Partial Pressures and Depth Question

[LanceRiley]

Absolute ambient pressure does not play a direct part in on/off gassing.

The only thing that counts is the difference between the inert gas partial pressures in the lungs and the rest of the body. This is why at the same absolute ambient pressure you offgass faster breathing oxygen than breathing air.

yes you offgas faster with oxygen. there is no more nitrogen being introduced into the body.

but if you’re on air….. you go shallower, lower ambient pressure. and your organs have higher pressure , you offgas.

so ambient pressure still plays a part.

bec if it didnt…. youd never offgas breathing air !

 

[David Carron]

yes you offgas faster with oxygen. there is no more nitrogen being introduced into the body.

but if you’re on air….. you go shallower, lower ambient pressure. and your organs have higher pressure , you offgas.

so ambient pressure still plays a part.

bec if it didnt…. youd never offgas breathing air !

This is correct: the inert gas partial pressure in the lungs when breathing air depends on the ambient pressure and is why I noted "does not play a direct part"

I think you may be confusing yourself a little by conflating these direct and indirect effects, especially since decompression safety does depend directly on ambient pressure.

 

[LanceRiley]

This is correct: the inert gas partial pressure in the lungs when breathing air depends on the ambient pressure and is why I noted "does not play a direct part"

I think you may be confusing yourself a little by conflating these direct and indirect effects, especially since decompression safety does depend directly on ambient pressure.

thats what ive been saying…
ambient pressure will affect offgassing

 

[LanceRiley]

Absolute ambient pressure does not play a direct part in on/off gassing.

The only thing that counts is the difference between the inert gas partial pressures in the lungs and the rest of the body. This is why at the same absolute ambient pressure you offgas faster breathing oxygen than breathing air.

this is what i reacted on

 

[rongoodman]

yes you offgas faster with oxygen. there is no more nitrogen being introduced into the body.

but if you’re on air….. you go shallower, lower ambient pressure. and your organs have higher pressure , you offgas.

so ambient pressure still plays a part.

bec if it didnt…. youd never offgas breathing air !

You offgas faster with oxygen because it has no inert gases in it, making the concentration gradient steeper and diffusion more efficient than if you were breathing 50% nitrox and even more efficient than if you were breathing air. The process is the same for each, it's just slower on air. There is no time at which your internal organs are at a different pressure than their surroundings, unless you're sitting in a submarine or the pressurized cabin of an airplane.

 

[LanceRiley]

You offgo

You offgas faster with oxygen because it has no inert gases in it, making the concentration gradient steeper and diffusion more efficient than if you were breathing 50% nitrox and even more efficient than if you were breathing air. The process is the same for each, it's just slower on air. There is no time at which your internal organs are at a different pressure than their surroundings, unless you're sitting in a submarine or the pressurized cabin of an airplane.

thats what i said… you’re not intaking nitrogen or inert gas.

but with regards to pressure i disagree… the moment you change depth to a shallower one…. ambient pressure becomes lower

You offgo

You offgas faster with oxygen because it has no inert gases in it, making the concentration gradient steeper and diffusion more efficient than if you were breathing 50% nitrox and even more efficient than if you were breathing air. The process is the same for each, it's just slower on air. There is no time at which your internal organs are at a different pressure than their surroundings, unless you're sitting in a submarine or the pressurized cabin of an airplane.

thats what i said… you’re not intaking nitrogen or inert gas.


“but with regards to pressure i disagree… the moment you change depth to a shallower one…. ambient pressure becomes lower”

An Introduction to Scuba Gas Laws - Part 3: Henry’s Law - DeepDive

Henry’s Law William Henry was an English physician and chemist who, in 1803, proposed what is now called Henry’s law. This law states that “At a constant

www.scuba.com

“Similarly, as a diver descends, nitrogen inhaled has nowhere to escape and, under pressure, gets soluble in the bloodstream, muscles, and tissues. This is no problem until the diver begins his ascent. As a diver ascends, the pressure is released, and like the soda bottle, the nitrogen in the body tries to escape and may form bubbles if the diver ascends too quickly, causing DCS or Decompression Sickness, also known as the bends. This is why divers ascend gradually, allowing the nitrogen to dissipate rather than form bubbles.”

 

[inquis]

on gassing and offgassing is about pressure

What you're saying is mostly correct, but the distinction others are trying to make has to do with that word "pressure". Gas transfer is actually driven by PARTIAL pressure. That extra word is very important. By definition, that is the product of ambient pressure and fraction of a particular molecule (e.g., 21% in air). The people talking about "concentration" are also correct, as partial pressure can also be viewed as a density (# of molecules per some volume).

Up thread, I believe the term "partial pressure" was intentionally not used because it's a bit confusing. It's not ambient pressure (alone). In fact, if you had said "partial pressure", I don't think there would have been disagreement.

 

[inquis]

the gas tanks were there as an example that without a pressure difference... it would take forever….

This is highly illustrative. There is zero total pressure difference, but a huge partial pressure difference. It will not take forever for molecules to even out the concentrations between the 2 tanks by diffusion. It will mostly be done in a matter of hours.

 

[rongoodman]

"This is why divers ascend gradually, allowing the nitrogen to dissipate rather than form bubbles.”

Nobody here is disagreeing with that. The nitrogen dissipates by the process of diffusion from the capillary beds into the alveoli and out of the lungs, the exact reverse of the way it got into the body in the first place. That process is driven by concentration gradients, not ambient pressure differences.

 

[boulderjohn]

Its like having 2 gas tanks on land. tank A is filled with pure oxygen, tank B is filled with Air.

Lets say both are 100bar. You connecr a hose in between them… open both tanks. Equal pressure… no transfer. Oxygen will not go to air nor air go to oxygen. Why? Both are in equilibrium…. Maybe theres a wee bit due to difference in density…. But bothe wil take forever to mix

the gas tanks were there as an example that without a pressure difference… it would take forever….

This is highly illustrative. There is zero total pressure difference, but a huge partial pressure difference. It will not take forever for molecules to even out the concentrations between the 2 tanks by diffusion. It will mostly be done in a matter of hours.

LanceRiley is correct on this. I once had two tanks with different blends, and I wanted them to be alike. I connected them with a transfill whip and left them connected for 3-4 days. When I tested them after that, they each had the same blends they started with. Whatever mixing had happened was undetectable.