Who's using 100% O2 for deco?

[lowviz]

Isn't 80/20 a heliox mix? As such there wouldn't be any N2 in it. .....//.......

Yes, written that way, it implies Helium. Right again, no N2. That will be next year for me, -expensive class.

I am discussing 80% O2 with the remaining 20% being N2, aka EAN80. Buoyancy is not the issue, I like to get onto a rich mix as deep as possible and 80% gives me more latitude in my deco plans. Which is also why 50/50 (O2 / He) looks so inviting on paper....

...//..... It was only until my 96th cold water dive that I tried a drysuit, even though buddies, dive shops and instructors had been pushing me to get on a drysuit much earlier......//.....

Get a P-valve installed if you don't already have one. Best safety device there is. Staying hydrated really is important. I don't care what anyone claims, dry divers without P-valves dehydrate themselves.

....//....I also see the merit of wanting to be deeper when the seas are rough. I'm prone to motion sickness and throwing up under water is no fun. I would also think that the motion sickness stress and the dehydration from throwing up would increase DCS likelihood.....//......

I, too am very prone to motion sickness. Yes, it gets rougher near the surface and EAN80 allows me to pick a deeper spot on the line where I can contemplate my mysery in private.

 

[Jax]

Okay, someone said they prefer to go on 100% because "they are not onloading inert gasses" . . .

Help me out with the physiology here . . . As long as the current aspiration gas gradient is less than the body's absorbed gas gradient, you WILL be off gassing, right? Which is why the 80% deco gas time is PD close to the pure O2 deco time.

But you do incur a much heavier CNS load on the 100%.

Must run some tables . . . .

 

[JeffG]

But you do incur a much heavier CNS load on the 100%.

So?..woop de do

 

[lowviz]

So?..woop de do

-you take air breaks.

Okay, someone said they prefer to go on 100% because "they are not onloading inert gasses" . . .

Help me out with the physiology here . . . As long as the current aspiration gas gradient is less than the body's absorbed gas gradient, you WILL be off gassing, right? Which is why the 80% deco gas time is PD close to the pure O2 deco time.

But you do incur a much heavier CNS load on the 100%.

Must run some tables . . . .

The direction of the concentration gradient for any gas that is in your lungs vs. the concentration of that same gas in your tissues determines which direction that particular "gassing" is going to take. The magnitude of the concentration gradient is one of the determining factors for how fast (efficient) the on- or off- gassing will be.

No N2 in your lungs gives you the biggest possible concentration gradient for N2 off-gassing. So pure oxygen is more efficient for N2 offgassing than EAN80 because EAN80 contains 20% N2.

-hope this helps.

 

[Jax]

The direction of the concentration gradient for any gas that is in your lungs vs. the concentration of that same gas in your tissues determines which direction that particular "gassing" is going to take. The magnitude of the concentration gradient is one of the determining factors for how fast (efficient) the on- or off- gassing will be.

No N2 in your lungs gives you the biggest possible concentration gradient for N2 off-gassing. So pure oxygen is more efficient for N2 offgassing than EAN80 because EAN80 contains 20% N2.

-hope this helps.

Okay . . . that's what I thought I said . . . . However, isn't it the magnitude of the pressure gradient that affects the dissolution of the gasses? I am thinking the amount of gas to come out of solution is a constant related to the current pressure of the gas.

Ugh, I hate chemistry.

 

[jimmy71]

Okay, someone said they prefer to go on 100% because "they are not onloading inert gasses" . . .

Help me out with the physiology here . . . As long as the current aspiration gas gradient is less than the body's absorbed gas gradient, you WILL be off gassing, right? Which is why the 80% deco gas time is PD close to the pure O2 deco time.

Correct that you will be off gassing in both circumstances. The rate of off gassing is also based on the size/delta of the gradient. The bigger the gradient, the faster the off gas. The question remains when the risk of 100% out weighs the benefit of a larger gradient. As you are seeing with your various run time calcs, the difference in the time is negligible or even shorter with 80%. That is why it is a personal decision much like the risks and benefits of all choices made while diving. (PS I am assuming that those calcs were done with 80/20 nitrox and not 80/20 heliox, but i have not worked out the different runs.)

jimmy

 

[jimmy71]

damn i need to type faster, my replys are often a few to late :depressed:

 

[rongoodman]

We talk about partial pressures of dissolved gases, but it's really describing a concentration rather than a literal pressure that's squirting gas out of the tissues. If you have a higher concentration of an inert gas on one side of a a barrier, and a lower concentration on the other, molecules of the gas are constantly moving both ways across the barrier, but the net movement is out rather than in, because there are more of them on the inside to travel outwards. If there are no inerts in the deco gas at all, all the movement is out, and none is being replaced by on-gassing from the deco gas. (Assuming that the inerts off-gassed into the alveoli are breathed away before they have a chance to diffuse back in the other direction. This isn't the case in a rebreather, with its closed loop and no place for the off-gassed inerts to go if the loop isn't manually flushed.)

 

[lowviz]

Okay . . . that's what I thought I said . . . . However, isn't it the magnitude of the pressure gradient that affects the dissolution of the gasses? I am thinking the amount of gas to come out of solution is a constant related to the current pressure of the gas.

Ugh, I hate chemistry.

For gradient, think of motion from high concentration to lower concentration. Or think of it like heat. Flows from hot to cold. More to less.

Absolute pressure directly determines the amount of gas (any gas) that will be dissolved. You are using pressure to force it into solution, no longer a gas, it is dissolved. If it is O2, your tissues can use it pretty fast so it may never become a problem. N2 is always a problem, it has to diffuse back out out your tissues in the dissolved state or become bubbles (bad).

So pressure forces it into solution and the gradient determines where it is going to go.

 

[Jax]

For gradient, think of motion from high concentration to lower concentration. Or think of it like heat. Flows from hot to cold. More to less.

Absolute pressure directly determines the amount of gas (any gas) that will be dissolved. You are using pressure to force it into solution, no longer a gas, it is dissolved. If it is O2, your tissues can use it pretty fast so it may never become a problem. N2 is always a problem, it has to diffuse back out out your tissues in the dissolved state or become bubbles (bad).

So pressure forces it into solution and the gradient determines where it is going to go.

So, I wonder how many pure O2 users understand that; or, do they just do it because everyone else does and "0%" N₂ sounds good?