Hypothetical question

[Dan]

@doctormike, let me preface my response with what you said "I'm (also) not a deco expert.." so If I'm wrong here, I hope someone corrects me!

I don't think pressure itself is really the issue. Gases move into and out of tissues by diffusion. The bigger the concentration gradient, the faster they move. What pressure does is just change the concentration gradient. You on gas at depth because you are inspiring inert gas at a higher concentration than what exists in your tissues.

So in your two thought experiments, I think the answer would be no, you would not get bubble formation. Let's make a caveat for scenario number 1. I'm assuming there is a layer of gas above the liquid and that layer of gas has a PN2 of 0.79. Now in both scenarios, even though you are changing pressures, you haven't changed the concentration gradient. So yes, you can change the pressure, but now, there is no concentration gradient for the gases to follow.

Again, if I've really missed something here, someone correct it!

That is what I learn in my Mass Transfer class (3rd year of Chemical Engineering). Fick's laws of diffusion - Wikipedia

Going back to my previous post about the nitrogen concentration in the blood at 9m depth with EAN60, assuming Henry's Law constant of nitrogen in the blood is the same as that of water, the concentration of dissolved nitrogen in the blood at that depth would be about 13.3 ppm (13.8 x 1.9 x 0.4 / 0.79). As you ascend to 0.1 m, the dissolved nitrogen in the blood would be about 7.1 ppm (13.8 x 1.01 x 0.4 / 0.79). So there is a concentration gradient from 13.3 to 7.1 ppm of dissolved nitrogen. 6.2 ppm (13.3 - 7.1) of dissolved nitrogen would come out of the blood as the ambient pressure reduced.

How that dissolved nitrogen come out of the blood and whether that would affect the diver, that would be beyond my comprehension. What I understand is some time after the diver takes of his / her regulator and begins to breathe the surface air, the dissolved nitrogen in the blood would slowly build back up to 13.8 ppm. Since a normal human body can handle that dissolved nitrogen and at any given time & depth that concentration has never exceed the normal concentration at the surface (13.8 ppm), I doubt if the diver would suffer DCS.

 

[dmaziuk]

Better tell this guy...

I know, but you have to keep diving for 6 hours to 90 feet depth, presumably without any surface intervals to speak of. On a single isolated dive my point should stand. With the usual disclaimers.

 

[Dan]

Going a bit out on a limb here, but I think we all tend to focus on bubbles in the blood. I don't know any reason why bubbles could not form in the tissues themselves. Diffusion would not be involved in that process.

To my understanding, tissues are like membranes. Deep tissue (internal organs) would have higher diffusivity than that of the shallow tissue (skin). It would take longer time for nitrogen to build up in or deplete from the deep tissue.

 

[doctormike]

I know, but you have to keep diving for 6 hours to 90 feet depth, presumably without any surface intervals to speak of. On a single isolated dive my point should stand. With the usual disclaimers.

OK, so how about this one. Lots of bubbles, from the very first echocardiogram in the session. Was it because of ongassing or ambient pressure drop?

Look, I'm not saying that every free diver is going to get bent. I'm not even saying that the diver in the OPs original question is going to get bent.

What I'm SAYING is that a reduction in ambient pressure is what causes bubble formation, and pretending that you can't bubble because you are breathing a gas with the same PPN2 as surface air is just not correct. So can those silent bubbles grow if you immediately ascend again in an air cabin? I don't know! And if they do, can they cause symptoms of DCS? I don't know, but I agree, it doesn't sound likely.

But that's very different from just saying that bubble formation on any ascent magically doesn't exist if you simply match your PPN2 at depth to the the PPN2 of surface air. Just because you aren't ongassing, doesn't mean that you have no inert gas in your tissues.

 

[BRT]

OK, so how about this one. Lots of bubbles, from the very first echocardiogram in the session. Was it because of ongassing or ambient pressure drop?

Look, I'm not saying that every free diver is going to get bent. I'm not even saying that the diver in the OPs original question is going to get bent.

What I'm SAYING is that a reduction in ambient pressure is what causes bubble formation, and pretending that you can't bubble because you are breathing a gas with the same PPN2 as surface air is just not correct. So can those silent bubbles grow if you immediately ascend again in an air cabin? I don't know! And if they do, can they cause symptoms of DCS? I don't know, but I agree, it doesn't sound likely.

But that's very different from just saying that bubble formation on any ascent magically doesn't exist if you simply match your PPN2 at depth to the the PPN2 of surface air. Just because you aren't ongassing, doesn't mean that you have no inert gas in your tissues.

You are forgetting that there is no promise that you won't get DCS in an airplane cabin even if you were golfing beforehand.

 

[doctormike]

You are forgetting that there is no promise that you won't get DCS in an airplane cabin even if you were golfing beforehand.

Well, golfing just shows bad judgement all around. Those guys deserve to get bent...

 

[dmaziuk]

OK, so how about this one. Lots of bubbles, from the very first echocardiogram in the session. Was it because of ongassing or ambient pressure drop?

Both. There's no full text on that one, I'm guessing we're talking repetitive dives without much SI and a noticeable amount of gas build-up here too:

Number of dives, bottom time and surface intervals were freely determined by the diver

.

You can get bubbles not flying after not playing golf too: just drive straight up Mt. Kea on the Big Island without stopping at the gift shop for a couple of hours, do the echo and see if it shows bubbles. That's your pressure drop with no on-gassing relative to sea level.

 

[RyanT]

Well, golfing just shows bad judgement all around. Those guys deserve to get bent...

There is clearly much that I don't understand, but this is something that I think most of us can get on board with!

 

[markmud]

@doctormike ,

How did you vote?

I have not read all of the posts, but most of them.

What did you recommend to the hypothetical diver?

thanks,
markm

 

[doctormike]

@doctormike ,

How did you vote?

I have not read all of the posts, but most of them.

What did you recommend to the hypothetical diver?

thanks,
markm

Hah! Forgot it was a poll. I'll go with DAN and PADI and say 12 hours.