Transcript Mac

Experiment 4. The Triple Point
The vacuum
manifold diagramed
schematically in the
ClassPak writeup is
shown here in the same
perspective, with stopcocks labeled the same.
A isolates the manifold
from the pump, while B
controls the vacuum to
the sample cell. C
normally stays shut
throughout the run; it can
be opened to admit air to
the manifold, or to
connect other devices,
like pressure gauges.
P gauge
A
B
C
2
In our current setups, the sample receptacle is fully
enclosed within a larger glass cell, which, under vacuum,
provides good insulation from the surroundings. The
thermistor feeds up from below and is embedded in
special epoxy to achieve good
thermal contact with the glass
and sample.
The first step in the experiment is to carefully evacuate the
system, after which the sample
is “freeze-dried” under vacuum.
It is essential that the sample not
“bump” while it is being
degassed and cooled. If it does,
and water is splashed on the cell
walls, the run is over!
3
In case we should need to use an earlier design of
sample cell, the lower part of that cell is shown
enlarged here. The thermistor probe is mounted
through the knurled plastic nut; the copper wire
surrounding the probe is there to facilitate thermal
equilibration throughout the sample.
Although this design has worked O.K. for years, it
is unfortunately prone to leak air through the O-ring
seal. If your team should work with one of these cells,
we will provide the special instructions needed for its
use.