In article , Mark Rand says...

It can be a tricky one to grasp at first. This is an adiabatic expansion. No
external work is being done and no heat is being added to, or taken from, the
gas. The reason it cools down is because it is doing work on _itself_ during
the expansion.

But in a real world regulator, it will not be quite a true
adiabatic expansion. Some heat will come in from the outside,
so it will want to be isothermal too.

Jim

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