Cobalt 1 inaccurate temperature indication

[Coral-Reefer]

Cobalt Serial Number: 1912-7388
DiveOS:V.1.13/2
Dive data storage every 30 seconds

I love my Cobalt computer.

During my last trip to Cozumel, from which I just returned, I was trying out a new 3mm suit and thought that it was not keeping me as warm as my old one because my computer indicated 81-82°F during every dive and I was getting cold. My 3mm keeps me warm between about 78-82°. Below that I wear a thicker suit and above that I wear a 3mm short suit.

Everyone else's computers were reading in water temperatures of 77-78°. Mine showed slightly lower air temp than the others and would drop to 81° during descent, stay at 81° through the dive and increase 0-1° upon surfacing.

Any suggestions on how to correct this inaccurate temp display? I am leaving for Cocos Island in 3 weeks and want to be sure my computer is operating correctly.


Thanks!

 

[drrich2]

This issue has come up in other posts. I've got a Cobalt 1 and an Oceanic VT3, and the latter consistently reads a bit higher. To summarize what I've read in other threads, and my understanding of it, the gist was that the Cobalt 1 has a good temp. sensor, but it's inside a fairly substantial body and thus insulated from its surroundings, and so make take considerably longer to reach the actual ambient temperature. Compare that to the drastically smaller body of the VT3 wrist unit, which probably 'chills' a lot faster.

I'm under the impression it's something to be aware of and compensate for in your mind, rather than re-calibrate. Be interesting to see if anyone says different. Your computer seems to give reading around 4 degrees above 'actual' (if there other computers were right).

Richard.

 

[Coral-Reefer]

Thanks for resonding, drrich2!

Yeah, I looked around a bit before posting and couldnt find anything on SB but didnt do a Google search. What lead me to think there is an issue is that the surface temps seemed correct and being at depth for over 45 minutes where the actual temp was 78° would have yielded a temp lower than 81°. Also, the indicated temp never went below 81°. My concern, albeit a very low level, is that the algorithm takes the water temp into consideration when doing calculations and displaying indications and warnings, which would not be accurate. I could increase the conservatism of the device to compensate, but for the price of the computer one might think the temp would be more accurate.

 

[RonR]

Richard is correct about the console case holding heat. Additionally I think that the Cobalt, being a console, tends to be held in the hand a lot and the temperature sensor is located right about where you would grip it- it's just under the lower battery door. It has very limited water flow, so it warm water from your hand gets in it tends to stay there. It's also possibly picking up some heat from the electronics or the display backlighting. We consistently get reports of Cobalts reading a few degrees higher during diving, I have on many occasions now taken these same Cobalts and put them in water with a calibrated thermometer, and once the Cobalt has time to reach equilibrium the temperature readings from the sensor have been within a degree or two of the thermometer- that is about as accurate as the sensor will get.

The algorithm actually doesn't take water temperature into account, so there is nothing to worry about there. We generally recommend cold water diving gets set to a higher exertion level to account for higher workload in keeping warm. But there is no way to know how warm an individual's tissues are based on water temperature. I've been very cold in warm water, and been overheated in very cold water. Temperature does factor into the depth sensor calibration, so it's important that the actual temperature of the sensor is used. It seems for many be a few degrees above the actual "outside" temperature.

Hope this helps.

Ron

 

[Coral-Reefer]

Thanks for the response Ron.

Would dimming the display help to achieve a more accurate temperature?

Can the display be turned off during a dive or just set tot he lowest display rate?

Chris

 

[RonR]

Dimming the display would produce less heat, I can't really say if that would alter the temperature reading in any particular case. I suspect most of the warming is from hands. You can't turn the display off during a dive, but if you use manual control (Cobalt 2- the Cobalt 1 always has manual control available through the Down button while in a dive) you can dim it.

Ron

 

[Coral-Reefer]

Thanks, Ron. The computer lives clipped to my retractor unless I need to check it, except for the 3 minutes of my safety stop when I do have it in my hand the whole time. When the 3 minutes is up it goes back to the retractor until I surface. OK, I am finished about this topic. Thanks to every one that helped me out with it. Still love my Cobalt.

 

[deeper thoughts]

The wife and I now just minus 4 degrees. We have compared it with numerous devices and it does read 4 degrees warmer

 

[Coral-Reefer]

Well, that explains why I was cold with my 3mm when it was 81 degrees. It wasn't, it was 77 degrees. Good thing, because I thought I was getting old and susceptible to the cold!!

 

[Tucsonwilly]

Ron,

I have a Cobalt 1 which I think is fantastic. I have one question regarding the temp reading. I am in a room at about 75F and my Cobalt is now reading -30F. Everything else is working fine. I start a weekend of diving tomorrow. Any advise on using/not using the Cobalt since it does not play into any other function if I understand correctly.