On Mon, 21 Jul 2003, Randall Clague wrote:
> >Be careful- this is true for nitrogen, but helium can actually get
> >hotter when going through a regulator (helium is usually above its
> >Joule-Thompson inversion temperature, 40 K).
>
> OK, I'm sure I'm not the only one here who doesn't follow this. But
> I'll be the first to admit my ignorance. Huh?
The Joule-Thompson coefficient, which determines *how much* cooling you
get when a gas expands through an orifice, is not a constant, even for a
particular gas. In particular, it is a function of temperature. Normally
there is a temperature at which it changes sign, the Joule-Thompson
inversion temperature. Above that temperature, the gas gets hotter, not
cooler, when expanded through an orifice.
(Note that expansion through an orifice is *not* the same as expansion
against a piston in a heat engine. Since the latter does work on the
piston, *it* always cools the gas, but the former is more complicated.)
The JTIT is high enough for most gases that we're used to thinking of
expansion through an orifice as always cooling them. However, helium
and hydrogen are exceptions to a lot of rules of thumb about gases,
including that one.
Henry Spencer
[EMAIL PROTECTED]
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