Has it ever dawned on you that you're the only one arguing that the gas in a tank being filled is experiencing "expansion"?
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 least as I interpret his comments and the non-Wikipedia link to "free expansion" calls it expansion.
There is no "heat of expansion"
We agree. That's why I asked the question. How can expanding gas heat up? I thought expanding gas cools down. It doesn't. If you add or remove zero energy, it just expands at constant temperature. I didn't know that when I started. Now I do. You still have to explain how the expanding gas gets hot, but it's a lot easier if you know that all the gas taken together shouldn't change temperature.
It's easy to see why we come down on opposite sides. It explains why this bothered me for years and it didn't bother you. I believe that gas starting out in one high pressure tank is expanding when it's released to fill two tanks at lower pressure. I believed that expanding gas always cools, so I saw an inconsistency. You believe that the gas entering the second tank is being compressed and it's being heated by compression, while the gas leaving the first tank is expanding/cooling and that fits naturally with your belief that expansion is cooling and compression is heating.
I respect your position. You, and others, believe it in good faith, but you are wrong. Gas that starts in one tank and ends up in two tanks and twice the volume is expanding. There's no way to call that compression. Joule called it expansion and it was named for him - Joule expansion.
If I accepted that gas expanding from one tank into two tanks is actually undergoing "compression" there could be no such thing as expansion. As soon as you make the container even slightly bigger, a first one or two gas molecules would enter that new volume, then all the rest of the gas would rush in and "compress" the gas in the new volume. It just doesn't work that way. You look at the starting volume and the final volume of all the gas and if the total volume has increased, it's called expansion. It
is expansion.