Way to Die thread.

 

[archer1960]

I saw a perfect example of people not paying attention to their PDC (or thinking) some years ago. The group of about 10 divers did a first dive, square profile, to 42m with one DM and then after 1h of SI we jumped in at the second site with bottom at 24m but with a second DM that had sat out the first dive and who's PDC was zeroed in terms of N2 loading!
The DM went straight to the bottom and stayed there. After just a few minutes we started going into deco on our PDCs. Only three of us noticed this and started going shallower to stretch out the dive. The remainder of the group stayed at depth even though we made signals to them to look at their PDCs and come shallower every time they looked up and we got their attention. The DM was off in the front of the group oblivious to what was going on behind him.
To cut a long story short most of the group accumulated a significant deco obligation which wasn't paid off. The DM went white when I pointed out to him what had happened when he surfaced - he simply hadn't realized that the group had gone so deep on the first dive and had accumulated a significant N2 loading.
He just followed his (clean) PDC and didn't think.
To this day I don't know why we didn't end up with 7 people going for a chamber ride.

At least he realized that this was a problem...

 

[fjpatrum]

In a word, no. Tools don't make bad divers. Complacency makes bad divers, whether it's a PDC, a depth gauge and a timer, or anything else.

 

[stuartv]

I wonder if that particular issue is down to whether it was reading the right "type of water" ie was it set for the wrong type (ie fresh water when it should have been seawater or vice versa).

I don't think so. I think, even at 40m depth, the difference in depth reading between a computer set to Fresh vs Salt is MUCH smaller than 3m. I think the difference would be a few cms at most.

 

[archer1960]

I don't think so. I think, even at 40m depth, the difference in depth reading between a computer set to Fresh vs Salt is MUCH smaller than 3m. I think the difference would be a few cms at most.

IIRC, the difference is ~2.5%, which is about 1m at 40m depth.

 

[Storker]

I think the difference would be a few cms at most.

For rather liberal values of "a few", sure. The difference is some 2.5-3%, so at 40m the error would be about 1m.

Not that it matters for on-and offgassing, though. Particularly since your "depth" really is measured in pressure units, and your on- and offgassing depend on pressure, not on real, physical depth.

 

[Hatul]

I don't think computers are a bad thing. Blind reliance on them is.

For shore diving tables are way too conservative. For square profile dives though they are great.

A sample shore dive (one I have done on a computer) -45 minute duration max depth 18m. Surface swim to buoy at A, descend to 5m, swim to 18m following the slope down to point B, run parallel to the shore at 18m to point C, ascend to 5 following the slope to D and return to A swimming at 5m. Doing this on tables would put you in group S on PADI tables as you would have to take the dive duration of 45 minutes at 18m.

Now that would be very conservative especially when you consider that a computer will have you with a huge amount of time left before any deco because it will give credit for the offgassing that will be done (the whole of the last leg is effectively a safety stop).

The other thing that a computer will give is the ability to vary a dive based on what is happening ie you decide to follow a seal/dolphin/fish deeper and the computer will vary it in real time instead of having to throw the entire plan out of the window. Unless you are very good or have tables with you you can't replan a dive on the hoof underwater

You're exactly right. A dive computer allows safe long multilevel dives not possible with dive tables. When I started diving in the late 1980's I went on a dive vacation with Brett Gilliam's Ocean Quest and we had to use dive computers for that reason. It was early in the evolution of computers but we were given the Dacor Microbrain to allow multilevel recreational diving. I take tables on dive vacations but never really use them.

 

[dberry]

And you thought that metric was some how superior.

99 times out of a hundred, it is That was the exception to the rule

Being comfortable with either (but generally preferring metric), I'd have to change that to "5279 times out of 5280 metric is better". But the "120" rule is definitely a worthwhile exception.

 

[EFX]

For rather liberal values of "a few", sure. The difference is some 2.5-3%, so at 40m the error would be about 1m.

Not that it matters for on-and offgassing, though. Particularly since your "depth" really is measured in pressure units, and your on- and offgassing depend on pressure, not on real, physical depth.

DC's can't measure depth directly. They measure pressure and given the type of water (fresh or salt) can calculate depth which is then displayed on your computer. Physiologically, inert gas loadings depend on pressure but this raises an issue: what does the algorithm actually use? According to Baker's paper's the pressures are converted to fsw (or msw) before being used in the calculations. The type of water entered is used to modify the calculations. If this is the case your computer will not calculate the correct NDL/deco for diving in the Dead sea which has a much higher density than typical ocean water. If the calculations are done in pressure units then it doesn't matter what the medium is.

I'm thinking of rec computers. Do any of the tech computers let you enter water density so that depth-based calculations can accurately calculate gas loadings?