Benefit of shallow Nitrox offgasing

Suppose I make Nitrox dives to recreational limits, is there benefit to using the rest of the Nitrox offgassing on the hangbar before heading up, if you have enough gas to safely hang more?

My guess is yes, that the extra oxygen at shallow depths would not be enough to cause O2 tox in most instances, but the higher O2 would decrease the Nitrogen bubbles in the blood.

I'm getting ready to make some Nitrox dives this weekend, followed by a flight home the next day. I don't plan to exceed the Do-Not-Fly timelimits, but I'd like to increase the safety factor (as the Do Not Fly times, in general, are coming down over time).

No. You're going of offgas most efficiently on the surface where there is less ambient pressure.

Great point.

PfcAJ:

No. You're going of offgas most efficiently on the surface where there is less ambient pressure.

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yes you will off gas quicker at surface but you will off gas safer at 15 feet where the bubble formation is retarded. So an extra long safety stop could be beneficial. I would not worry about the flight home as long as you have 18 hours before the flight and 24 hours preferable but for sure if you come even close to a deco obligation

Surface off-gassing is more efficient than UW off-gassing. So rest stops and extending those rest stops is not faster off-gassing but may be safer off-gassing especially if you have been pushing the NDL limits. But there is another way to look at it. What is your hurry to get to the surface and begin the most efficient off-gassing? If you are planning a subsequent dive, then the sooner you get to the surface, the sooner your surface interval begins. And, while it may only be a matter of minutes, a similar situation may exist with time to fly.

Bottom line, if there is not reason to get to the surface quicker, extra time on a rest stop will not hurt. If you are diving conservatively anyway, the benefit of a longer rest stop may be almost non-existent - but it does not hurt. If you are pushing limits, the benefit may make the difference between another good dive and an incident. Also, remember there are probably others diving with you who are left waiting if you extend your rest stop.

BTW, the answer is the same whether you are using air or nitrox.

As stated, your ascent profile really isn't impacted by what gas you breathed -- that had most of its impact on the allowable bottom time.

There are advantages to slowing your ascent as you get shallower, if you look at the bubble models. Keeping ambient pressure relatively high controls bubble growth. But since, in theory, if you are within your no-deco limits, you are safe to ascend directly to the surface at the ascent rate prescribed by your model (which for the RDP is 60 fpm, a VERY fast ascent) there is probably very little benefit to spending inordinate amounts of time hanging on a bar, and there are some disadvantages. If the bar is being yanked up and down, you're going to have more wear and tear on your ears, and you have to be very careful to make sure you don't hold your breath at any time.

The three minute safety stop, or, if you were close to NDLs, perhaps extending it to give, is probably as much 15' time as you are going to get benefit from. Remember to ascend slowly from there, too!

RogerAGrimes:

My guess is yes, that the extra oxygen at shallow depths would not be enough to cause O2 tox in most instances,

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I prefer not to guess about that stuff!

What's the pO2 of 32% at safety stop depth?

ianr33:

I prefer not to guess about that stuff!

What's the pO2 of 32% at safety stop depth?

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Good point. P02 is something like .55 according to this chart: http://www.ndc.noaa.gov/pdfs/nitrox32.pdf

I haven't done the (simple) math in a few years, but I probably should do that. I'm guessing the math would prove that I would be more likely to run out of gas volume before I exceed P02 worries. But no need to do the math, it makes better sense to do it above water anyway...if I was so inclined.

RogerAGrimes:6267823:

ianr33:

I prefer not to guess about that stuff!

What's the pO2 of 32% at safety stop depth?

Click to expand...

Good point. P02 is something like .55 according to this chart: http://www.ndc.noaa.gov/pdfs/nitrox32.pdf

I haven't done the (simple) math in a few years, but I probably should do that. I'm guessing the math would prove that I would be more likely to run out of gas volume before I exceed P02 worries. But no need to do the math, it makes better sense to do it above water anyway...if I was so inclined.

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Its .46 at 15 feet. Seems like a waste of gas if your staying within the recreational limits. As stated above you off gas the best at the surface.

Teller:

Its .46 at 15 feet. Seems like a waste of gas if your staying within the recreational limits. As stated above you off gas the best at the surface.

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Yeah but there are no pretty fish on the boat. : )