Mixing Nitrox with a Partial pressure system

[Dive-aholic]

What gas law says that diffusion is instantaneous? If merely dumping gasses together immediately resulted in a homegenous mix, there would be no need for the baffles in a "stick" system.

I'm not saying a reading should ever be 6% out, but it will take some amount of time for the mix to settle.

Where did I say diffusion is instantaneous?? I said there's no need for rolling. Gas molecules are not like marbles. They're not going to mix just because you roll a tank on the ground. Yes, time is your friend in blending. You seem to have assumed a lot from my post.

 

[Hickdive]

But you're in Scotland. Everything moves at a more relaxed pace there. One reason I'd so love to visit. Just one dive in Loch Ness is all I ask. Just the chance to see would be worth it.

Hmm, I've 'dived' Loch Ness. In the diver lockout chamber of a mini-sub formerly used in the North Sea oilfields.

It was quite an experience but there was very little to see and the water was the colour of tea with no ambient light.

 

[NAM001]

I have never seen any "layering" in a nitrox fill. You would have to fill real slow before teh o2 falls out of the dip tube. as it is the fill rate sprays it out the dip tube and mixes it. If you did fill o2 at say 5psi / min the sir top off will stir it up. I cant speek for trimix fills.

 

[Searcaigh]

Hmm, I've 'dived' Loch Ness. In the diver lockout chamber of a mini-sub formerly used in the North Sea oilfields.

It was quite an experience but there was very little to see and the water was the colour of tea with no ambient light.

Hi Hickdive,

Was it this one by any chance?

 

[Doppler]

... At the high pressures of a scuba tank, the gas acts much like a fluid...

Gas is a fluid... all the time... and behaves as such.

As pressure and temperature inside a pressure vessel increases, the VISCOSITY of the gas(es) being filled (or emptied) changes... gas viscosity increases as temperature rises (the opposite of a liquid... another fluid). Pressure also increases the viscosity of a gas -- unlike a liquid which remains unchanged in practical terms -- therefore increasing its resistance to flow as well.

Gases will expand to fill the vessel it's being pushed into in a uniform manner, always: however, when internal pressures get above, I think it's either 100 bar or 80 bar but don't have a reference handy, the time that mixing takes increases... a little. Rolling does bugger all. It simply passes time.

 

[wedivebc]

To those who say rolling does nothing I say I didn't believe it until I saw it with my own eyes. I have seen it with high O2 nitrox blends and with trimix. We recently did a experiment during a gas blenders course where we blended 2 identical sized tanks of 50% and I had the student leve the room while I stood one tank on its valve for a few seconds. When he came back he analyzed both tanks and one was 50% and one was 35%. As soon as I tipped the other tank it came up to expected reading.
I am no scientist I just knows what I sees.

 

[Dive-aholic]

I'm with Dave. Actually, here's a better one. I did a blending class this year and we were mixing trimix. We had the analyzer hooked up and the readings were coming back way off. I saw the calculations, confirmed them, and watched the mixing process. The mix shouldn't have been off that much unless the supply tanks were off. Well, we took the tanks and laid them down on their sides and watched the analyzer read out change immediately. Set them back up and watched the read out change again. This happened instantly. While rolling may not do much to "mix" the gasses, the gasses will not mix immediately when partial pressure blended and changing the orientation of the tanks will affect the analyzer reading. This probably happens more with helium mixes than nitrox. It was pretty cool to watch, though.

 

[PfcAJ]

To those who say rolling does nothing I say I didn't believe it until I saw it with my own eyes. I have seen it with high O2 nitrox blends and with trimix. We recently did a experiment during a gas blenders course where we blended 2 identical sized tanks of 50% and I had the student leve the room while I stood one tank on its valve for a few seconds. When he came back he analyzed both tanks and one was 50% and one was 35%. As soon as I tipped the other tank it came up to expected reading.
I am no scientist I just knows what I sees.

I thought it was a bunch of baloney, too.

Watched my doubles get filled with all the right stuff to make 18/45. Analysis wasn't 18/45. Scoffed at the notion to flip 'em over a few times, but did it anyways. Analysis read 18/45. Interest piqued. I considered the possibility of residual gas in the diptube/valves as being the culprit (because anything sounded better than the need to roll tanks. After all, rolling tanks is BS, right?). Filled buddy's tanks, cracked the valve and bled out some gas, then tested again. Not 18/45. Flipped tanks over a few times and I'll be damned if it didn't come out to 18/45!

Moral of the story: Its a real thing.

 

[waterpirate]

I continuous blend nitrox and dump it onto helium to get lean mixes. Immediately folowing that proccess the mix is generally off. I choose to test again in the morning, as my back ain't what it once was. Mix is allways acceptable in the morning. Is it a factor of time rather than agitation??
Eric

 

[Hickdive]

Hi Hickdive,

Was it this one by any chance?

It doesn't look familiar.