Mixing gases in a cylinder for a partial fill

[Steve_C]

at cold enough temperatures gasses will seperate. but you will not see those temps unless your tank is on pluto

 

[Brett Hatch]

If we were to take a tank of air and let it sit for a long time... does the heavier gas (oxygen vs nitrogen) settle in the tank? Requiring the tank to be remixed?

That was basically my question on page 2. The answer is, no. This answer was not intuitive to me -- after all, oil will settle on the top of water for this reason, and it's not like the gas is immune to gravity. So I asked a physicist buddy what was going on here.

His response was that the particles in a gas are too hot to be able to settle in this way. Yes, the gas does see the force of the gravity, but it is not the dominating factor -- these little particles will be zooming around at hundreds of meters per second at room temperature. That's just what gas is, a substance that's too hot to behave like a liquid under the circumstances. If you cooled the gas down to a sufficiently low temperature, it would be able to settle due to gravity. But the temps we're talking about here for nitrogen and oxygen are in the 100~200 Kelvin range, which is approximately negative 75~175C and negative 100~300F, respectively (similar to lowest recorded surface temps on Earth, in the winter in Antartica). I dunno about your shed or local dive shop, but mine just ain't that cold.

 

[BlueTrin]

at cold enough temperatures gasses will seperate. but you will not see those temps unless your tank is on pluto

Get your kit ready

Potential for Life
... Pluto's interior is warmer, however, and some think there could even be an ocean deep inside.

 

[Rich Keller]

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 ?

The instructor was correct. It helps if you view the gases as liquids like mixing oil and water. You want to start with the heaviest gas first then add the lighter gas after. If you pour water through oil the water will go straight to the bottom without much mixing. Pour the oil into water and you get a better mixture. When the tank is full just roll the tank to finish the process. I have done this with 300cu/ft tanks and it works fine.

 

[wedivebc]

Helium does not preferentially "leak" past the valve seat or anywhere else for that matter.

The bailouts I filled with 9/72 three years ago are still full and still 9/72
The helium supply cylinder I bought 4 years ago is also still full

That has been my experience as well

 

[Rich Keller]

If we were to take a tank of air and let it sit for a long time... does the heavier gas (oxygen vs nitrogen) settle in the tank? Requiring the tank to be remixed?
From a Google search...
Nitrogen gas has a molecular formula of N2 and a molecular weight of 28 g/mol. Thus, 1 mole of nitrogen gas weighs 28 g. Similarly, oxygen gas has a molecular formula of O2 and a molecular weightof 32 g/mol. ... This means that there is more oxygen than nitrogen in the same volume of gas.

Nitrogen and oxygen are very close in weight so rolling a tank like that shouldn’t be needed. It is the helium mixes that benefit from tank rolling.

 

[rjack321]

It definitely leaks out of Lola manifolds.

those things have all sorts of issues

 

[PfcAJ]

those things have all sorts of issues

They look cool tho