There was an issue with Cobalt 2 production, early on, with some potentially bad depth sensors that passed all testing but fail in the field. If these fail, they will read a slightly deeper depth than is accurate. In many cases this would not be noticeable, but if the discrepancy is over 3', it can mean the computer won't come out of the dive- a major irritation. It sounds very much as if this is what you are experiencing. It may appear to correct if out of the water for a long while, but we need to regard this correction as temporary. The only fix is to return the Cobalt to Atomic for a PCB transplant that will give it a new depth sensor. I would encourage you to contact Adan at Atomic
adan.campos@huishoutdoors.com - they are very good about service. This issue was covered extensively in the Computers section of SB on the Cobalt 2 thread.
http://www.scubaboard.com/forums/computers-gauges-watches-analyzers/475970-atomic-cobalt-2-a.html
This isn't an ongoing problem, except in the sense that all dive computer depth sensors will have some failures. This identical sensor is used by many dive computers, and has been for years.
If anyone has a question about their Cobalt 2. You can easily check the accuracy of the depth sensor by comparing the atmospheric pressure reading on the System Info screen to a known local reading at your altitude. It should be plus or minus 30 mbar or so (a foot of sea water = about 30 mbar), and the temperature reading should be within reason. Most places have a local weather station you can access online- we use one from a local university. Unfortunately most of the commercial weather services in the US display the pressure in inches of Mercury, To convert inches of mercury to millibars, multiply the inches value by 33.8637526. The value the Cobalt displays should go up and down with actual atmospheric pressure, and not oscillate by large amounts within a few minutes. Changes of several mbar over a short span of time are normal, however, as atmospheric pressure does fluctuate.
Note that if you are above sea level you will need to convert local weather barometric pressure data to remove the altitude correction that is usually added by weather services. You need to know your actual altitude to do this. The Cobalt displays
actual pressure data- the surface ambient pressure is what is needed for decompression calculations. Most weather services provide a
"sea level corrected" number. Unless the value is specifically listed as "station pressure”, this correction must be removed to determine the physical atmospheric pressure. Online calculators such as
Above Sea Level Barometer Reading Correction Calculator can do this for you.
Please feel free to contact me if you have any questions, but Atomic should be able to fix this for you quickly.
Ron
