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I dropped back to see where this thread had gone. I'm not here to restart a flame war, but I did ask the original question, and I worked hard to understand the answers given. Windapp's answer quoted above and his other answers are the best I saw. I agree with all he wrote. The only...

I'd say it's more - 1) I have questions 2) Answers, some good, some very bad, none complete 3) Research leads to more complete answers. 4) More complete answers presented to see whether others agree. 5) Most others don't agree. 6) Careful explanation of why answers seem to be more complete...

So make it 0.01 seconds, or open the valve a lot less :) Yeah - good point. I can see you have a good feel for reality. Ray, thanks for this. I think you're wrong, too, but no more wrong than I was, and you seem like a decent guy, too. I do think I understand it now, but I also recognize...

I still think it's cool that the fundamental reason a filled tank gets hot is different when filled from the compressor directly (where the compressor adds energy to the gas) and when the tank is filled from another tank (where no energy is added). I'm going to add one more detail to the...

No I am not. I'm no scientist - just a long term recreational shallow water diver. But, I had asked this question (several times) before and I settled every time for the claim that gas was being compressed and that was the source of the heating. Before coming here to ask the experts I thought...

If I close the fill valve and stop the process of expansion into the "vacuum empty" tank after 10 seconds, the gas will be hotter in that tank. We now have gas in the originally vacuum empty tank, just like the gas in a not-vacuum empty scuba tank. Now I reopen the valve and let the process...

Yes, I've thought of it. Except that Wikipedia calls it expansion. Joule called it expansion - that type of expansion was named for the guy - Joule expansion - and his name was given to the fundamental units of energy, so he must have known something about it. Doppler called it expansion, at...

You are the one arguing that expanding gas always cools off. I'm the one arguing it doesn't. The expanding gas in the receiving tank heats up. We're discussing why. If you don't think expanding gas always cools, then your original argument only makes sense if you still believe that gas...

At least this is a new way to express the same argument - graphically. It's creative thinking like this that makes it worthwhile. You are making the residual air argument by leaving air in the second tank. I agree the residual air in that second tank is being compressed. The escaping air is...

Doppler gave the same answers I did - I noticed you didn't try. He was the only one here who has even tried. (I'm not saying Doppler agrees with me, but he at least understands the process well enough to give the right answers. He claims to have taught physics and it appears to me at least...

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...

I offered to go through the mathematics and physics to show how and why the tanks heat/cool without any compression. No one asked to see that, but eventually someone may read this thread and wonder if I could show what's happening mathematically. I thought I'd at least lay out the basic...

Thank you for the link. It does add to my understanding. :) The equations cover an "isothermal" process as you say - i.e constant temperature. The "Joule expansion" or "free expansion" process described in my link above is also an isothermal process. No energy is added or extracted during...

A compressor is provided with energy by plugging it into the wall, or by using muscle energy to press down on the plunger. The added energy appears as heat and increased pressure. The heated air flows into the tank and remains hot. That entire process involves adding energy to the gas. When...

Filling the empty tank by letting the high pressure air in the donor tank expand into the empty receiving tank. (As before, I continue to agree with the other part of your post).

The equations don't care how quickly the process is done, only whether we add any energy. There are no "rate of change" terms in the gas laws. We are taking the gas in one tank and letting it expand into two. No, We have gas that starts at 3000 psi and let it drop pressure as it expands...

I keep getting hammered on. Why not hammer on Doppler? He's got a clue. That's the post I would point to to explain what's happening. @Doppler: Sorry if that tars you with what seems to be my bad boy/stubborn rep here. I've really tried to be polite and just figure out the answers...

We agree. The entire "system," meaning all the gas, gets no energy added, and the reason the empty tank gas gets hot is that energy is lost from the full tank and that energy is gained by the gas going into the empty tank. I agree 100%. If you let the two tanks freely mix, since no energy was...

This would be so easy if this were true. I weld a vacuum empty tank onto the side of a high pressure tank and drill a hole between them. The gas in the high pressure tank rushes into the vacuum empty tank and the pressure equalizes as the volume doubles. The gas in the high pressure tank...

I do believe it has energy, but the sum of the energy in the scuba tank plus the energy in the donor tank is unchanged. That's the point. We don't add any energy to the gas and some gets hot and some gets cold.