Computer Failure

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zboss

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Hello,

My brother asked a very good question last night. He is a dyed-in-the-wool advocate of analog guages (keep in mind that he has never owned a computer but that is another matter) - he asked "What happens if the computer fails on a dive?"

My answer to him was "If the computer dies and you are only diving an NDL profile, simply abort the dive, being sure to make your safety stop." He didn't seem to understand the concept that a computer is more accurately determining your nitrogen tissue saturation levels - and regardless of if the values displayed disagree with manual tables - you are still within the NDL.

However, his next question was interesting... he asked "how do you know that it's counting right?" My initial reaction was that it wouldn't start miscounting... but after thinking about it (I am an independant verification and validation manager for software) I began to wonder if there were verification routines coded into the computer. In other words - do the computers use redundant calculations to ensure that the values are correct? Does is check it's own math? Does anyone know?

Also - has anyone even heard of a computer extending out the NDL due to a functional or performance defect? How often do you perform a safety check of the actual vs. real depth displayed? How would one do that - check it at multiple levels, at the begining of the dive or the end of the dive?

I know my aeris atmos II temp sensor is screwy but that doesn't bother me because I am not diving in anything very cold, but it makes me wonder about the other sensors...
 
I have had my computer go haywire on me once. I left it out in the sun for about an hour before we dove, and it gave me bad depth readings the entire dive. I know this because I was in a quarry I've been in many times before, and I know where 10, 15, 20, 40, etc. is.

This is why you always have an alterior way of determining your max dive time and why you should always have a contingency plan. The computer failed me during the dive, I had a back-up table profile in my head for what we expected to do, I had a bottom timer & a depth gauge.

People have become way too dependant on computers....there is no reason not to have a contingency plan and be prepared to use it.
 
zboss once bubbled...
"how do you know that it's counting right?"
Does is check it's own math?
Also - has anyone even heard of a computer extending out the NDL due to a functional or performance defect?
How often do you perform a safety check of the actual vs. real depth displayed?

It's very unlikely to screw up on the math, but there are other problems you need to watch for. They all center around the accuracy of converting the strain gauge pressure sensor signal to depth.

Big-T mentioned a typical problem, one that I have also seen, where a sun-warmed computer goes crazy --- the case I saw it started a dive when first immersed in water. The fast temp change triggered a dive start with a +6' depth error (that reduces NDL's). Even at the surface, the computer thought it was still 6' under.

It is also possible to descend so fast that, before the computer does it autocalibration (which determines what the computer calls 0'), you are a few feet under the water. That will be bad, since you are deeper than what the computer thinks. In the case of this I saw, the error was about 10 or 12'. Forgetting to push the button on a button-activated computer until you've started your descent can also cause this problem (don't ask how I know).

Both of the above errors are easily detected by checking your depth a few feet below the surface, at the same time you do your first gross bubble check.

A more subtle problem can be caused by salt crystals on the pressure transducer. In this case, it shows up as an error that increases with depth --- and may be hard to detect. The only way to check it is to compare it with another computer or depth gauge.
If you rinse out your computer you will never see this problem, but some console mounted computers may crust up with salt if you never take them out of the console for a rinse. A few hours in lukewarm water should fix the problem.

Note that electronic bottom timers are also subject to all of the above problems since they use the same type of pressure sensor.

Checking your computer isn't difficult -- when you and your buddy agree to start the dive at 80' and he levels off at 74', you had better compare gauges. Even if you don't suspect anything, it only takes a second to glance at your computer, then look over at his.

Charlie
 
Thanks both of you for your complete responses! I am very good about making sure my equipment gets dunked after a dive, however I had not thought much about leaving them exposed during a surface interval.
 
We deal with the possibilities of computer failure everyday in the cars we drive, the lights at intersections, the planes we fly on, the hospital wards where babies are born.

There will always be ludites, but fear of technology does not change the fact that we use and depend on technology and must have contingency plans to deal with failures when they happen. Failure to address these issues with the entry-level diver will not make them go away or reduce their severity but it might save a life.

It is a fact that the average income of recreational scuba divers is significantly higher than that of other sports, which implies that the jobs they hold pay more, jobs that pay more generally require clarity of thought and the capability to adjust to changing requirements on the fly. If we take this to be true then we are short changing the intelligence and capabilities of the average divers by not requesting more performance from them and then rewarding them for their performance. I strongly suggest that organizations teach contingency planning for computers for the sole reason that they happen and the average diver is intelligent enough to deal with them if we have all the facts of how these accidents occur (assuming a good and caring instructor).
 
zboss once bubbled...
There will always be ludites
luddite has two d's... but I'm sure that is what you meant.

Actually zboss... being dependent doesn't make you a cutting edge kinda guy. :D

But I apologize... this is your thread and I shouldn't have intruded.

You are right... if you are dependent then you do need a contingency to deal with the failures when they happen.
 
However - the point being made is that the reliability of computers when used in normal recreational diving is extremely high. However - knowing when and how failures may occur, and then practicing drills to accompany that knowledge is key.
 

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