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This question has spawned from a discussion between myself (a novice diver but an engineer), and a diving instructor.

The training materials indicate that if you (having not dived yet) are driving and go from sea level, to an arbitrary higher altitude (assuming the gas mix remains constant), and then return to sea level after a short period, it is possible to have more total dissolved nitrogen in your body than when you started your ascent from sea-level. Which would indicate that your blood/tissue has become super-saturated with nitrogen going from a lower to a higher pressure.

My understanding tells me that this is impossible.

When you start ascending (higher to lower pressure), you become super-saturated as the solubility of nitrogen decreases at a lower pressure. This causes you to off-gas and lowers the total amount of nitrogen in your tissues until you reach the saturation/equilibrium point of the higher altitude (or start descending again).

When you're descending on the other side, the solubility of nitrogen increases, so you're no longer saturated and your tissues take on more nitrogen. This process continues until you hit your bottom altitude (sea level) and eventually become saturated at that new altitude.

As such, on the other side you should have less than or an equal amount of dissolved nitrogen as where you started.

Plotting altitude, pressure, and dissolved NO2 against time on a graph by my reckoning should give you something like this.

Plot

Now, this guy is far more experienced at diving than me, and this isn't my field of expertise so am I missing something or is the training material mistaken?

I originally asked this question over on Physics SE and recieved a rather speculative answer that this might have something to do with adsorbtion but was advised that this is probably more of a biology question than a physics question, so I'm looking for a more confident answer.

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    $\begingroup$ Your understanding is how I understand it too. I think they are conflating the "don't fly after diving" rule with nitrogen absorption. The partial pressure of N2 at altitude should be less, so there should be less in your blood is how I understand it. Dalton's law. However, there might be a biological mechanism I don't know about at play here. $\endgroup$
    – bob1
    Commented Apr 24 at 0:26
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    $\begingroup$ @JiminyCricket. - It could reduce your no deco time for your first dive potentially, but it's not a huge difference. I guess my main issue is that I want to understand why the process we're being taught works and unless this is definitely a mistake, then I don't understand it and have a gap to fill $\endgroup$ Commented Apr 24 at 10:22
  • $\begingroup$ What training materials are you referring to? Can you link them and/or quote from them? $\endgroup$
    – Eonema
    Commented Apr 29 at 15:51
  • $\begingroup$ It's the BSAC training materials. I don't believe that I'm allowed to share them. I have paraphrased the section in question though. There's a transfer diagram where you start at a Tissue Code A, go up a hill taking you to a tissue code B, return to sea level still at a tissue code B $\endgroup$ Commented Apr 29 at 17:15

2 Answers 2

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After a lot of discussion with my diving officer, we've agreed that the following might be true. I'm not going to accept this as an answer until I've had some affirming opinions though.

The diagrams in the answer are accurate when considering purely the nitrogen that is actually dissolved in tissues/fluids in your body.

However, what it doesn't consider is that when you first ascend, you will have formed (micro)bubbles of nitrogen in your tissues. These may not have had time to actually escaped from your tissues by the time that you descend again. Once you have descended again, there's also no guarantee that these bubbles will have been broken down/re-dissolved. So, whilst your dissolved nitrogen load will be less than or equal to before you went over the hill, you may still have extra bubbles in your tissues.

This lowers your no-deco time for your next dive because it would take a smaller number of bubbles (as they are being added to the already extant bubbles) to increase your risk of DCI unacceptably than had you not already had these residual bubbles, regardless of the actual amount of dissolved nitrogen.

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  • $\begingroup$ As I understand it (but probably a gross oversimplification), gases in equilibrium with their dissolved forms may come out of solution and form bubbles upon a sharp pressure decrease. These guys seem to think it can occur with nitrogen at high altitudes. $\endgroup$
    – user338907
    Commented May 1 at 19:51
  • $\begingroup$ @user338907 Yes it can occur at high altitude, but only if there is a relatively sharp depressurization event - going up in a non-pressurized plane for instance. I think driving wouldn't count here as the ascent/descent is so slow. $\endgroup$
    – bob1
    Commented May 1 at 20:44
  • $\begingroup$ @ScottishTapWater This also doesn't make much sense to me. The bubbles would be pressurized by diving and become smaller, therefore dissolve more quickly into the bloodstream/tissues. I could see that being a problem if you were nitrogen saturated and the bubbles couldn't dissolve, but I think driving would be too slow to cause bubble formation. $\endgroup$
    – bob1
    Commented May 1 at 20:47
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Normally the proportion of nitrogen does not change perceptively with altitude/depth. Divers tanks run off compressed air usually, so the composition doesn't change.

Altitude doesn't change the gas composition. The effect above water surface is small since the entire pressure range is just 1 atmosphere, several atmospheres in an extreme dive. (34 ft -> 1 atm).

Our atmosphere contains approximately 79% nitrogen and 21% oxygen, a constant ratio until you reach an altitude of about 270,000 feet.

FAA Airmen Education

The solubility of nitrogen comes from the partial pressure of nitrogen (usually a constant) from the gas breathed. Which does not produce bubbling in the blood for quite a while. So ascending altitude is similar to coming up from about 30 feet depth. The nitrogen degassing is there but it does not create decompression sickness. Not suprising, land animals are acclimated to pressure changes of up to 1 atm. It makes you think about diving ocean mammals and birds who can go down 120 or more feet in depth. They have adapted so that decompression is not an issue for them!

its the total pressure in the blood. above sea level

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