Depth differences

[LouieLouie]

Keep in mind that weather will affect dive computers. Atmospheric pressure is typically in the 930-1030 mbar range, which roughly translates to a margin of error of +/- half a meter at any depth. Altitude gauges are typically calibrated to reflect current weather conditions, but as far as I know, no dive computer is. Not that it matters much, though.

 

[miketsp]

As a general rule an electronic depth gauge (computer) will be a lot better than an analog one.
A good Bourdon pressure gauge will have an accuracy of +/- 2.5% at Full Scale Deflection and a poor one, +/-5% or even +/-10%. The % error will be greater at less than FSD.

An electronic one, on the day it leaves the factory will generally be a lot better.
Example my Suunto Vyper computer:
Depth Gauge:
• Temperature compensated pressure sensor
• Salt water calibrated, in fresh water the readings are about 3% smaller (calibrated complying with prEN 13319)
• Maximum depth of operation: 80 m [262 ft] (complying with prEN 13319)
• Accuracy: ± 1% of full scale or better from 0 to 80 m [262 ft] at 20°C
[68°F] (complying with prEN 13319)
• Depth display range: 0 … 150 m [492 ft]
• Resolution: 0.1 m from 0 to 100 m [1 ft from 0 to 328 ft]

Now this can be thrown right off after a physical shock or a repair.
I have a Citizen dive watch that matched my Vyper perfectly (within 0.2m) until I had to have it repaired. What happens is that the pressure sensors are graded in ranges and the electronic modules come in similar ranges. Unfortunately when my module blew, the repair shop didn't have the same class and I was faced with a choice of replacing both the sensor and the module to get a new matched pair (cost more than a new watch) or use a module that was not matched to the sensor or wait a long time for the right module class to arrive. I opted for the next nearest module class. The watch now differs from the Vyper by about 1m at 30m but it's still a lot more accurate than any of my analog gauges.

 

[miketsp]

I did a chamber dive to 50m at the Wesley Hospital in Brisbane. It was pretty cool (actually - it was very very hot!) I love the concept of being able to talk and eat pizza on the deco stop...

Anyway, I digress, we took our dive computers with us to see how accurate they were. I have a Uwatec SmartCom and it read 51.5m! I was pretty surprised with the variance, my friends mostly dive with Galileo's and they were within +/- 1%. I figured I'd rather have it read deep than shallow, that makes it a little more conservative per dive, but it was a good way to gauge the accuracy.

I'm just curious if you had your Uwatec in a bucket of water inside the chamber.
One of the problems of taking computers on a chamber ride is that the seals are designed to stop water ingress and not necessarily air ingress.
So there are 2 issues. One is that if air penetrates past the seals to the interior of the watch it can affect the components and shift the calibration significantly. The other is that if the interior of the computer does get pressurized, when you leave the chamber you may have a miniature bomb. This is the reason that watches designed for deep saturation diving (like Rolex) actually have a pressure release valve to stop the face plate blowing off. While the divers are in the bell the helium penetrates past the seals quite easily.
By putting the computer in a bucket of water during the ride you avoid both issues.

 

[gryder]

I'm just curious if you had your Uwatec in a bucket of water inside the chamber.
One of the problems of taking computers on a chamber ride is that the seals are designed to stop water ingress and not necessarily air ingress.
So there are 2 issues. One is that if air penetrates past the seals to the interior of the watch it can affect the components and shift the calibration significantly. The other is that if the interior of the computer does get pressurized, when you leave the chamber you may have a miniature bomb. This is the reason that watches designed for deep saturation diving (like Rolex) actually have a pressure release valve to stop the face plate blowing off. While the divers are in the bell the helium penetrates past the seals quite easily.
By putting the computer in a bucket of water during the ride you avoid both issues.

Hi Mike, yes, the computers were all placed in a bucket of water.