Exceeding NDL on an al80?

[DavidFL]

Routine two tank (AL80) drift dives in Jupiter / West Palm on air would be limited on NDL for both dives. On Nitrox 32 site dependent. On Nitrox 36 and HP100s not likely but site dependent.

 

[tursiops]

I used the dive planner app which is based on PADI tables to come to some estimation of ndl under air and nitrox for a deep dive with an actual average depth provided by my dive computer. How else would you recommend I make the comparison?

If you are using tables, then the depth you enter is the max depth on the dive, not the average. So you should have entered 90 ft. You get a much shorter NDL that way, because the assumption in the tables is that you spent all your time at 90 ft. You didn't, but your tables won't help you; they don't know how to deal with average depth. That is why you have a computer. There is often confusion on this, because air consumption is linear with time and depth, so you can use an average depth without any issues. But nitrogen uptake and off-gassing -- which is what your NDL is all about, is NOT linear with either depth or time, so you can't use averages.

 

[RayfromTX]

Devils throat. Cozumel. Both my wife and I had plenty of air to go deep into deco. My tank on the left and hers on the right.

 

[boulderjohn]

This was with my Shearwater @ 50/80 on air.

A Shearwater in tech mode on 50/80 will put you into deco faster than just about everything else on a first dive. When it does that, it will want you to do a 1 minute decompression stop. By the time you get all the way to the point that it wants you to do a 3 minute stop (like everyone else's safety stop) you are at a more comparable bottom time. (BTW, that is how I usually dive myself, so I am very familiar with it.)

 

[scubadada]

I used the dive planner app which is based on PADI tables to come to some estimation of ndl under air and nitrox for a deep dive with an actual average depth provided by my dive computer. How else would you recommend I make the comparison?

Avg depth is good for gas consumption, not for nitrogen exposure. You must use the real profile for nitrogen exposure.

 

[TMHeimer]

I assume this is a question to each of us as individuals, based on what our own SAC rate is. So some will answer yes and some no or rarely. A lot of stuff here on various computers, apps, table use, etc. as well as nitrox vs. air. Needless to say, your uw breathing rate is different each dive, even if slightly so. But much different if you are a photographer hanging around vs. someone covering some serious ground. So, I'm not sure what the answers to the question mean. I always use AL80s now and the rare times I boat dive to say 80-100', I usually ascend with no less than 5 minutes remaining to my NDL. I usually use Air. I usually find that I end the dive around 500 PSI--maybe a bit more at times. Is that info. of use?
My usual shore dives are to 20-30+ feet. Of course my air consumption, like anyone's, can vary quite a bit if I am covering a lot of ground vs. taking it easy. Sometimes I am still amazed at the difference. It would seem (at least in my case here) my SAC rate is of no importance. And of course my NDL is like 2-3 hours, whatever. But I cite this to show that with varying activity on normal deeper dives, every dive is different--thus at times you could wind up with less than 500 PSI, yet be well within your NDL.

 

[mi000ke]

There is often confusion on this, because air consumption is linear with time and depth, so you can use an average depth without any issues. But nitrogen uptake and off-gassing -- which is what your NDL is all about, is NOT linear with either depth or time, so you can't use averages.

Avg depth is good for gas consumption, not for nitrogen exposure. You must use the real profile for nitrogen exposure.

Thanks for setting me straight on this. I have my profiles stored in Subsurface. Is there a way to use those profiles to compute nitrogen exposure for air vs. EAN?

 

[tursiops]

Thanks for setting me straight on this. I have my profiles stored in Subsurface. Is there a way to use those profiles to compute nitrogen exposure for air vs. EAN?

I don't use subsurface; can't help.

You can certainly get an idea from tables for each gas, or the dive-planner mode on your computer, but I don't know how to do it on an actual stored profile.

 

[Centrals]

Within recreational dive limit, 30m or 40m?
Al 80 = 11L and at 200 bar = 2200 free litres of breathing gas.
My average SAC is about 8L/min or even less but I would call it 10L/min for easy calculation.
At 40m the consumption is 5 x 10L = 50L.
Depends on the algorithm of your computer, 10min of ndl is probably quite generous at 40m. So 10mins at 40m, I would need minimum of 10 x 50L = 500L.
So on a square profile dive at 40m, I normally surface with at least half a tank of breathing gas left.

However, on a multi level dive, 2200L is never enough.

 

[CWK]

My SAC varies between 0.8 and 1.1 on an AL80.

On my last trip to the Maldives I was doing lots of channel dives where we hook onto the reef at about 30m and I was always limited by NDL even on the least conservative computer settings. I was unhooking when my NDL showed 1 min. The divemaster winked at me and said that when NDL reaches 0 min, I'd still have up to 59 seconds before going into deco. This is adequate time to unhook and shallow the dive to find more NDL.

I was actually diving with two computers. My Perdix (set to 50/80) always showed a ceiling by the time my Suunto (set to 0) reached NDL.