Air Tank Covers

[shockman]

Yesterday when I was at CPX one of the guys told me not to use a tank cover because it could ruin the fiber wrapped tanks. Is there any truth to this?

[shartley]

LOL I don't think so. How could it? I would like to hear more of why this "guy" said that.....

[shockman]

He said something about if I didn't have the tank cover on I would get a true fill of 2900 opposed to a fill that my gauges would read as 2300. Also he said something to the effect that expansion/contraction with the cover on would not be permissable. I kept it on anyway and when I went for a fill he said, "Still have that cover on huh?"

I figured it sounded fishy.

[Shirow]

I've been told to take the cover off for a fill, too. I always do anyway, so they can see the hydro date.

[shartley]

I can spin my tank cover from side to side, it does not fit THAT tight. How can a cloth tank cover restrict the tank (made of MUCH harder material) from expanding that small amount? And does the TANK expand, or does the air IN the tank expand?

Sorry, I think this sounds like total BS. I would think that because tank covers CAN move, the person who fills the tanks may want them off so that they have a direct contact with the tank when handling them..... and are using this other stuff to try to justify their "wants" to the public.

I have never taken my tank cover off to have it filled, and will never do so unless there is a REAL reason stated. And none of the places I have had my tank filled have said a word to me about it being on. (and the hydro date would be a good reason, but they can always pull the cover back to see it themselves)

Can anyone offer actual information to support this?

[battlegroup]

The tank does not expand. If the metal and carbon fiber were that flexible for it to expand even a little bit, they wouldn't be able to hold the pressures they do. Take a coat hanger, what happens when you bend it back and forth a couple times? It breaks because of the constant moving. The same would happen here if the tanks expanded. Not as fast as a coat hanger but it would eventaully, especially with 4500psi behind it.
When air is compressed it is heated. The covers, especially neoprene, do not allow the tank to dissapate heat as fast therefore when you get a 3000 psi fill that drops a few minutes after you leave the fill station, it's because the temp went down. That's why they fill scuba tanks in water. To dissapate the heat immediatly while filling. Water has much better heat transfer properties than air.
The equation PV=nRT proves that the temp will affect the pressure. P is pressure, V is the volume of your tank (what it can hold), n is the number of air molecules in the tank, R is a constant, and T is temp. Setting two equations equal to each other, we get PV/nRT = PV/nRT Assuming you don't fire the gun after a fill) the number of mulecules in the tank is the same. The volume of your tank stays the same. And the constant is well.....constant. So rearranging the equation. P/T=P/T Pressure over the fill temperature is equal to the pressure over the cool temp. If the temp is higer when you fill, the Pressure will be higher. Lets assume the pressure of the fill is 3000 and the temp is 100 degrees. 3000/100=30 so 30 = equals P/ambient temp of the day lets say 80 degrees. so 30 * 80 equls real pressure in the tank after letting it cool, or 2400 psi.
I know this is long but it shows a simple 20 degree warming of your tank will drop the pressure significantly. If you are filling your own tank and can wait until the tank colls before you top off after the first fill, keep the cover on. I prefer to have my tank filled slowly to allow for the heat to dissapate. If you sit at the fill station and wait for the pressure to drop they may top it off for you after the tank cools.

[shartley]

Thank you for the post. :)

Geee.... I thought the tanks actually expanded though... like balloons.

On a side note.... this would then apply to ALL tanks and not JUST Fiber Wrapped. The stated reason for it "ruining" the fiber wraped tanks IS BS.

[petefol]

when they hydro a tank dont they fill it over the limit causing it to expand a little?

[shartley]

Originally posted by petefol
when they hydro a tank dont they fill it over the limit causing it to expand a little?

I believe that is true to a point. I think they fill them past what would be "normal" use and to put added stress to the entire tank/setup. I am sure someone more experienced in the process can "expand" on this.....

As for expanding in general... everything expands and contracts to some degree.

[Kevmaster]

yes. in hydro testing they fill the tank to 5/3 capacity while it is in a measured quantity of water. then they measure hte amount of increase in the water's height. this measures the volume that the bottle increased. if its over a certain amount (based on a ratio for inc vol to inital vol) then they fail it, if not, then they pass it


but, yeh, the tank cover thing is crap-o-la. dont worry about him, hes a retard. the only reason you would need to remove the tank cover is so the filler can see the hydro information

[xmetal2001]

Tanks can actually expand slightly when being filled, but the tank cover thing is BS.