Experiment 4. The Triple Point

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Experiment 4. The Triple Point
The vacuum manifold diagramed schematically in
the ClassPak writeup is shown here in the same
perspective, with stop-cocks 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
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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 experi-ment 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!
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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.