Oceanic computer algorithms

[redacted]

My buddy dives a Veo 250 and I dive a Data plus. We were comparing PDPS output at the end of a one week dive trip to Bonaire. While he did skip a dive early in the week, we basically dove very similar profiles for the last 4 days (12 dives). I say similar because he was often a few feet shallower than I was as we traveled along the slops in the 50 to 80 ft range but we were basically the same in the 2nd half of the dive done in the flatter areas in the 20 to 35 ft depths.

I was surprised to see that we got clearly different PDPS outputs in both 21% and 32% EAN settings. The Veo 250 was allowing deeper dives and longer bottom times for deep dives that both computers allowed. The 2 computers then had pretty good agreement in the 70 to 90 ft range. But for shallower dives, my Data plus was allowing much longer bottom times (in most cases of little use because we are talking both times much in excess of 1 hour.

So, are the two computers running somewhat different algorithms/inputs for subsequent dives? And why the change(s)? I did look at the Veo 250 manual and confirmed that both use a modified Haldanean algorithm and both manuals list the same PDPS times (except for obvious misprint) for first dive NDLs.

 

[D_B]

Oceanic Personal Dive Computer Algorithm
Decompression theory can be trusted only as far as it has been demonstrated by actual test dive data. In 1987 a unique series of experiments were conducted by Diving Science and Technology (DSAT). Commissioned by the Professional Association of Diving Instructors (PADI), these Doppler ultrasound monitored human experiments conducted by Dr. Michael Powell produced the most comprehensive data set that exists for recreational divers to this day. These data were used to validate the PADI Recreational Dive Planner and are the basis of the algorithm used in all Oceanic Personal Dive Computers (PDCs).

It is posted the same for both computers

If you go to the Scuba Diving magazine site, they tested several computers and the Oceanic VEO250 will typically be the less Conservative one
And one reason for that (that I've heard) is that the sampling rate the computer uses to calculate your nitrogen loading is more times per minute than the others (note, this has nothing to do with the sampling rate for displaying/downloading dive data)
I like mine ... personally, I want my "gages" to be as accurate as possible and for me to be the one deciding on how conservative to dive, not my computer.
I wonder why the differences? is it because one has your gas consumption information?
I'll be interested in the answer

 

[Doug Krause]

The basis (modified Haldanean validated by DSAT database) is unchanged for both the Veo 250 and the earlier Data Plus. Modifications based on new studies may occur over time - such as the relaxation of restrictions based on reverse profiles or repetitive deep dives - or flying after diving rules, etc.

That being said - the most likely cause of the differences is that despite "very similar profiles" - you're not attached at the hip, so differences will occur.

 

[fppf]

Doug
If Oceanic makes changes to the algorithms, can they make those updates avalible for owners of current computers? My wife and I own both the Atom 2.0 and Pro Plus 2 (4 computers total). We are very happy with both.

To add some real world cases, we do mainly deep square dives in lake Erie. There can be a large difference between my wifes and my computer on some dives. At deeper depths, a few feet can affect you bottom time and nitrogen loads a lot. Even comparing our Atom 2.0 with the Pro Plus 2 we see differences of a few minutes. The Pro Plus 2 tends to hang a foot or two lower than the Atom on the wrist.

 

[redacted]

The basis (modified Haldanean validated by DSAT database) is unchanged for both the Veo 250 and the earlier Data Plus. Modifications based on new studies may occur over time - such as the relaxation of restrictions based on reverse profiles or repetitive deep dives - or flying after diving rules, etc.

That being said - the most likely cause of the differences is that despite "very similar profiles" - you're not attached at the hip, so differences will occur.

Doug

Thanks for the response. It is good to hear that the algorithms are updated on newer model computers based on new data. The nature of the differences we saw makes me suspect it involved an algorithm update (or a malfunction) rather than just differences in out profiles. We are making another trip in a few weeks and we will probably put my spare Data+ on his wrist next to his Veo 250 to control the dive profile variable and see what happens. We normally keep an eye on the differences of our computers as we did pick up a problem with his computer when it was new. His computer was behaving like it was stuck in an altitude mode and was replaced by Oceanic.

Thanks

 

[Charlie99]

One feature of the Oceanic/DSAT algorithm that might cause readings to be other than what you would expect is the "surface credit control". Also sometimes called "asymmetrical offgassing", this means that the faster compartments are treated as if they are 60 minute compartments when surfaced.

The real life effect of this is that if one diver spends more time at a safety stop or shallow, then the fast compartments will will offgas more than the diver that surfaced earlier.

What you describe in post #1 would be what you would expect if the the VEO 250 diver stayed in shallow water for a while after the Data Plus diver got out of the water.

 

[redacted]

One feature of the Oceanic/DSAT algorithm that might cause readings to be other than what you would expect is the "surface credit control". Also sometimes called "asymmetrical offgassing", this means that the faster compartments are treated as if they are 60 minute compartments when surfaced.

The real life effect of this is that if one diver spends more time at a safety stop or shallow, then the fast compartments will will offgas more than the diver that surfaced earlier.

What you describe in post #1 would be what you would expect if the the VEO 250 diver stayed in shallow water for a while after the Data Plus diver got out of the water.

We'll play with it some more to control the profile differences on a trip to the Flower Gardens later this month.

In our case, there were 3 of us diving together. I (Data+) did one solo dive but that was fairly early in the week. The last 12 dives (4 days) were all done together. Entries and exits were within a minute of each other. As I said, the Veo diver tended to stay a bit shallower (0 to 5 ft) during the deeper first 1/3rd of the dives and then we were pretty much the same. I would have understood the Veo diver having somewhat longer NDLs across the board due to my tending to be a little deeper early in the dives. What makes me believe this is an algorithm difference is that the Data+ was giving longer NDLs for shallower dives, but shorter for deeper dives.

It 's really not that important as NDLs rarely limit our dives but I do want to be sure that we are not seeing a problem with one of the computers. And my wife is talking about a new computer and I'm seriously thinking about the Veo 250 for her.

 

[MarineResearch]

My wife and I just finished a week of diving in Bonaire and use backup computers. She had a VT Pro and VT3, while I dove with a VT Pro and Atom 2.0. We dove the same profiles, give or take a few feet and seconds, and usually had the same mixtures (EAN32 to EAN34). I did notice some curious differences between the computers that I used (VT Pro and Atom 2.0), as well as my wife's. Some differences can be attributed to the different display formats (5 vs 8 bar TLBG), but others are quite odd. Also, the temperature profile on my Atom was consistently reading way too low (75F versus low to mid 80s on all other computers). I'll post a comparison next week when I have time to look over the data. On a positive note, all four computers were generally consistent in the parameters that matter and sounded alarms at about the same time (max depth, max PO2, tissue loading, elapsed dive time, etc.).

 

[redacted]

OK, the FGB trip was great. For the first 6 dives, my buddy wore my spare Data+ right next to his Veo 250. the plan was to do a dive and then record samples from the dive planning outputs. All dives were done on 32%. The first dive was 85 feet for 58 minutes.

Dive planning outputs after specified surface interval for various depths:

SI = 48; Depth = 40; Data+ = 3:15; Veo250 = 3:23
SI = 50; Depth = 60; Data+ = 1:11; Veo250 = 1:13
SI = 51; Depth = 80; Data+ = :36; Veo250 = :37
SI = 56; Depth = 100; Data+ = :23; Veo250 = :24
SI = 57; Depth = 120; Data+ = :16; Veo250 = :17

Dive #2 had an SI of 2:04 and ran 83 feet for 56 minutes. Dive planning outputs:

SI = :59; Depth = 40; Data+ = 2:41; Veo250 = 2:53
SI = 1:00; Depth = 60; Data+ = 1:05; Veo250 = 1:10
SI = 1:01; Depth = 80; Data+ = :36; Veo250 = :37
SI = 1:02; Depth = 100; Data+ = :22; Veo250 = :23
SI = 1:03; Depth = 120; Data+ = :11; Veo250 = :17

We did not record again until after the 6th dive. Dive #3 had an SI of 2:19 and went 73 feet for 49 minutes. Dive #4 had an SI of 2:18 and ran for 59 minutes at 70 feet. Dive #5 had an SI of 1:58 and ran for 63 minutes at 73 feet. The next morning, dive #6 had an SI of 10:29 and ran for 55 minutes at 76 feet.

Dive planning outputs after that dive were all O2 limited except for the 120 ft dive which was N limited on the Data+ and O2 limited on the Veo250.

SI = :58; Depth = 40; Data+ = :44*; Veo250 = :45*
SI = :59; Depth = 60; Data+ = :30*; Veo250 = :30*
SI = 1:00; Depth = 80; Data+ = :22*; Veo250 = :22*
SI = 1:01; Depth = 100; Data+ = :17*; Veo250 = :17*
SI = 1:02; Depth = 120; Data+ = :08; Veo250 = :14*

* limitation established by cumulative O2.

We also observed during dives that the Veo250 tended to provide more NDL time than the Data+.

So, I'm concluding that the nitrogen related algorithms for subsequent dives are a bit more liberal for the Veo250 than for the Data+, especially for deeper dives. And the cumulative O2 algorithms appear to be the same. Not a very big or important difference unless you are wanting to push toward maximum depths which we avoided.