Underwater off-gassing equivalent to a surface interval on air

[Jacob BV]

If you're breathing pure O2, ppN2 in your lungs is 0. At any depth.

Yep exactly, sorry i wasn't clear with this.

And yeah after some thinking I am wrong about offgassing quicker on 100% at 10' than 20'. Good to brush up on some theory! Bubble formation is driven by how much tissue tension you have above ambient, while off-gassing is driven by the difference in tissue tension and inert gas pressure. So on 80% a 10' vs 20' would promote quicker off-gassing, however, with 100% there is no difference in off-gassing so ignoring hyperoxia you are safer staying at 20' as you are less likely to drive bubble formation by ascending.

I guess the OP is looking at a sort of Equivilant air depth for deco gas, but slightly modified for equivalent off-gassing depth compared to air at the surface. With a bit of maths i get (equivalent surface depth - made that up):

ESD = (7.9/FN2)-10

With depth in metres, and FN2 the fraction of N2 in your deco mix.

 

[Duke Dive Medicine]

On-gassing and off-gassing are based mostly in Dalton's Law, which states that at a given temperature, the amount of gas that will dissolve in a liquid is directly proportional to the partial pressure of that gas on the liquid. With the gas/liquid interface in the human body being the lungs, if you are off-gassing N2 after breathing some mix of N2 and O2, including compressed air, the most efficient way to do this is to minimize the partial pressure of N2 in the breathing mix. Doing this safely is essentially what technical diving is all about.

It might be an interesting thought exercise to see what percent of helium, O2, and N2 you'd have to have in trimix at a given depth to duplicate the partial pressures of O2 and N2 in air (I think that's one of your questions), but the risk of isobaric counterdiffusion makes this impractical in real-world diving.

Other things being equal, a diver will off-gas more quickly with higher partial pressures of O2, so 100% O2 at 20 fsw will result in faster off-gassing than 100% O2 on the surface.

Best regards,
DDM

 

[inquis]

a diver will off-gas more quickly with higher partial pressures of O2, so 100% O2 at 20 fsw will result in faster off-gassing than 100% O2 on the surface.

I don't understand how the same nitrogen gradient in both cases can result in a different rate of off-gassing. Can you please elaborate on the mechanism behind this (how higher O2 pressure influences the N2 rate)?

 

[LFMarm]

Thank you. What is isobaric counterdiffusion?

On-gassing and off-gassing are based mostly in Dalton's Law, which states that at a given temperature, the amount of gas that will dissolve in a liquid is directly proportional to the partial pressure of that gas on the liquid. With the gas/liquid interface in the human body being the lungs, if you are off-gassing N2 after breathing some mix of N2 and O2, including compressed air, the most efficient way to do this is to minimize the partial pressure of N2 in the breathing mix. Doing this safely is essentially what technical diving is all about.

It might be an interesting thought exercise to see what percent of helium, O2, and N2 you'd have to have in trimix at a given depth to duplicate the partial pressures of O2 and N2 in air (I think that's one of your questions), but the risk of isobaric counterdiffusion makes this impractical in real-world diving.

Other things being equal, a diver will off-gas more quickly with higher partial pressures of O2, so 100% O2 at 20 fsw will result in faster off-gassing than 100% O2 on the surface.

Best regards,
DDM

 

[dmaziuk]

Other things being equal, a diver will off-gas more quickly with higher partial pressures of O2, so 100% O2 at 20 fsw will result in faster off-gassing than 100% O2 on the surface.

I don't think it's modeled, I believe in the formula the off-gassing will be driven by time only because 0% inert gas will zero out everything else. In Schreiner's equation oxygen is only counted in adjusting alveolar pressure for respiratory quotient, but even that is then multiplied by fraction of inert gas. I.e. if higher ppO2 makes you off-gas faster -- on pure O2 -- then your computer knows nothing about it.

Well, maybe if it's because of bubble formation, and your computer is running VPM or RGBM... but I don't know much about those.

 

[lowwall]

Other things being equal, a diver will off-gas more quickly with higher partial pressures of O2, so 100% O2 at 20 fsw will result in faster off-gassing than 100% O2 on the surface.

The model's not that sophisticated. If you're breathing pure O2, ppN2 in your lungs is 0. At any depth.

The planner I showed above agrees with dmaziuk. It shows no difference in either time or tissue loadings between 100% at 6m vs. stops at 6m and 3m even with an absurd dive of 40m with 60 minutes bottom time on air.

Of course that could be a result of how the programmer implemented the algorithm. But let's take a look at the algorithm itself:

Bühlmann decompression algorithm - Wikipedia

en.wikipedia.org

It appears (to me) that ZHL-16 does not vary the inert gas offgassing rate within a tissue compartment if the breathing gas contains 0% of that inert gas.

The calculation for each compartment for any given time period is:

Po is the initial partial pressure of the inert gas and Pgas is the partial pressure of the inert gas in the breathing gas. You don't have to worry about the rest (except to note that the exponent is the exposure time divided by the compartment time), because simplifying the equation with Pgas = 0, gives you P = Po - Po + 2^time or simply P = 2^time.

Edit: see posts below. The "time" in "2^time" is shorthand for the full exponent, i.e, exposure time divided by the tissue compartment time.

 

[dmaziuk]

Nitpick: I think T1/2 is compartment's half-time whereas Texp is elapsed time so it doesn't reduce to 2^time, exactly, but the point still stands: it's driven only by time and inert gas loading.

 

[lowwall]

Nitpick: I think T1/2 is compartment's half-time whereas Texp is elapsed time so it doesn't reduce to 2^time, exactly, but the point still stands: it's driven only by time and inert gas loading.

You are correct. I kept rewriting it, but couldn't come up with a way to represent it that wasn't confusing, e.g. "2^(adjusted time)". I ended up putting in the parenthetical statement to make it explicit what "time" meant in my simplified equation and then hoping for the best . As you noted, the point of the exercise was that the Po's cancelled out. I went ahead and added an edit to explain it.

The more interesting question is whether not adjusting outgassing speed for varying levels of PP02 is a bug in the algorithm that should be addressed, at least for high 02 technical deco mixes. Or is it a "no-harm, no foul" simplification that can be safely ignored.

 

[dmaziuk]

The more interesting question is whether not adjusting outgassing speed for varying levels of PP02 is a bug in the algorithm that should be addressed, at least for high 02 technical deco mixes. Or is it a "no-harm, no foul" simplification that can be safely ignored.

I think if tech divers using high O2 deco mixes don't come up bent all the time, it suggests the latter.

As I mentioned before, the only place oxygen is involved is when you adjust the pressure of your breathing gas for CO2 and water vapour in the lungs, but since this is applied only to the inert gas fraction in the breathing mix, and that is 0, it also cancels out. I.e. you'd have to do a significant rewrite of the model to account for O2, with all the trials and exploding goats, so I don't see that happening. Until/unless someone proves that the simple stupid model is broken and needs fixing.

 

[Alaskan Scuba Dude]

The concept of 90 to 360-minute Surface Intervals is to clear the Nitrogen bubbles, and to allow Central Nervous System toxicity to decay over time. It would be inefficient (cost wise) and inadvisable to off-gas underwater after the dive plan is completed. Live-aboards work to a schedule, and are responsible for safety. Plan for a longer dive, or dive a different plan.