Checking Cobalt's depth sensor

[RonR]

You can easily check the Cobalt's depth sensor accuracy by using the atmospheric pressure reading on the System Info screen.


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.

The Cobalt displays actual pressure data- the actual surface ambient pressiure 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 conversion, and will also convert units to mbar if the data is inches or mercury or other units.


As a rule of thumb, the correction is about -35 mbar per 1000 feet of elevation, or -50 mbar per 400 meters. This is about what is applied for the first few thousand feet. But the calculator will be more accurate.


We have had a few users become concerned over this apparent discrepancy in readings, when the issue was the local pressure data had been corrected to sea level equivalent.

Ron

 

[gonnacall]

Add another Cobalt 2 to the depth sensor issue, first dive no issue second dive ended with a 4-6' difference. On the boat (4 feet above water) still showed I was at 2 feet depth for about 4 minutes. Still waiting for AA to call me back, good thing I was running 2 computers and haven't put my trust into the Cobalt yet.

 

[RonR]

Add another Cobalt 2 to the depth sensor issue, first dive no issue second dive ended with a 4-6' difference. On the boat (4 feet above water) still showed I was at 2 feet depth for about 4 minutes. Still waiting for AA to call me back, good thing I was running 2 computers and haven't put my trust into the Cobalt yet.

This is the issue we are concerned with, and why Atomic stopped shipping Cobalt 2's until it was fully understood and fixed. The was a problem with some of the depth sensors, or more accurately with the assembly packaging that came from the sensor manufacturer. If present, it will cause the sensor to register slightly too deep in a dive and after- as you experienced. We have not seen vary many, but it is a known issue.


This is a real aberration- we and many other dive computer manufacturers have used these sensors for years and they have been very accurate. It isn’t caused by anything Atomic did, or the design, it is an issue with how these parts were handled, and it has been corrected. I expect you will hear from Atomic soon- probably they are busy after the holiday weekend.

Ron

 

[gonnacall]

Ron

Do you have a serial number where the good ones start at? Mine is/was a 1408- (2 days old to me). Also is there anything in the works to warn users of older affected units of the possible issue? With buddy diving you always go by the most conservative computer and we would up spending extra time at 13' waiting for the cobalt to clear after staying for 3 minutes at 20 with the cobalt not even seeing me in the safety stop range. I was also with a guy in a re-breather so it wasn't good for him to be hanging around at that depth for too long.