Oceanic NDL Algorithm Change?

[Charlie99]

Surprise - his was showing shorter NDLs even with zero N2 loading. For example, my Data plus showed 50 ft for 1:20 - his showed 50 ft for 1:07. I suspect he might be stuck in an altitude mode (We were in central TX diving at an elevation of 620 ft).

Bingo! It looks like the VEO180 thought it was at 2,000' altitude. 8% Cross correction of 50' to adjusted depth of 54' would generate something around 1:07 for NDL --- the DSAT model has 1:05 NDL for 55'.

DandyDon has played around with Oceanic computers and their automatic altitude adjustment, and the +/-1,000 altitude accuracy. IIRC, he posted some info on the NDL the computers displayed at the various altitudes around his hometown.

Edit: http://www.scubaboard.com/showthread.php?t=38880&post=#7 is a thread where this same sort of problem was discussed. It turns out that 1:07 is the 2,000' altitude NDL for 50' on my Oceanic Data Plus, so probably is also for the VEO180.

As noted in that thread, a long soak in warm water may remove salt crystals from the depth/pressure sensor and get the altitude sensing back to normal.

 

[redacted]

Bingo! It looks like the VEO180 thought it was at 2,000' altitude. 8% Cross correction of 50' to adjusted depth of 54' would generate something around 1:07 for NDL --- the DSAT model has 1:05 NDL for 55'.

That was about the altitude of Balmorea that we dove earlier this year.

 

[wedivebc]

our air integrated computers do not increase tissue loading with increased respiration

Can you (or anyone) tell me what algorithm Oceanic uses?

 

[cerich]

Can you (or anyone) tell me what algorithm Oceanic uses?

Here ya go!

http://www.oceanicworldwide.com/p_computers_atom_specs.html#algorithm

 

[wedivebc]

Here ya go!

http://www.oceanicworldwide.com/p_computers_atom_specs.html#algorithm

So if I understand correctly this computer is based on a single compartment table using a 60min controlling tissue, per the PADI RDP rather than an actual algorithm? When was this change made? Do other Pelagic based computers use this table?

 

[Charlie99]

So if I understand correctly this computer is based on a single compartment table using a 60min controlling tissue, per the PADI RDP rather than an actual algorithm? When was this change made? Do other Pelagic based computers use this table?

No, it is based upon the DSAT model, which has compartments with halftime from 5 minutes to 480 minutes. Interestingly, Oceanic only uses 12 of the compartments, not bothering to track the 30 minute and 60 minute compartments. (In practice the 20, 40 and 80 minute compartments cover that span pretty well). The algorithm though, does have asymmetrical halftimes for offgasing during the SI, where the halftime for the faster-then-60 minute compartments is increased to 60 minutes.
During a dive, the halftime off gassing is the same as that compartments ongassing halftime.

The PADI RDP repetitive pressure groups are based upon the 60 minute halftime compartment, but the NDLs are based upon whichever one of the compartments hits the limit first. That's why, for example, that when you hit the 80' NDL of 30 minutes, that you aren't in pressure group Z.

The Erik Baker article, Understanding M-Values lists the compartments and M-values of the DSAT model as well as many others.

 

[cerich]

Thanks Charlie, you saved me some typing.

Pelagic computers have always used DSAT based tables in the algorithm.

Best,

Chris

 

[wedivebc]

OK thanks guys.