Randall Young writes:
>Geo :
>
>The bulb mounted in the Tstat housing contains a very volatile liquid
(which
>I believe is diethyl ether), whose vapor pressure is proportional to it's
>temperature. The gauge is then basically a pressure gauge that reads the
>vapor pressure.
Well Put.
>Since the vapor will condense (giving off considerable
>heat) if it touches a cooler surface, the entire capillary tube (and the
>guts of the gauge) wind up being heated very close to the same temperature
>as the bulb. (This is the same principle used to cool sodium filled
exhaust
>valves.)
<Randall
>59 TR3A
Randall,
With all due respects, I ,ust disagree. The pressure is equal throughout
the system and the temperature effects the pressure due to Boyle's
(I think) law which states PV=nRT. Since the volume is constant and n
and R are constant then pressure is porportional to the weighted average
temperature of the system. I say "weighted" because majority rules here.
Now this system has a bulb located in the cooling system subject to the
coolant temperature. The rest of the system is in ambient temperature
(Much of the capillary tube is subject to under hood ambient and the rest
of the capillary and the gauge is subject to cabin ambient). However the
internal volume of the bulb is significantly greater than that of the cap
tube
and the gauge reducing the influence of the ambient tempature to
insignificance. The fluid doesn't circulate much since the ID of the
capillary
is mighty small.
This is different than sodium filled valves in which the fliud circulates
through the
valve due to the rapid movement of the valve.
Dave (I got an "A" in thermodynamics) Massey
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