Backup Computer went into Deco

[DON-PNW]

Looking for some insight and confirmation whether this was a normal event or should I consider that I have a problem with my Cressi Leonardo Dive Computer. Here are the details:
I went on a dive with 5 other divers - all friends in a fresh water lake. I had two dive computers, my backup was a Cressi Leonardo and the other was a air integrated Mares Icon HD Net Ready with 4.1 firmware, not sure on the Cressi. I paid complete attention to the Icon throughout the dive. What happened was that on my return at 45 feet I glanced at the Cressi and found it had me in deco, the Mares did not. I was diving a HP120 on Air. I decided to follow the Cressi mainly as practice. Note none of the other divers went into deco, they also all had AI Computers. In any case the Leonardo directed me at a 44 minute decompression. I followed through with it and after the Cressi cleared me I surfaced with no signs of DCI. Dive time before deco was 45 minutes, total time underwater was 89 minutes. Average SAC rate according to the Icon was .3

 

[KeithG]

Maybe it would be helpful to try and verify if the surprise deco obligation made sense? You were asked to do 44 minutes of deco after 45 minutes of bottom time. In what world does this make sense? Different algorithms / settings on your two computers could put you slightly into deco on one and not the other. 44 minutes seems to be way past "slightly".

How deep did you go? Was it a square profile?

 

[DON-PNW]

I agree, also being that the AI computer knew my SAC doesn't it also give credit for nitrogen intake? The Cressi has no idea. The only reason I bring this up is because it wasn't just a few minutes of deco obligation that the Cressi wanted, 44 minutes seems excessive. We did go deep, max depth was 118fsw for 3 minutes, then 60 feet for the majority from there. Average depth 34 minutes (remember 44 minutes at 15 feet reduced that average depth).

 

[oldschoolto]

And "THIS" post is why a diver needs to "KNOW" tables in his/her head.. The simple 60/60=120/140 rule would have told him his computer was wrong... Just don't understand why people blindly follow computers ? I need to write a book called " DECO on the FLY" How to do simple deco in your head with navy tables and live....

Jim....

 

[stuartv]

I agree, also being that the AI computer knew my SAC doesn't it also give credit for nitrogen intake? The Cressi has no idea. The only reason I bring this up is because it wasn't just a few minutes of deco obligation that the Cressi wanted, 44 minutes seems excessive. We did go deep, max depth was 118fsw for 3 minutes, then 60 feet for the majority from there. Average depth 34 minutes (remember 44 minutes at 15 feet reduced that average depth).

Your SAC does not make any difference to your deco obligation. Your system absorbs the same amount of nitrogen whether you inhale and exhale 3 times in 1 minute or 6 times. It's the partial pressure of nitrogen in your lungs, the ambient pressure, and the time that matters.

Using even a modestly conservative deco model (Buhlmann with GF50/80), your dive would have had you in deco. But, 44 minutes is definitely over the top too much deco time. However, with a somewhat conservative deco model setting (GF30/70), the deco would be 20 minutes at 20' (or 11 minutes at 10').

What depth were you hanging at for your 44 minutes?

Bottom line is, this seems to be a perfect example of why NOT to use a recreational computer for a technical (i.e. decompression) dive.

 

[stuartv]

And "THIS" post is why a diver needs to "KNOW" tables in his/her head.. The simple 60/60=120/140 rule would have told him his computer was wrong... Just don't understand why people blindly follow computers ?

Because a lot of people don't have any training to do things any other way? And because the statistics show that, for sport diving, that is a very safe way to do it?

 

[gfaith]

@DON-PNW Do you know what was the safety setting on the Cressi. SF0, SF1 or SF2?

 

[DevonDiver]

When you first noticed the deco obligation, had you already 'blown' a stop, or was the indicated stop significantly above your depth at that time?

Was this a repetitive dive? Could you have had deco on an earlier dive and not noticed it?

 

[dmaziuk]

In any case the Leonardo directed me at a 44 minute decompression. I followed through with it and after the Cressi cleared me I surfaced with no signs of DCI. Dive time before deco was 45 minutes, total time underwater was 89 minutes.

max depth was 118fsw for 3 minutes, then 60 feet for the majority from there. Average depth 34 minutes

Have you tried downloading the log and looking at it in like subsurface? The only thing I can think of is was stuck in dive mode from previous dive and though your dive time was quite a bit longer than actual -- the log would show that. I'd consider contacting cressi or throwing the computer out: as others have said 40 minutes deco is way too much, it can't be right.

The closest I have on mine is a dive to 100' for about 3 minutes with average depth slightly below 50' and total runtime of 43 minutes. On a 3rd day of 3-tank dives. No deco on sf0, IIRC after a couple of minutes above 80' or so it was back to lots of NDL.

 

[redacted]

I would repeat the dive but pay more attention to both computers.