Partial Pressures and Depth Question

[LanceRiley]

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.

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.

your explanation is very cottect

but i had to use pressure…. because thats whats happening

You descend to 99fsw or 4 atmospheres.

surface is 1 atmosphere

by going down to a depth of 99fsw you are exposing yourself to 4atm of pressure while your body could still be at 1 atm . well not exactly… but lets just assume you went down so fast…. that your body was still ar 1 atm. and the ambient is 4atm at 99fsw.

when you breathe in the tank… to fill in your lungs and expand it… it will be filled with 4atm. which is 4x the normal air you breathe in at the surface. what is 4atm. thats pressure…. and yes that means 4x air. (21% oxygen and 79%Nitrogen)

at a certain point in time staying in that depth of 99fsw. your body and organs… will be equal to 4atm. which is also equal to the ambient Pressure of 4atm. at this point there is No more Ongassing Nor offgassing….

go up… to 3atm. or 65fsw
your internal body and organ pressure will still be at 4atm. while the ambient pressure is at 3atm.

this is what i meant . Internal pressure is 4atm . ambient at 3atm. higher pressure moves to lower pressure. offgassing is occuring.

on gassing and offgassing is about pressure

 

[rongoodman]

your explanation is very correct

but i had to use pressure…. because thats whats happening

You descend to 99fsw or 4 atmospheres.

surface is 1 atmosphere

by going down to a depth of 99fsw you are exposing yourself to 4atm of pressure while your body could still be at 1 atm . well not exactly… but lets just assume you went down so fast…. that your body was still ar 1 atm. and the ambient is 4atm at 99fsw.

when you breathe in the tank… to fill in your lungs and expand it… it will be filled with 4atm. which is 4x the normal air you breathe in at the surface. what is 4atm. thats pressure…. and yes that means 4x air. (21% oxygen and 79%Nitrogen)

at a certain point in time staying in that depth of 99fsw. your body and organs… will be equal to 4atm. which is also equal to the ambient Pressure of 4atm. at this point there is No more Ongassing Nor offgassing….

go up… to 3atm. or 65fsw
your internal body and organ pressure will still be at 4atm. while the ambient pressure is at 3atm.

this is what i meant . Internal pressure is 4atm . ambient at 3atm. higher pressure moves to lower pressure. offgassing is occuring.

on gassing and offgassing is about pressure

Your body is essentially a liquid, except for a few small air filled rigid spaces, such as the sinuses. When you go to 99 feet, everything in your body is at the same pressure, 4 ATM, when you come back up to 33 feet, everything goes to 2 ATM, and those changes are immediate. There is no time when the internal pressures in your body don't match that of the depth you're at or the gas you're breathing. On the chance that it does happen, it's going to be very uncomfortable. Think about how it feels when you can't equalize on descent or get a reverse block on the way back up. What does change is the amount of inert gases which can be held in solution and there is a time lag involved there before that equilibrium is reached. On and off gassing are about diffusion of inert gases across the capillary beds in your lungs. Decompression injuries are a result of supersaturated tissues no longer being able to hold those gases in solution due to a decrease in ambient pressure. They are two different processes.

 

[rongoodman]

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.

Especially if the tubing connecting them is the size of a tennis court, which is about the surface area of your lungs.

 

[LanceRiley]

Air goes to lungs, lungs to arterioles, to bloodstream…. to heart …. distributed to organs

diffusion of gas. to blood stream.

No its Not immediate it takes
time. thats why you have your NDL. why can you stay x no of mins before running out of NDL? because it takes time for the theoretical 16 chambers…. or 16 different kinds of organs to absorb the Nitrogen…. like Bones takes the least i think, fats the fastest

when you go to 2atm or 33ft . ehy do you have to offgas? when its immediate? bec its NOT immediate… you wait mins….. for the gas to diffuse again from blood stream to lungs… and exit
.

 

[LanceRiley]

Your body is essentially a liquid, except for a few small air filled rigid spaces, such as the sinuses. When you go to 99 feet, everything in your body is at the same pressure, 4 ATM, when you come back up to 33 feet, everything goes to 2 ATM, and those changes are immediate. There is no time when the internal pressures in your body don't match that of the depth you're at or the gas you're breathing. On the chance that it does happen, it's going to be very uncomfortable. Think about how it feels when you can't equalize on descent or get a reverse block on the way back up. What does change is the amount of inert gases which can be held in solution and there is a time lag involved there before equilibrium is reached. On and off gassing are about diffusion of those gases across the capillary beds in your lungs. Decompression injuries are due to supersaturated tissues no longer being able to hold those inert gases in solution due to a decrease in ambient pressure. They are two different processes.

this is a different thing. air in sinuses.

this is different from decompression. if it was as easy to decompress as blowing of your nose…. then there would be no technical diving.

everyone can go down to 50m stay there as their air supply hold out and go up…. since its immediate…

but the truth it it takes time… to offgas the nitrogen… feom higher pressure to lower pressure. we’re in the organ and bloodstream level. and Not the air sinuses

 

[LanceRiley]

Especially if the tubing connecting them is the size of a tennis court, which is about the surface area of your lungs.

yes yes….. but what about the organs ? Buhlman 16compartments? some organs absorb faster and some slower? what is absorb? Nitrogen!

 

[rongoodman]

The pressure changes are immediate, but they aren't what is driving diffusion. Diffusion is purely a statistical process, driven by the differences in concentrations of a substance and it does take time. Think about dropping a cube of sugar into a cup of tea. How long before the cube is gone and the tea is sweet? It won't happen any faster if you put the tea cup in a pressure chamber(but it will if you make the water hotter).

 

[LanceRiley]

The pressure changes are immediate, but they aren't what is driving diffusion. Diffusion is purely a statistical process, driven by the differences in concentrations of a substance and it does take time. Think about dropping a cube of sugar into a cup of tea. How long before the cube is gone and the tea is sweet?

lets assume thats its immediate

go 4atm. at a rate of 4mins.

so your at 4atm or 30m/99fsw.

breathe 4atm of air. so you have 4x Nitrogen in your lungs.

the 4atm air. is in your lungs. its not easily absorb by your organs. otherwise if it is. No need to countdown NDL. it would mean in 4mins your body is saturated with Nitrogen.

if its immediate…. as you go up . Nitrogen is expelled IMMEDIATELY. and there is No reason for Nitrogen to remain in your organs. since Ongassing and Offgassing is immediate.

So what you’re trying to say… that performing Decompression. is a Myth?

In recreational diving the agencies simplified it. by asking you NOT to break NDL. what does this means. it theoretically computes that…. YOU NEVER Reach Saturation. thats what the NDL Countdown is….. the Max Saturation wherein you can still safely go up.

But what about technical diving? can we do the same? No we cant. because our organs are Saturated. Maxed out. going up immediately, would kill or maim the diver? why? bec it takes time for gas to leave the body…. the same way it took time to fully saturte the body when descending

 

[lowwall]

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

This is incorrect and I think the crux of your misunderstanding of the processes.

When you bring two different gasses into contact, it's their relative concentration, aka concentration gradient, that drives diffusion. Concentration simply means the number of molecules of each substance per given volume. Differences in pressure only matter to diffusion because, for gases, higher pressure means a greater difference in concentration.

It may help to bring this back to first principles. Unlike solids, gas molecules are constantly zipping around in random directions. If you've got only x molecules in one spot and y molecules in another, then from the first zip they are going to start intermingling. With each subsequent zip, around half the molecules will further move away from their initial point of concentration. It doesn't take that many zips before the concentration differences have disappeared. We call this equilibrium, but it's a mistake to think of it as static. The molecules continue to zip around, causing variations in concentration in any particular tiny volume.

I think I should link this here: Fick's laws of diffusion - Wikipedia

Of course, if you put some sort of barrier between the two gasses, this process will slow. But it won't stop if there is any possible movement across the barrier. In your case of the gas tanks connected by a hose (of reasonable length), the tanks will reach equilibrium within several hours, maybe even a day, but certainly not "forever".

 

[rongoodman]

But what about technical diving? can we do the same? No we cant. because our organs are Saturated. Maxed out. going up immediately, would kill or maim the diver? why? bec it takes time for gas to leave the body…. the same way it took time to fully saturate the body when descending

It isn't necessarily that they're saturated or "maxed out", just that the amount of overpressure in the leading compartments would be greater than their M-Values if you didn't give them chance to off-gas, so the potential for injury would be there. It would literally take days to fully saturate the slower compartments, but it does became an issue in commercial diving.