Depth gauge and computer discrepancy

Does anyone know how computers and depth gauges are calibrated? I was in PV diving Los Arcos in the Devil's Canyon (deep shelf dropping off to 1600ft). I was diving the wall with an instructor (was completing my PPB cert, but not on this dive) and my Oceanic depth gauge on my console max depth measured 120ft, while my loaner dive computer (Genesis) recorded a max depth of 98ft. Kind of freaked me out, is this normal? As the computer was a loaner, I do not know the history or the configuration. Which one can be trusted? I haven't invested in a computer yet, an have been using my console depth gauge to log my dives. What gives?

First of all, I cannot answer your true question about "Do I know how they are calibrated" however I personally would suggest having both checked out. I have read (not confirmed personally) that the manufacturers allow for up to 10% variance (tolerance) which I think is horrific but your 22% is even worse.

You were within Rec limits for both so I am sure there was no major problem but to have a variance like that is scary. What else is wrong with them is something you have to ask yourself. AND, which one is wrong. Did you figure out which one better matched the instructors?

My console depth gauge has a max depth needle and I only noticed it when I was resetting for the next day's dives. I used the computer's measurements for my log the first day without knowing that my gauge measurement was so far off. Should have compared with the instructors' other computer and redid my log. I don't think there was any problems as you mentioned. We completed the dive as multi-level and never approached NDL.

Making a Scuba depth gauge:

YouTube - making a scuba depth gauge

Jang:

Does anyone know how computers and depth gauges are calibrated? I was in PV diving Los Arcos in the Devil's Canyon (deep shelf dropping off to 1600ft). I was diving the wall with an instructor (was completing my PPB cert, but on this dive) and my Oceanic depth gauge on my console max depth measured 120ft, while my loaner dive computer (Genesis) recorded a max depth of 98ft. Kind of freaked me out, is this normal? As the computer was a loaner, I do not know the history or the configuration. Which one can be trusted? I haven't invested in a computer yet, an have been using my console depth gauge to log my dives. What gives?

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It's certainly not normal. Both of them should be showing withing a couple of feet of the actual depth.

A couple of observations:

In my experience mechanical guages can, but don't usually, start showing incorrect depths after long and intensive use. It probably has to do with metal fatigue or something of that nature. In any case I've had one that started showing incorrect depths after about...hhmmmm maybe....800-900 dives or so.

Computers can also be subject to showing incorrect depths. Compared to mechanical guages this is actually relatively common. The awful thing about computers is that *when* they malfunction they usually read "shallower", which needless to say, sucks. (my guess would be to suspect your computer in this case too).

Why computers do this has to do with any build up of salt crystals, debris or damage to the depth sensing transducer. On some computers, like the Sunnto Cobra, that sensor is positioned in such a place that you have to be very careful when cleaning it or salt crystals can start to cake over the sensor and make it read shallow. In other cases I've seen computers malfunction because the sensor was damaged or just not functioning correctly.

The lesson here being -- good cleaning after every dive is really necessary.

So what to do:

- First things first, try to figure out which one to trust. Check with your buddy and see how deep their guages say it is.
- If in doubt take the deepest one as the truth and if you're really not sure you should end the dive.

hope this helps

R..

First of all, is it possible that the 120' came from a previous dive where you never reset it meaning that particular dive you actually did not hit 120'? Also meaning that both may actually be closer to matching that you thought? If not, then I go on to say:

Out comes the overly cautious side of me. Given that difference, I think you really should have redone your tables using the Consoles depth (which was deeper) to be safe. Calculating your pressure groups at 100' versus 120' makes a difference....especially on repetitive dives. But, if you were done all diving by the time you established the problem then there was nothing could be done. Have you dove since to see if the problem remains?