Old air in tanks.

[Centrals]

I know this is off topic.
Mixing of non reactive gases is NOT a chemical reaction. The chemical and physical properties of the individual gas molecule is the same before, during and after the mixing. Therefore no equilibrium is disturbed so I don't see any change of entropy.
If I still have access to GC/MS, I will mix He with CO2 in a vessel, partially evacuated it so there is more empty space for the gas molecules to move about. Let it stand for few days and analysed the gas content of the top layer.The gravitational pull might separate the two gases because of the large weight differences. CO2 is 22 times heavier than He.

 

[Storker]

The chemical and physical properties of the individual gas molecule is the same before, during and after the mixing. Therefore no equilibrium is disturbed so I don't see any change of entropy.

I'm going to assume that you really meant that there won't be any change in the system's enthalpy. Because if you take two pure compounds that don't interact chemically and mix them, you'll have a significant increase in the system's entropy. That comes from the basic definition of entropy.

I will mix He with CO2 in a vessel, partially evacuated it so there is more empty space for the gas molecules to move about. Let it stand for few days and analysed the gas content of the top layer.The gravitational pull might separate the two gases because of the large weight differences.

It won't. But by all means, do the experiment.

BTW, here's a nice video showing that heavy molecules (in this case Br2, which has a molecular mass of 160, more than three and a half of that of CO2) certainly diffuses upwards in spite of the gravitational pull:

 

[dberry]

I know this is off topic.
Therefore no equilibrium is disturbed so I don't see any change of entropy.

Whether you see it or not, mixing of gases increases the entropy. Either you meant enthalpy, or you don't understand what entropy is.

If I still have access to GC/MS, I will mix He with CO2 in a vessel, partially evacuated it so there is more empty space for the gas molecules to move about.

This isn't the first time you've mentioned "empty space" for the molecules to move about, and it reveals another fundamental misunderstanding. Gas molecules are really small compared to the volume of a container, except under VERY high pressures. At 1 bar the container is already >99.9% "empty space". That's why gases are easily compressed, whereas liquids and solid are not.

Let it stand for few days and analysed the gas content of the top layer.The gravitational pull might separate the two gases because of the large weight differences. CO2 is 22 times heavier than He.

Nope. This is exactly the sort of situation we pose as a problem in class. Unless your container is REALLY tall, you won't detect a difference in gas composition between the top and bottom. And not that it matters, but CO2 is only 11x more massive than He.

 

[Peter69_56]

Cycling through those 30b is not going to kill a tank in a few years imo. And if it was stored inside, it hardly felt any change.

If it did kill tanks we would all have dead tanks in OZ seeing we cycle all our tanks by their test pressure yearly. Not that I think doing that is a good thing, personally I think the less you significantly cycle tanks to their test pressure (overpressurise by a significant amount) the better so say 3 or 5 yearly is ok (but thats a post thats already been done to death here a number of times). My point being I agree, cycling a tank by 5-7% is not a big deal unless cycling it quickly continually over long periods, and even then if a steel it will cope well.

It might have been this site where I saw a post about 50 litre steel storage tanks that had a series of boxes stamped on the neck. What they were, were German submarine air cylinders that had their swaztika emblem overstamped to cover it up. So in service since 1945 and still passing test.

It also reminds me of some of the wrecks I have seen that still have steel cylinders on board and presumably full. I always wonder how long they will last until they finally explode from corrosion. 70 years under the sea is not bad.

 

[Peter69_56]

I'm going to assume that you really meant that there won't be any change in the system's enthalpy. Because if you take two pure compounds that don't interact chemically and mix them, you'll have a significant increase in the system's entropy. That comes from the basic definition of entropy.


It won't. But by all means, do the experiment.

BTW, here's a nice video showing that heavy molecules (in this case Br2, which has a molecular mass of 160, more than three and a half of that of CO2) certainly diffuses upwards in spite of the gravitational pull:

I am struggling to remember to breath in breath out while underwater, let alone soak up and understand all the chemistry wizzing about here

 

[dberry]

Guilty as charged. But it's tough to let some statements pass unchallenged. This escalated faster than most threads. The funny thing is this got rolling because of a joking comment about shaking tanks with "stale" gas.

 

[RayfromTX]

This off topic detour is probably the best thing that could have happened to this thread short of falling off at a post count of perhaps 2. No offense intended to the OP. I've enjoyed the discussion.

 

[kelemvor]

I have had a tank with nitrox in it for over a year that tested considerably lower on o2 than when it was filled. I have no idea of what exactly happened to the o2 but given that it would have worried me during the dive at a minimum I chose to dump it and pay the $12 for another nitrox fill. Since it had been sitting so long it also needed a VIP, which showed no signs of rust or anything else wrong.

So, personally, I'll test any tank that's been sitting for a long time. Probably 3 months or longer. If I don't get back close to what I expect, I think I'd dump it and refill. Even if it really was safe to use, it's just not worth the worry that would be in the back of my mind to save $12. If you live somewhere that nitrox is more expensive (I think I paid $25 for a nitrox fill in the keys once) it might be a tougher decision.

 

[Peter69_56]

I have had a tank with nitrox in it for over a year that tested considerably lower on o2 than when it was filled. I have no idea of what exactly happened to the o2 but given that it would have worried me during the dive at a minimum I chose to dump it and pay the $12 for another nitrox fill. Since it had been sitting so long it also needed a VIP, which showed no signs of rust or anything else wrong.

So, personally, I'll test any tank that's been sitting for a long time. Probably 3 months or longer. If I don't get back close to what I expect, I think I'd dump it and refill. Even if it really was safe to use, it's just not worth the worry that would be in the back of my mind to save $12. If you live somewhere that nitrox is more expensive (I think I paid $25 for a nitrox fill in the keys once) it might be a tougher decision.

Maybe when yours was filled it hadnt mixed properly prior to first measurement. I have had 50Nx read 80 until I roll them and 80Nx read 50. I still dont understand how it happens and its not just a bit trapped in the valve that causes it as I have also blown through the valve before measuring. I demonstrated this to a friend not long ago as he didnt believe it could happen either.

Once mixed correctly I have then never had a tank read anything other than the original measurement.

 

[Storker]

I still dont understand how it happens and its not just a bit trapped in the valve that causes it as I have also blown through the valve before measuring. I demonstrated this to a friend not long ago as he didnt believe it could happen either.

Once mixed correctly I have then never had a tank read anything other than the original measurement.

I guess you're talking about pp blending here. It's a bit like when you mix yourself a G&T by filling the glass half full with ice cubes and tonic water, splashing some gin on top and immediately taking a sip. The gin is sitting on the top with only a little tonic water mixed in, so the sip won't be properly blended. Stir it with a spoon or let it sit long enough for the diffusion to do its job, and it'll be very nice. Except if you decide to wait for the diffusion, all the ice will have melted and your nice G&T will be lukewarm.

The O2 you put on top of the air behaves basically like the gin. And the valve is at the end of the tank, so your gas isn't a mixture from all places inside the tank. At high pressures, the viscosity of gas is rather high, and diffusion is rather slow. But the mixing due to handling of the tank after the fill, and the time for diffusion will do their job, and when you have the same mix everywhere in the tank the gases won't separate again.