Dive computer accuracy-Depth

[bigduke12]

Unlike a lot of my buddies, I always dive with more than one depth measurement device.

Currently, I use an Aeris Manta computer and a Citizen Aqualand watch. The watch gives a digital readout of current depth, max depth, dive time and temp.

There has always been a discrepancy between current depth of the 2 of .3m (30 cm).

All the documentation does give a +-accuracy range.

For example, the watch instruction manual states:

Depth measurement accuracy:
• 0 m to 100 m: Within ±(1% of displayed value
+ 0.3 m) (when used at a constant temperature)
• 0 ft to 300 ft: Within ±(1% of displayed value + 1 ft)
(when used at a constant temperature)
• Guaranteed accuracy temperature range: 0°C to
+40°C (water depth range: 0 m to 100 m)
32°F to 104°F (water depth range: 0 ft to 300 ft)
* The accuracy of water depth measurement is
affected by changes in ambient temperature.

For example when my watch shows 15m, my computer will show 15.3m. The logs of both will show the same difference after a dive.

Anyone else have a similar experience or offer any insight?

 

[TSandM]

Absolutely. I have seen variations of up to 3 feet at 100 feet (3%) between gauges. Good thing decompression is not that exact a science!

 

[bigduke12]

Absolutely. I have seen variations of up to 3 feet at 100 feet (3%) between gauges. Good thing decompression is not that exact a science!

I agree. For interest sake, I am trying to determine which one is the accurate one (if any).

I do know that the watch samples atmospheric pressure every 5 minutes, as a reference for 0m.

I believe the Manta computer, only does this when the dive surface mode is activated, and then every 15 minutes until a dive is made, or 2 hours if a dive is not made.

I wonder if this can account for the difference?

 

[damdim]

There is also a difference if the gauge is calibrated for fresh or salt water...

 

[Gilldiver]

There can also be a variance due to salt water density and temperature layers. The water in the Red Sea can be much more salty than in the North Atlantic, so is denser and has more weight per meter depth. So, you will read shallower in actual linear depth in the Red Sea than off of North Carolina. Decompression wise this doesn't make a diffrence as it is the true gas pressure, not the true depth that affects decompression

 

[squid509]

I have worked calibrating gages and I can say none are 100% accurate. the expansive high accuracy gages that are used to calibrate other gages have a tolerance or +- 1% it is normal to see different gage reading the same pressure give slightly deferent reading as long as they within acceptable tolerance

 

[bigduke12]

I have worked calibrating gages and I can say none are 100% accurate. the expansive high accuracy gages that are used to calibrate other gages have a tolerance or +- 1% it is normal to see different gage reading the same pressure give slightly deferent reading as long as they within acceptable tolerance

What would be an acceptable tolerance.

There is always the same difference with mine. 30cm.

I would think the watch is accurate, as before I got a computer I used the watch with a mechanical depth gauge on my console as back up.

They were always the same.

I was thinking it was maybe a "safety" feature built into the computer, so it reads marginally deeper than you really are.

 

[michaelw]

I just sent a Mares Mission console back for replacement that was off by 11' at 92' if I remember correctly. I would have been just fine with it being off two or three feet.

 

[skankpile]

Find someone with a Cochran and dive next to it if you want to know the actual depth. The Cochran automatically compensates for salinity and measures every 1S from the transducer. My shearwater stays pretty close, but when I want to know my actual depth I always turn to Cochran.

If you send you log to cochran they can tell you how big the swells were 100' above you as they actually record internally in 4 or 6" increments.. I can't remember which, but its in inches.