Deco in Geo, not in VT3?

[memorex77]

Normally I dive with my VT3. Last week I also took my Geo along for a deeper dive, basically as a back-up. Right arm VT3, left arm Geo.

My Geo went into decompression after about 17min, while the VT3 didn't. After 17min the VT3 was at TLBG 6, at 19min at 7, at 32min back to 6. Both computers where not on "conservative". The screen shots of the same dive with both computers are attached.

In the second dive the same day which was slightly deeper and a deco dive, the VT3 was out of deco for 10min when I surfaced, but the Geo was on violation. A fun-"dive" in a decompression chamber last year showed that the depth measurement of the VT3 is 100% accurate, so the Geo is about 1m off.

Since I don't believe that the 1m depth calibration difference makes the difference, what is it then? Any ideas?

 

[AndyDragon]

One possibility...does one of the computers think it is at a different elevation? (one might say 'SEA' and the other 'EL 2')?

 

[memorex77]

One possibility...does one of the computers think it is at a different elevation? (one might say 'SEA' and the other 'EL 2')?

I just tried to get the elevation out of my VT3, but can't figure out how to read this out? The Geo says "SEA" right now, as it should because my current location is at about 300ft. Same as the dive location in question.

Any other idea?

 

[TSandM]

Any other ideas? Yes.

I am not familiar with the two computers in question, but it is quite possible they are using different decompression algorithms that result in different no-decompression times. You see, decompression is not an exact science. We do not really understand what is happening with gas that is absorbed into the body, and how it exits. We have some ideas, and some models that have resulted in huge decreases in decompression sickness compared with the pre-Haldane days. But each model makes slightly different assumptions about tissue half-lives, M values, diffusion, bubble behavior, etc., and the result is that we have a variety of algorithms that give different answers.

What you should learn from this is that there is no "bright line" dividing no-decompression from staged decompression diving. You are not "safe" on one side, and in trouble on the other. As you get close to the limits, the advisability of a controlled ascent with stops becomes greater and greater.

 

[memorex77]

The bright line dividing no-decompression from staged decompression doesn't exist. That's even scarier when I tell you the two computers are supposed to have the very same algorithm! That may also be an explanation why Oceanic don't answer as they are aware that product to product variation would be the reason.

This was an important lesson for me, I'm going to respect this knowledge appropriately.

 

[Muad'dib]

Just a wild guess,is it possible that one of them was switched on at a different elevation.
Oceanic computers take an atmospheric reading when they start up.
There's afaik no function on them to do this manualy

 

[Jim Lapenta]

You could try removing/replacing the battery on both and starting them up at the same location. And both are pelagic products so they should be using the same algorithm unless something changed which I'm not aware of.

 

[scubafanatic]

...well, like 3-4 years ago, I'd bought a Veo 250 to use as a back-up to my DataMax Pro Plus II's, thinking it was a good way to have consistent behavior across all my computers....same manufacturer, same algorthim, right ? WRONG ! On my first post-purchase dive trip to the TX Flower Gardens, my buddy and I BOTH quickly 'bent' our "Veos", while our primary DataMax Pro Plus II computers were perfectly 'happy'.....and YES I know how to do the 'settings' and 'read' Oceanic computers...as had/have been diving them for many years.....also, I own 4 Cochrans, which I can successfully 'program', and which are WAY more complex to set up than an Oceanic. We immediately retired our 'Veos', never dove them again...I eventually used mine a year or so ago as 'trade-in-credit' towards a Cochram GEMINI. (in 2005 I'd borrowed a Cochran EMC-20H to test as a back-up computer while wreck diving in North Carolina....which worked fine with my primary DataMax Pro Plus II while there...after I'd 'retired' my Veo, Scubatoys was having a close-out-sale on Cochrans where I picked up a good deal on my first Cochran, the EMC-20H....bought it due to the good experience I'd had with my 'borrowed' Cochran in NC.) Originally I'd gone for the Veo as a back-up...cheaper than a Cochran and same manufacturer (Oceanic)....turned out to be a mistake......I was told......well after purchase, finding out the hard way....the algorthims are different, which does me no good as a back-up computer! So, while I REALLY like my DataMax Pro Plus II units......the Veo 250 'S*CKS" !!! (if I'd wanted a computer that's easy to violate, I'd just bought a Sunnto in the 1st place!)

 

[memorex77]

.....the Veo 250 'S*CKS" !!! (if I'd wanted a computer that's easy to violate, I'd just bought a Sunnto in the 1st place!)

Two highly interesting points you mention.

1. The Veo didn't tell you what you wanted to hear (i.e. everything roger). This is my problem, too - in a way! How do I know that the VT3 tells me "the truth", but the Geo (in my case) doesn't? Same for the Veo.

2. I had a number of dives with Suunto guys and definitely share your experience. Suunto seems to be VERY conservative compared to the VT3...

Anyway, according to Oceanic, both the Geo and the VT3 (and the Veo and all the other Oceanic air integrated computers!) use "Modified Haldanian/DSAT Database":

http://www.oceanicworldwide.com/pdf/compare_computers.pdf

 

[scubafanatic]

Two highly interesting points you mention.

1. The Veo didn't tell you what you wanted to hear (i.e. everything roger). This is my problem, too - in a way! How do I know that the VT3 tells me "the truth", but the Geo (in my case) doesn't? Same for the Veo.

2. I had a number of dives with Suunto guys and definitely share your experience. Suunto seems to be VERY conservative compared to the VT3...

Anyway, according to Oceanic, both the Geo and the VT3 (and the Veo and all the other Oceanic air integrated computers!) use "Modified Haldanian/DSAT Database":

http://www.oceanicworldwide.com/pdf/compare_computers.pdf

.....well, I 'know' my ProPlus II tells me the truth as I'd been successfully and safely diving them for quite a while prior to buying the Veo......also, I'd used a Cochran with my 'Pro' on a dive trip and it tracked very well alongside the 'Pro'...and the Cochran has a user-adjustable 'conservative level' capability so I can adjust it to parallel the 'Pro'.

I've never personally owned a Sunnto but have dove with many divers who used one and based on what I've seen in the field, I wouldn't ever use one myself....Sunnto is beyond 'conservative'...approaching 'paranoid' !!!

Karl