SUUNTO Cobra ascent rate warnings...

[beachnik]

'Trust But Verify' - I may have been trusting too much and not verifying enough.

I've been working at keeping my SUUNTO Cobra happy. Still, I get too many 'ascent rate' warnings. SUUNTO gives you a 'slow' warning and puts a mark on your logs if it believes you have exceeded the 33 ft/min rate. Once you've exceeded this rate, you've pushed the 'microbubble' envelop and need to do an extended surface interval. If you get back in the water too soon, you'll get another warning. For all I know, these warnings my lead to more-and-more cumulatively conservative recommendations from SUUNTO on repetitive dives.

'bleeb' gave me a tip to avoid moving the computer too fast.

That suggestion was just enough of a nudge to get me to go back and, once again, study my logs. This time, instead of assuming the computer had exercised good judgement, I started double checking with a calculator. Here's a typical example of what I discovered (refers to attached screen capture):

- For the 20 seconds preceding the first ascent warning, I was actually ascending at a rate of 9 ft/min.
- For the 20 seconds preceding the second warning, I was actually ascending at a rate of 3 ft/min
- The average ascent rate for this segment of the log (from trough to peak) was 15.5 ft / min

In other-words, I'm no where near breaking the speed barrier.

I then did similar checks/calculations on 4 other dives that had 'ascent rate' warnings - the results were comparable. I had ascent rates of 5.6 ft/min to 18 ft/min - and SUUNTO was logging an ascent rate violation.

I can only guess that the problem is what was suggested by 'bleeb' - moving the computer too fast. And I thought I was already being careful about this!

 

[Bubbletrubble]

You're arguing that the average ascent rate for a given time interval is still under the Suunto-specified 33 ft/min rate. I think you're basing your argument on incorrect assumptions.

I'm fairly certain that the computer samples more frequently for ascent rate violations than for saved datapoints in the divelog. Of course, the sampling rate for divelogging purposes can be set by the user (as set forth in the manual).

I'm not sure over what time interval the Suunto computer uses to calculate ascent rate for violation purposes. It could be as frequently as one datapoint every second during an ascent. If that's the case, it's entirely possible that you stayed within ascent rate limits for the 20 seconds preceding the first ascent warning as well as for the 20 seconds preceding the second warning...yet still violated the Suunto recommended ascent rate (whenever it said you did).

Why not try clipping off the Cobra to your left chest D-ring? The hose will route under your left arm. In this way, you can just look down without touching your computer to check air pressure, depth, and ascent rate. A side benefit is that it frees up your hands to dump air from your BC or drysuit.

From your safety stop at 15 fsw to the surface, ascend very slowly. I typically take about 2 min to get to the surface from a depth of 15 fsw. This ascent rate works for me. I have yet to trigger a Suunto ascent rate violation since adopting this method. YMMV.

If you're still annoyed by Suunto ascent rate violations after making all of these changes, I'd recommend getting a different dive computer.