Help.. my recently replaced Cobalt wont read air guage

[chillinme]

We have 2 cobalts... purchased around 18 months ago in Australia. Both had around 40 dives on them before they failed. One was a sensor and it was replaced with a brand new one. The other was just repaired.

The brand new one now has 10 dives on it. 4 in the last 2 days. Our first days diving everything was fine. Today I entered the water with 208 bar. 15 minutes into the dive it read 0 bar and we aborted the dive buddy breathing because we had no idea what the issue was.

The second dive the guage read 198 bar. Less then 5 minutes in it read 30 and falling fast. We let it run to 0 and basically snorkled through the dive at 5 metres holding my buddies hand in case the guage was somehow right. I left the water and swapped computers with my buddy. There was 120 bar left in the tank.

The worst part is we're on holidays in fiji. Diving Namena island and today was probably the only chance we'll ever have to do the dive. The other divers enjoyed hammerheads and blacktips and pigmy seahorses while we followed there bubbles and swore at the cobalt.

We have 5 days left in fiji so have bought a new air guage. There was nowhere to hire one where we are. The cobalt will be used for the remainder of our dives unconnected from the first stage and we'll send it back when we get home. Can anyone advise why the air guage might fail and is the other one safe to rely on. We seem to have a lot of aborted dives due to these computers failing during our dives. Why can't they do it when we are at home...lol

 

[HIGHwing]

Why can't they do it when we are at home...lol

You might be careful what you wish for.

 

[RonR]

So if I understand properly, this Cobalt reads tank pressure correctly on the surface and at the beginning of a dive, but heads to zero- not instantly but with rapidly dropping pressure- when you start breathing from the tank?

That scenario to me points to a possible problem in the hose, QD, or regulator port, something blocking the continued supply of high pressure air to the Cobalt's HP sensor once you start breathing from the tank. If the sensor or electronics were failing you would most likely see an immediate change, and correct readings would be unlikely to come back. Have you tried swapping hoses or QD fittings with the other Cobalt for a dive to see if the issue is there? What happens if the Cobalt sits for a time with HP connected, say post dive, without breathing from the tank, does the pressure come back up? Or post dive, when you swapped Cobalts to get the correct tank reading, did you put yours back on, and if so did it read the correct pressure or zero? By that I mean does it read correct pressure when the air is first turned on, but drop afterwards?

I can't, of course, diagnose at a distance effectively, it's possible this is a problem with the HP sensor, but this is not an electronics failure mode we have ever seen. The sensors typically either work or don't, with no sometimes mode. While there is no problem running the Cobalt disconnected from the HP hose, it might help figure out what was wrong, and a possible fix, if you could check the Cobalt against the gauge.

I'm going to PM you with instructions for checking the calibration of the HP sensor, I don't quite see how it could be a calibration issue, but it would not hurt to check. Keep me posted.

Ron

 

[chillinme]

Thanks Ron... I have replied to your message but the cibalt doesn't just drop the air reading... it fluctuates - even when disconnected from the HP hose. It does the same whether connected to a tank or not although on the surface it seems to read correctly (200ish bar on a rank and zero if disconnected) . We have tried the computer on both hp hoses and have the same result.
We'd stopped diving with that computer even disconnected because everytime it hit zero the alarm would go off anoying other divers and nothing I can find in the settings will turn that alarm off. The low air alarm is turned off.

---------- Post added October 31st, 2013 at 04:26 AM ----------

Just looking at the logs... my last dive with computer disconnected. The dive started with zero bar and ended with 251 bar. Dive before that also disconnected. I started with zero and finished with 46. During these dives the air reading contantly jumped up and down and back to zero.

The two previous dive the computr was connected to the hp hose. Those readings on the log say started with 189 bar finished with 0. And 204 bar at the start and finished with 12 bar. We aborted the dive that read zero... the other one we stayed very close to the surface but and came up after 25 minutes unsure how much air I actually had. The other gauge said that tank had 120 bar at the end. Hopefully that all makes sense.

 

[RonR]

Ok, I see. If it's jumping around rather than just slowly dropping to zero, and it's doing it on the surface when disconnected, then it does sound more like a sensor problem, or possibly a water leak. But electronic in any case, so scratch my earlier comments about air pressure. The signal coming from the HP sensor is very small, so even a tiny bit of moisture getting onto the contacts could cause erratic behavior. Apart from that these sensors have been very reliable.

If the HP calibration were totally wrong, something like this could possibly happen, so using the instructions I sent is at least worth a try.

There isn't, unfortunately in this case, any way to turn off the audible out of air alarm. Future firmware updates are planned that could enable an "expert" mode where you could turn off all audible alarms. I guess this is one case where our making the alarms louder is not ideal.

Keep me updated on how this is to get repaired, and let me know if you need help. Sorry this happened, particularly on a trip.

Ron

 

[chillinme]

Thanks Ron. I recalibrated the sensor 3 times and got a different number every time. Once was 860 then 72 then 12. I dont think that's our problem. We'll send it back once we get home and I'll let you know what they find since it's not a common failure. Our trip has been fantastic and we made it back to the reserve 3 times after our botched dives so no hard feelings there now either. We love the cobalts and the company has been great sorting out any issues we have had with them.

 

[RonR]

It sounds like it's just as erratic in calibration as you are seeing in the dives, so this is almost certainly a sensor issue. We hate to hear of any failures, but I know Atomic is very good about taking care of problems. Glad you had a good trip otherwise.

Ron