Mixing gases in a cylinder for a partial fill

[BlueTrin]

Some time ago, I saw an instructor roll a cylinder on the ground.

When I asked why the instructor was doing this, the instructor said that he checked quickly the gas contents and they got two very different readings when the measured it (apparently the first reading looked wrong so they read again immediately).

The instructor said that the gas wasn't probably mixed (this shop is doing partial fills), so he was trying to help it getting mixed.

This seemed a bit strange to me because I thought that with gases, as long as there would be some movement inside the cylinder, I thought that the gas would start mixing and stay that way (I am not good at physics so correct me if wrong).

Was the instructor correct or is it some kind of urban myth ?

 

[michael-fisch]

My experiences with getting a homogenius mix after filling are:
1/2 hour after a fill > forget it
24 hours after laying a set of doubles down > homogenius mix
48+ hours after leaving freshly filled doubles standing > homogenius mix
Load freshly filled doubles in your car and go for a 1/2 hour drive > homogenius mix
I assume a half hour boat drive will give similar results.

Given enough time Brownian motion will result in a homogenius mix, but the question remains what is enough time.
Just for shits and giggles I once put O2 and Air in the left tank of an empty set of doubles with the manifold closed, then put the helium in the right tank. Pressure in both tanks was the same. I then opened the manifold and let the tanks sit there. Each week for the next month, I'd close the manifold and analyse each tank, then reopen the manifold for another week. Even after a month I had not gotten to a homogenius mix, nor did it become homogenius after 2x 280km drives laying down in my car.

Michael

 

[BlueTrin]

My experiences with getting a homogenius mix after filling are:
1/2 hour after a fill > forget it
24 hours after laying a set of doubles down > homogenius mix
48+ hours after leaving freshly filled doubles standing > homogenius mix
Load freshly filled doubles in your car and go for a 1/2 hour drive > homogenius mix
I assume a half hour boat drive will give similar results.

Given enough time Brownian motion will result in a homogenius mix, but the question remains what is enough time.
Just for shits and giggles I once put O2 and Air in the left tank of an empty set of doubles with the manifold closed, then put the helium in the right tank. Pressure in both tanks was the same. I then opened the manifold and let the tanks sit there. Each week for the next month, I'd close the manifold and analyse each tank, then reopen the manifold for another week. Even after a month I had not gotten to a homogenius mix, nor did it become homogenius after 2x 280km drives laying down in my car.

Michael

When you say 1/2 after a fill, do you mean in twins or any cylinder ?

In my case, it was a single stage cylinder.

 

[michael-fisch]

When you say 1/2 after a fill, do you mean in twins or any cylinder ?

In my case, it was a single stage cylinder.

My experiments were with doubles, but when I started filling 3liter 300 bar diluent tanks after 24hrs lying down they were homogenius. Since I'd already determined that nothing useful (using doubles and Al80 Stages) could be shown after a half hour, I did not test untill the next day.

Michael

 

[Graeme Fraser]

Good luck rolling twins .

We partial pressure fill, so I rock and roll stages after a fill to get a more accurate reading. When topping off with air I also do a couple of quick blasts half way through to stir the blend up a bit. Obviously still analyse prior to the dive to ensure I'm diving the the correct mix and it's had a chance to fully homogenise.

 

[RyanT]

I just give them a short drive in the back of my truck. That seems to do better than just about anything I've seen for mixing them. It also gets around that pesky problem that @Graeme Fraser mentioned above!

 

[Superlyte27]

I'm ashamed to say that for decades mocked people rolling tanks. No way that works

Then a few years ago we had 8 tanks PP mixed for 32%. I rolled 4 and didn't roll four. After 5 minutes the 4 that we rolled were perfect. The 4 that didn't get rolled were way off. I haven't mocked anyone since. Lol. Well, at least for this.

 

[BlueTrin]

I'm ashamed to say that for decades mocked people rolling tanks. No way that works

Then a few years ago we had 8 tanks PP mixed for 32%. I rolled 4 and didn't roll four. After 5 minutes the 4 that we rolled were perfect. The 4 that didn't get rolled were way off. I haven't mocked anyone since. Lol. Well, at least for this.

Well I am glad I asked ...

How come the movement from the fill is not enough to start mixing the gases ?

 

[Graeme Fraser]

You should be filling very slowly for safety and to keep the temp down. I usually fill at a rate of 4bar per minute for O2 and 6bar for air. A by product of this is that the gases won't initially mix that much. Hence the reason I give a couple of short bursts during top-off.

 

[Jack Hammer]

I recall a thread on TDS where people were tossing around the science of how gasses behave and spinning tanks being a voodoo waste of time. The people blending were essentially saying, "we dont get how or why spinning tanks works, but it works". When I started mixing I found I would get wonky readings until i spun my tanks too.

ps: piano dolly with a strap or a mechanics creeper works great for doubles.