I don't agree that tanks being filled always get hotter.
I love when all physical evidence contradicting your "science" is ignored!!
You said you had a transfill whip. I'm sure you have a camera somewhere too. Let's see your frosty filling tank.
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I don't agree that tanks being filled always get hotter.
Here is the physical evidence of what happened when Joule did this test (from the Joule expansion page):I love when all physical evidence contradicting your "science" is ignored!!
I learned here that helium has a slightly negative index as compared to an ideal gas and He and hydrogen would heat up slightly if allowed to expand. Those are second order effects as compared to the underlying process explained by assuming the gas is an ideal gas.Joule performed his experiment with air at room temperature which was expanded from a pressure of about 22 bar. Air, under these conditions, is almost an ideal gas, but not quite. As a result the real temperature change will not be exactly zero. With our present knowledge of the thermodynamic properties of air [6] we can calculate that the temperature of the air should drop by about 3 degrees Celsius when the volume is doubled under adiabatic conditions. However, due to the low heat capacity of the air and the high heat capacity of the strong copper containers and the water of the calorimeter, the observed temperature drop is much lower, so Joule found that the temperature change was zero within his measuring accuracy.
I offered to go through the mathematics and physics to show how and why the tanks heat/cool without any compression.
Look. No one here seems to dispute the Wikipedia article on "free expansion."
If this example applied to our scuba tanks you would have one tank holding 10 liters of gas (pressurized donor tank) and a receiving tank holding ZERO liters of gas (because it is evacuated there is no volume of gas in the tank) so your starting volume of gas is 10 liters (10 + 0 = 10) When the partition/valve opens you do in fact end up with expansion of the gas from donor into receiving tank. If you closed the valve/partition as soon as the receiving tank pressure was 14psi/1atm you would then have 20 liters of gas - 10 liters at 2986psi and 10 liters at 14psi. And - you are correct AT THAT POINT IN TIME, AND ONLY IN A NONEXISTANT HYPOTHETICAL SITUATION
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Too bad that's not what's happening when you fill a scuba tank from a compressor or another tank. You have no vacuum receiver to start with and you fill it beyond the brief moment of expansion. You fill it all the way up to about 1500psi (assuming the donor is an AL80 at 3000psi)Here is the physical evidence of what happened when Joule did this test (from the Joule expansion page):.
hate to rain on your parade, but in the close system you are describing, there are two tanks at 10 liters each (10+10=20)
one tank has 3,000psi, the other has 0 psi. You can't magically have a tank appear in a closed system. Because it is a closed system. The tank is there whether it has 0 psi or 14 psi, or 3,000 psi. PhD's from cambridge love to bitch slap people who make foolish assertions. Most (perhaps not all) tank-monkees would bitch slap you for calling them tank monkees...
the second tank is "there" of course, and the exterior of the tank displaces 10 liters of volume/air in the room it is in. however, because in this hypothetical situation the second tank is a perfect vacuum, it has zero volume INSIDE the tank. I can show the the formula, if you like...
:d
PS - as a part time tank monkey myself (+/- education) I used that term with all due respect and affection.
Too bad that's not what's happening when you fill a scuba tank from a compressor or another tank. You have no vacuum receiver to start with and you fill it beyond the brief moment of expansion. You fill it all the way up to about 1500psi (assuming the donor is an AL80 at 3000psi)
Unless you are flying a TARDIS, I can't see the logic of the arguement. Saying it has zero volume in a perfect vacuum and then popping into existance after opening the valve. Following your arguement, the addition of a single molecule of gas would magically make the 10 liters pop into existence. The mass of gas in the empty cylinder would be zero, the energy in the empty cylinder would be zero, but the volume of the cylinder would be zero? what you are describing is the big bang, not a tank fill. Add a little matter and BOOM a 10 liter universe? A quantum SCUBA tank?
Of course all this is relative. From the perspective of the vacuum tank, you add ANY volume of gas, you are increasing the pressure and energy. From the perspective of the donor tank the pressure is always going down. From the perspective of the outside observer the you are shifting gas with in a system with no change in the overall volume or energy of the system... With no external force acting on the components, the system remains the same. If you want to prove your point, draw and post a graph of what is going on in the system, and each tank respectively. You will immediately see why your zero volume arguement doesn't hold up.
It is what's happening if you start with a vacuum empty receiving tank. I've done that with an oxygen tank fill and the expanding oxygen still gets hot. If you start with a tank of residual room temp air, then you have to add in the heat from the residual air that is compressed. I don't disagree with anyone that wants to say that residual air gets compressed and that makes it hot. I agree with that, but it's just some additional heat that gets added in. It doesn't explain why the expanding gas from the donor tank also gets hot.Too bad that's not what's happening when you fill a scuba tank from a compressor or another tank. You have no vacuum receiver to start with and you fill it beyond the brief moment of expansion. You fill it all the way up to about 1500psi (assuming the donor is an AL80 at 3000psi)
If the receiving tank started out empty, there is no gas to be compressed. If the receiving tank gets its gas from the full tank, then according to your argument, shouldn't that gas have gotten colder due to expansion before getting hotter due to compression? By your logic, shouldn't the fact that it expanded first more than it was compressed second mean that it should end up colder, not hotter? Edit:For those interested, that's the answer to question #5 below - extracting energy causes the gas expanding into the second tank to cool.)All of this marginalia aside, let's get back to the original question:
Q: "Why do tanks get hot when you fill them from higher pressure tanks?"
A: "Because the gas in that tank has been compressed*, and the simple, immutable laws of physics tell us that when a gas is compressed its temperature rises."