Incorrect depth on Suunto Cobra

[cleomagic]

I have a Suunto Cobra that I purchased from my authorized LDS last August. This is my backup computer to my D9. On my first dive trip with this computer, I noticed during my safety stops (15-20 feet) that this computer was consistently off by about 2' in depth. This was confirmed against my D9 and my wife's Cobra and D6i (all were held next to each other). All three of the other computers were in agreement on depth. This 2' differential was consistent at depth as well.

I sent this back for warranty repair 4 weeks ago and I followed up with my LDS today who followed up with Aqualung warranty repair. The message I got back was that a difference of 2-4 feet was typical and there were no problems with this computer.

I've checked my manual for the Cobra and the specs for this computer is that it will be within 1% of depth. 2' at 20' is 10% which is well outside of the expected tolerances. Keeping within a 3' range for safety stops, unless you are hanging on a line, is not enjoyable. Even at 80' the difference should be less than 1 foot.

Has anyone else experienced this problem and behavior?

 

[NCadiver]

I would ask what the depth reading was at full scale, the manual say +/- full scale then back tracks on itself. Reading deeper is better than shallower.

 

[cleomagic]

At my LDS request, Frankie from Aqualung called me. He has run a complete test of my computer compared to another Cobra and they tracked in depth from 10' - 130'. He will be sending a copy of the report back with the computer. According to him, anything less than 1-2' in difference is acceptable. Anything more than 3' is unsafe and would be replaced under warranty. Obviously something was lost in translation between Aqualung and my LDS and myself.

One thing I did learn from additional searching on the web, and discussing with Frankie, is that if you don't turn the dive computer on before the start of the dive, it is possible the water contacts may not activate immediately upon entering the water. When this occurs, the computer calculates and records "0 feet" as the depth it is currently at (which could be as deep as 3'). As this is a backup computer, it is possible that it may have timed out between the time I updated the Nitrox settings to the time I actually dove. I'll have to keep an eye on this on my next trip.

From the Suunto FAQS (Diving | Suunto)

I have a Suunto dive computer. During a recent dive it was consistently showing depths that were 2 meters too shallow. This was first noticed during a safety stop in which I was deeper than I should have been. On the following dive the depths were accurate again. What could have happened?
It sounds like the water contact was not activated immediately but at 2 meters depth. In this case the depth sensor would read the pressure at 2 meters as zero depth and that's why the computer may have shown 2 meters too shallow all through the dive. When the dive computer is being activated it measures the surrounding pressure as reference pressure. For example air pressure changes can sometimes cause that the unit shows 0.1 meters as depth a few seconds before it goes to 0 meters, when it is being activated. It is recommended to activate the unit manually before a dive, even though there is an automatic water contact on the computer. The water contact may not always activate immediately after inserting the computer into the water and in this case the computer would read the wrong surrounding pressure as zero depth.
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[Hendrik@Suunto]

cleomagic,

The information that you were given by Frankie at AquaLung is all correct. Your Cobra is within specification, and the depth reading being off by 2' is not enough to compromise your safety. I would continue to keep an eye on your Cobra. If you notice that your computer starts to read off by any more than 2-3', I would strongly advise you to send it in for warranty replacement.

Best Regards,
Chris from Suunto

 

[sharkshark]

Weird, didn't find this thread yesterday, I tried resuscitating this one...
http://www.scubaboard.com/forums/co...nsor-inaccurate-possible-fix.html#post5124503

In my case, the comp was definitely on (had check air pressure before jumping in), and did the soak overnight in warm water thing, but still a good 15ft off. First time I've ever noticed an issue, the thing reads 0ft 'till you hit 15ft, then it starts counting in regular increments.

Proves the need for backup computers, in the least... Annoying this sport is, sometimes...