Partial Pressures and Depth Question

[lowwall]

And the higher the altitude (with lower atmospheric pressure) the lower the temperature it takes to reach a boil

With the result that you need to cook wet things longer at high altitudes. My late father told me that as a child living in a Hoboken tenement he would read the high altitude cooking instructions on some packaged foods and dream of life in the mountains. He managed to retire to Wyoming so got to put those instructions to good use.

 

[halocline]

If you want to observe gas bubble formation firsthand without getting soda all over your arm, all you need to do is heat up a pot of water. A fluid's ability to hold dissolved gas diminishes as its temperature increases. At a high enough temperature, but still short of the boiling point, the gases will start to come out of the water in the form of bubbles that spontaneously appear at scratches or pits at the bottom of the pot.

Edit: I went ahead and did this myself. It's kind of fascinating.

View attachment 897791

So you’re saying don’t dive in hot springs, lol!

 

[LanceRiley]

Not to go too much into technical diving….

Lets separate the saturated tissues with nitrogen.

The only way to release those gasses in the tissues is when

Ambient pressure < internal pressure. - so gas moves from higher pressure (from inside the body to out) . Pressure will always want to equalize.

As your gf99 drops…. The rate of decompression slows down. So you need to go up the column again…. The difference in internal pressure and ambient widens again. And off gassing rate increases again

Too wide a difference from internal pressure and ambient, the nitrogen over expands and cant exit. Hence the bends

Now all of this is happening while youre breathing air 21% and 78% nitrogen.

By breathing nitrox…. You take in less Nitrogen…. Helping you off gas more.

Nx32 has 68% nitrogen. Vs air 78% …. This is a little difference but you have less nitrogen intake

Technical divers use nx50 which has 50% nitrogen and 100% oxygen whic is zero nitrogen

Less nitrogen intake

 

[Dsix36]

Not to go too much into technical diving….

Lets separate the saturated tissues with nitrogen.

The only way to release those gasses in the tissues is when

Ambient pressure < internal pressure. - so gas moves from higher pressure (from inside the body to out) . Pressure will always want to equalize.

As your gf99 drops…. The rate of decompression slows down. So you need to go up the column again…. The difference in internal pressure and ambient widens again. And off gassing rate increases again

Too wide a difference from internal pressure and ambient, the nitrogen over expands and cant exit. Hence the bends

Now all of this is happening while youre breathing air 21% and 78% nitrogen.

By breathing nitrox…. You take in less Nitrogen…. Helping you off gas more.

Nx32 has 68% nitrogen. Vs air 78% …. This is a little difference but you have less nitrogen intake

Technical divers use nx50 which has 50% nitrogen and 100% oxygen whic is zero nitrogen

Less nitrogen intake

This is one of the simplest and easiest explanations I have ever read. Good job and thanks for sharing it.

 

[rongoodman]

Not to go too much into technical diving….

Lets separate the saturated tissues with nitrogen.

The only way to release those gasses in the tissues is when

Ambient pressure < internal pressure. - so gas moves from higher pressure (from inside the body to out) . Pressure will always want to equalize.

As your gf99 drops…. The rate of decompression slows down. So you need to go up the column again…. The difference in internal pressure and ambient widens again. And off gassing rate increases again

Too wide a difference from internal pressure and ambient, the nitrogen over expands and cant exit. Hence the bends

Now all of this is happening while youre breathing air 21% and 78% nitrogen.

By breathing nitrox…. You take in less Nitrogen…. Helping you off gas more.

Nx32 has 68% nitrogen. Vs air 78% …. This is a little difference but you have less nitrogen intake

Technical divers use nx50 which has 50% nitrogen and 100% oxygen whic is zero nitrogen

Less nitrogen intake

This explanation does not take into account diffusion, which is what's really driving the process. It's the inert gas concentrations which are trying to equalize, not physical pressures. You will off gas inerts more quickly if you switch to a breathing mix containing less of them, even if you don't change your position in the water column. That's why technical divers switch to higher O2 mixes as soon as they can safely do so, not because their bodies have any use for the extra O2, but simply because those mixes have less or none of the inert gases they're trying to get rid of. Another way to get a breathing mix with less inerts than those present in the body is to ascend to a shallower depth, in which case the concentration(aka partial pressure) of the inerts in the breathing gas is less than that of the supersaturated tissues, again driving diffusion across the capillary beds into the alveoli. Ascending too rapidly has its own dangers however, if the ambient pressure is reduced to the point where the supersaturated tissues begin to release their inert gas load directly, before its being cleared in the lungs, leading to inflammation and blockages.

 

[LanceRiley]

This explanation does not take into account diffusion, which is what's really driving the process. It's the inert gas concentrations which are trying to equalize, not physical pressures. You will off gas inerts more quickly if you switch to a breathing mix containing less of them, even if you don't change your position in the water column. That's why technical divers switch to higher O2 mixes as soon as they can safely do so, not because their bodies have any use for the extra O2, but simply because those mixes have less or none of the inert gases they're trying to get rid of. Another way to get a breathing mix with less inerts than those present in the body is to ascend to a shallower depth, in which case the concentration(aka partial pressure) of the inerts in the breathing gas is less than that of the supersaturated tissues, again driving diffusion across the capillary beds into the alveoli. Ascending too rapidly has its own dangers however, if the ambient pressure is reduced to the point where the supersaturated tissues begin to release their inert gas load directly, before its being cleared in the lungs, leading to inflammation and blockages.

Lets simplify things some more….

At depth you are saturated with Nitrogen. depth x time. Say recreationally… 30m 15mins…

You go up to. 20m. The pressure inside you will be 30m worth…. And the ambient is 20m

Pressure will always equalize… so the higher pressure inside yourbody will attempt to go out. Includinf the nitrogen in your organs. To match the pressure at 20m

Staying at 20m extra time wont off gas anymore.. 20m ambient and 20m worth of pressure inside you will never offgas…

You need to go up say 15m. The pressure worth 20m inside you will offgas again.

Theoretically if you have unlimited air tanks…you can offgas up to 3m.

How did i know…. I tried it… with a perdix…. Looking at gf99. Id let it hit 20-30% hold depth…. When it drops to like gf99 = 15-20% , i would go shallower… until gf99 is 30%

What were really offgassing is the nitrogen in our body. The oxygen part… we use it… some of it…. But nitrogen we dont really need it…

The nx50 and oxygen just REDUCES the Nitrogen you inhale which in turn goes into your system.

So you see you’re trying to expel excess pressured nitrogen OUT of your system. By Reducing your NITROGEN INTAKE. You make the process fasttteer.


The example above is being simplified…. Those arent the exact numbers. If you wish to further understand. I would suggest you take up a technical course …. This is for the OP

 

[LanceRiley]

This explanation does not take into account diffusion, which is what's really driving the process. It's the inert gas concentrations which are trying to equalize, not physical pressures. You will off gas inerts more quickly if you switch to a breathing mix containing less of them, even if you don't change your position in the water column. That's why technical divers switch to higher O2 mixes as soon as they can safely do so, not because their bodies have any use for the extra O2, but simply because those mixes have less or none of the inert gases they're trying to get rid of. Another way to get a breathing mix with less inerts than those present in the body is to ascend to a shallower depth, in which case the concentration(aka partial pressure) of the inerts in the breathing gas is less than that of the supersaturated tissues, again driving diffusion across the capillary beds into the alveoli. Ascending too rapidly has its own dangers however, if the ambient pressure is reduced to the point where the supersaturated tissues begin to release their inert gas load directly, before its being cleared in the lungs, leading to inflammation and blockages.

PP involves both the gases. oxygen and Nitrogen. But since oxygen we can process we dont look into that. Only nitrogen

Lets say pressure inside you is equal to ambient pressure. nitrogen and oxygen…


Changing gas to NX50 hardly changes anything. Offgassing will be very slow.

You will need a wider difference in pressure between internal and ambient.

Lets put this in numbers…. You stay at 30m for 30mins wirh air.
That will set you beyond NDL. need to Deco

You go to 21m. Just air deco…. Soon ambient and internal pressure is equal.
You switch to nx50. Will offgassing happen? Maybe but it will reallly be very slow if ambient pressure and internal pressure is equal. Maybe a bit due to less nitrogen intake.
You still need to go shallower


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

 

[David Carron]

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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 equal absolute pressures in your example will not prevent diffusion. Oxygen will diffuse from the oxygen tank to thr air tank and nitrogen from the air tank to the oxygen tank.

 

[LanceRiley]

The equal absolute pressures in your example will not prevent diffusion. Oxygen will diffuse from the oxygen tank to thr air tank and nitrogen from the air tank to the oxygen tank.

Yes but at a very slow rate…. Only by its density….

But going back to offgassing…. Ambient pressure < internal pressure….at least 20% … some prefer 50% . Will have significant offgassing..

If internal and ambient pressure is almost equal…. Deco gas nx50 Will Still create offgassing but at a very slow rate. Ineffective use of offgassing

 

[boulderjohn]

You go up to. 20m. The pressure inside you will be 30m worth…. And the ambient is 20m

Pressure will always equalize… so the higher pressure inside yourbody will attempt to go out. Includinf the nitrogen in your organs. To match the pressure at 20m

Ron's language made the process of diffusion more clear. The average reader looking at this will not understand what is meant by the pressure differential. It is not like a wind blowing.

It's the inert gas concentrations which are trying to equalize, not physical pressures.

Let's say you have been on the surface for days, and you have achieved equilibrium. Your lungs have a certain concentration of nitrogen molecules. These go through your body, mostly through perfusion of blood going through the tissues. The nitrogen molecules both enter and leave the tissues constantly through this process, and the concentration stays about the same.

You then descend to 99 FSW, or 4 atmospheres of pressure. To maintain their volume, your lungs now take in 4 times as many nitrogen molecules as before, so 4 times as many nitrogen molecules are entering the body as exiting. This raises the concentration of nitrogen in the tissues rapidly. As time passes and the amount of nitrogen in the tissues increases, the difference in concentrations lessens, and the process slows down. Eventually there are just as many nitrogen molecules leaving the tissues as entering it. Those tissues are at equilibrium again.

You ascend, and the concentration of molecules in the lungs decreases. Now more molecules are leaving the body than are entering it. If you stay at that shallower depth for a while, the tissues will once again be in equilibrium. When you ascend again, once again more nitrogen leaves the body than enters it.

The word "pressure" is confusing because it is not what the average person thinks of by pressure. When I open a faucet in my sink, the water pressure forces water to spurt out. That is not what we mean by gas pressure in the body.