Variation in NDL values: Comparing different dive tables and dive computers

[Mr Carcharodon]

There was a comparison of NDL v. dive time for repetitive dives on different computers at deco.org. Their main website may not still be up but they have a FTP site. Also in Karl Huggins course on decompression from Wrigley Marine Sciences (you can find it on the web too) you will find another comparison. Some algorithms, like RGBM, have substantial penalties for short surface intervals, rapid ascents or reverse profile dives. In general you will see larger differences when those conditions occur. Also you will see large differences in ~35-50 fsw range where the slope of the curve is steep. Finally of course VPM will give you credit for deep stops if you are over threshold values but the Buhlman based computers will penalize you for those stops.

Oh and the previous poster is right about UWTEC having a penalty for cold water. I have personally seen that result in a 20 minute discrepancy between computers on a repetitive dive. That penalty in particular seems ill advised since presumably the computer does not know what the diver is doing for exposure protection. Large differences computer to computer make it hard to keep a team all on the same plan. So as usual it is better to have a plan before getting into the water and to have the computer in gauge mode.

 

[fisheater]

@fisheater: Why did you ascend to the surface without your buddy?

Lost buoyancy control by inadequately venting my drysuit (task loaded with navigational errors, reeling in the line and a constant, unexplained, unstoppable mask leak). We'd finished our our safety stop and were headed up together. I was surprised and very worried when he didn't surface with me.


Sent from my iPhone using Tapatalk

 

[lowviz]

............Can anyone give an example of how two different computers taken down on the same dive would "vary widely"? In other words, are there two different computers out there that if I were to take them down on the same dive I'd get a big difference in bottom time or whatever? ...................................

Yes, but it depends on how you feel about deco. When I take my Uwatec along with my VR3 there will often be a seemingly huge difference in bottom time due to their NDL's. My VR3 will always time out first. At 60', my Uwatec gives me 48 min while my VR3 times me out at 37 minutes. However, all my VR3 did was to add a mandatory 1 minute stop at 20'. Mandatory stop means being in deco. Your post got me wondering exactly how they do compare. See my chart for details, just don't dive the info as it is unchecked simulations from both computers.

Both on air, "Safety" = zero or L0, VR3 doing BUH:

Clearly shows what I knew from experience, I'll be in the water about five minutes longer on a much safer profile.

Here's a problem: On a deeper dive, I give one computer to my buddy and take the other for myself. We both stay down until the Uwatec NDL's out and the VR3 is in deco. If we both follow our computers to the surface I lose my buddy at the VR3's first stop and we remain massively separated all the way to the surface.

............I can't quantify it for you, only report having seen it. I did a dive with a buddy once (first of the week for both of us) that my old Oceanic considered 'within NDL' for which his VR3 wanted something like 20 minutes of deco (don't recall the exact amount, just that it seemed inordinate). Shrug. ............

Maybe it was the difference between their NDL's, not 20 minutes of additional obligation??? NDL's could easily vary that much due to how VR Technology does things. I'll be the first to admit that VR3's are difficult to understand, but I really like them.

 

[Rhone Man]

Just curious here. Can anyone give an example of how two different computers taken down on the same dive would "vary widely"? In other words, are there two different computers out there that if I were to take them down on the same dive I'd get a big difference in bottom time or whatever?

I ask not to challenge what is being said but because I really have no idea and I'm not really directing my question to Thal but to anyone who might have an example.

Every dive I do I carry a Suunto D9 and a Sherwood Profile. It is pretty routine for me to note on my ascent that the NDL on the Suunto is down 2 or so minutes, whilst the Sherwood shows me good for another 15-20 minutes.

 

[Rhone Man]

This table may help. But it actually understates the differences. Where the conservatives computers really kill you is how they treat off-gassing and penalising repetitive dives.

 

[CT Sean]

I always thought this chart from Scuba Diving magazine was helpful in illustrating the differences in NDL over repetitive dives (assuming it's accurate of course and I don't know why I'd think it wasn't). Can pretty clearly see on dive #2 about 35 minutes into the "dive", the Pelagic based computers show 40 mins NDL where the Mares shows zero.

http://www.scubadiving.com/files/old/images/pdf/scubalab_200706_10_new_comps.pdf

 

[BDSC]

I just wanted to say this has been a really good thread and I appreciate all of you folks who have posted. That table is really interesting to look at. I think it would be a good idea if SB could post it as a sticky maybe.

 

[notivago]

Well, I had not considered of the different ways of dealing with degassing from computer to computer. As it seems, if you want to stick to your buddy, you have to either do it by tables, establishing a hard bottom max being conservative or every buddy have to have the same computer.

 

[explorer08]

As it seems, if you want to stick to your buddy, you have to either do it by tables, establishing a hard bottom max being conservative or every buddy have to have the same computer.

Or you just both stick to the most conservative of your computers!

 

[Puffer Fish]

Rhone, That is the same chart posted at the beginning of the thread, and at least two of the computers show have different number than the chart... not sure about the others, but would guess there is more wrong than the Uwatec and Nitek.