Underwater off-gassing equivalent to a surface interval on air

[LFMarm]

As I study to get ready for the CCR class, I discovered something that may be obvious to many but was definitely not to me. “Surface intervals are not always the most efficient way to off-gas nitrogen after a dive”. Depending on the blend, there is an optimal depth at which the off-gassing is fastest and a range of depths at which it is faster than air on surface.

For OC:

• As long as you have a blend with more than 21% O2, there is always a depth that gives you ppN2 lower than .79 bar and hence is more efficient that being at the surface breathing air. Some examples are 6m for EAN50, 14m for EAN67 (the most efficient nitrox mix for off gassing), 6m for O2.
• Adding He increases the optimal depth for off gassing. E.g., 8m for 21/35, 11m for 18/45, 30m for 10/70

For CC (assuming ppO2 of 1.3 bar):

• When not using He, the optimal depth is 11m
• When using He, the more He the deeper is the optimal depth. E.g., 17m for 21/35, 21m for 18/45, 39m for 10/70

As long as you have enough gas/scrubber available, these are faster strategies for offgassing compared to surface interval on air. I wander if this is something that technical and CCR divers use at their advantage.

 

[Addison Snyder]

That might be true for the more porous tissues like blood, but won't slower on/off-gassing tissues (like bone) be likely on-gassing during this decompression?
Not sure how all the other technical divers do it, but I try to breath the richest gas at the shallowest depth. Practically that's almost always 100% O2 at like 20ft (and slowly going up to the raw surface gradient if I'm itching to get out).

I'm going to be terrible at explaining it, but the partial pressure of nitrogen in your mix is essentially the actual factor in the rate of on-gassing/off-gassing, not depth or mix alone. The dive computer simulates the partial pressure of the nitrogen inside you (in like 17 odd tissues or something like that) and does magic to simulate the gradient between that and what you're breathing.

 

[LFMarm]

That might be true for the more porous tissues like blood, but won't slower on/off-gassing tissues (like bone) be likely on-gassing during this decompression?

Interesting point. In all the cases I mentioned the partial pressure of nitrogen is lower than on surface while breathing air so I guess the off-gassing should be true for all tissues?

 

[Addison Snyder]

Another point is your value of time vs. gas. I _could_ breathe 100% O2 at 20ft, and that would be optimal, but I'm curious if it would save me money to breathe it at shallower depths as my ceiling rises.

 

[inquis]

Optimal means no better, so the optimal depth (for a given gas other than oxygen) is actually at the surface, regardless of gas (assuming at least 21% nitrox). The "best of the best" off-gassing would occur when on O2 at any depth within MOD, including the surface.

 

[LFMarm]

Optimal means no better, so the optimal depth (for a given gas other than oxygen) is actually at the surface, regardless of gas (assuming at least 21% nitrox). The "best of the best" off-gassing would occur when on O2 at any depth within MOD, including the surface.

Isn’t a ppO2 higher than surface better for off-gassing?

 

[inquis]

Adding He increases the optimal depth for off gassing. E.g., 8m for 21/35

I don't see how your assertion holds for trimix, as helium is also an inert gas subject to absorption/deco considerations.

 

[inquis]

Isn’t a ppO2 higher than surface better for off-gassing?

It's the difference between inspired and tissue ppN2 that controls the rate of off-gassing of nitrogen. While higher ppO2 necessarily decreases ppN2, it's the latter that matters.

Off-gassing rate of N2 is identical/maximized when on O2 because inspired ppN2 = 0 regardless of depth. (The gradient can't get any larger.)

 

[LFMarm]

It's the difference between inspired and tissue ppN2 that controls the rate of off-gassing of nitrogen. While higher ppO2 necessarily decreases ppN2, it's the latter that matters.

Off-gassing rate of N2 is identical/maximized when on O2 because inspired ppN2 = 0 regardless of depth.

So breathing pure oxygen is the best gas for off-gassing and the off-gassing rate will be independent of the absolute pressure you are breathing oxygen at, right?

 

[inquis]

the off-gassing rate will be independent of the absolute pressure you are breathing oxygen at, right?

This part is true.

pure oxygen is the best gas for off-gassing

This part is true when breathed on the surface. More generally, though, the choice of deco gas(es) *during a dive* can more complex since you can start off gassing sooner/deeper with something else.

As to your original question, yes you are correct: the rate of off-gas is a central consideration in technical diving. Not too high (ceilings are based on this) nor too low (progress must be made).