Thanks for pointing that out and giving me something to think about. It should be "driven by the difference in the absolute pressures of the tissue and the capillaries".We absolutely agree on the part I bolded. What is the partial pressure of N2 in each of the capillaries?
Let's say you have a small tank of 100% N2 at 10atm. It is sitting in a giant hyperbaric chamber filled with 100% 02 also at 10atm. Now fully open the valve on the N2 tank. What happens in the first second?
Very little. Molecules meander here and there (Brownian motion) so eventually the whole system comes into equilibrium. But it isn't fast.
Now rerun the experiment except with the chamber at 1atm. What happens in the first second? The N2 comes screaming out of the tank. The diffusion of N2 into the chamber driven by the difference in absolute pressure completely dwarfing the diffusion due to Brownian motion.
Deco algorithms are a series of approximations, this is just another to add to the list. The deco algorithm that Mr. Baker is discussing simply ignores this scenario. It's probably reasonable to do this because most of the time the differences between absolute pressures in the various tissues are low and the inert gas percentages in the tissues don't vary greatly. Thus most of the diffusion is driven by Brownian motion and thus and thus the rate is going to largely depend on concentration which is stated as partial pressures. The big exception is exactly what we are discussing here, when the diver switches to a very high percentage of O2. But even then the fairly narrow limits on the ppO2 that a diver can handle mean this effect will not be large.