Nitrox Under The Boat?

[HJeffreyM]

I recently returned from my first dive trip where I used EAN(32) for all dives. My initial reaction is very positive with the "anecdotal" effects of post-dive wellbeing vs. air seeming true for me at least. I also enjoyed watching the nitrogen loading indicators on my DC dissapear almost before my eyes!

I also followed some advice that was not in the PADI EANx manual, but was suggested by the instructor I took the class from:

I generally still had 700-900psi left in the tank when I hit my 15fsw safety stop. After the stop, I would proceed to about 10fsw for a couple of minutes, then 5fsw...in other words, breathe the tank down as close to 500psi as possible while at very shallow depth. Theoretically, I would then be off-gassing faster on EAN32 than I would breathing air (EAN21) at the surface?

Aside from slowing my ascent from 15-0fsw to a snail's pace (good), is there an off-gassing advantage in the above end-of-dive profile? Would I be better off doing a normal ascent from 15fsw and sucking down the rest of the tank while sitting on the boat?

Happy diving!

 

[GuardTrash]

That is interesting. I have not got my Nitrox cert yet so I have no idea, but would like to know also.

 

[aku321]

Me too

 

[GrumpyOldGuy]

I don't think there is an inherent advantage to off gassing slightly faster on Ean32 vs 21 if you do not have a deco obligation. I too often screw around near the boat (away from the props) at 15-20' or so, but this is more to stretch out a dive if I have extra air and conditions are favorable.

 

[paddler3d]

The pp (Partial Pressure) of Air (21%) at sea level is .21. The pp of EAN 32 at sea level is .32.

The pp of Air at 15fsw is .30. The pp of EAN 32 at 15fsw is .46.

You'll definately off gas faster using the higher concentration of O2.

The tec folks decompress with 100% O2. At 20fsw ppO2 is 1.6, which is the max accepted level.

If you diving in recreational limits, the benefits of continuing to breath the higher concentrations of O2 on the boat wouldn't be a great as getting out of your gear, relaxing and hydrating. If you're concerned, I'd just extend your safety stop as long as you can under the boat. At least you get to see something down there!

 

[Dr Deco]

Hello HJeffreyM:

Breathing the tank down before climbing onto the boat is a good idea. Some call it the “hidden stop.” It reduces nitrogen loads before you exert muscle strain and reduces the chance for decompression bubble formation/growth.

Dr Deco :doctor:

 

[IceBergSlim]

Not to be pedantic but there is no such thing as EAN21.

Or, I guess to be really pedantic, it is EAN.

Enriched by .1%

 

[ianr33]

Its an interesting question .

You need to look at the partial pressures of nitrogen.(pN2)

pN2 at the surface is 0.78
At 10 feet breathing nitrox 32 it will be about (1+ 10/33 ) X 0.67 = 0.87
(Remember that air is about 1% Argon)

So you would actually be offgassing dissolved nitrogen FASTER at the surface breathing air than you would at 10 feet breathing nitrox 32.

However, staying at 10 feet will help keep any bubbles that may have formed smaller,giving your lungs a better chance of eliminating them.

Definitely safer to do the extended stop on nitrox if you are only doing the one dive. Might not be if it reduces your safety stop before the next dive.

In the big picture though the difference is pretty small whichever way you go.

 

[Rhone Man]

Yes, the key is not ppO2, it is the ppN2. If you are breathing pure oxygen, your ppN2 is zero, whether you are at 20 feet or on the surface. And thus you will off-gas faster than you would if breathing air at the surface.

If you breathed EAN32 at 5 feet, your ppN2 is in fact (just) smaller than the ppN2 of air at the surface (0.78 ATA against 0.79 ATA). But if you really want to off-gas faster, just keep the regulator in your mouth for 5 minutes after you surface and breathe the EAN32 on the surface.

 

[MaxBottomtime]

The slow ascent from 15-0 feet is the real reason you felt better after diving. Try making the same ascent rates with air and you will see no difference.