FIND: Describe the conditions necessary to establish phase equilibrium points. Identify
the effects of elevation, weather and material purity.
SOLUTION:
Other than the vapor-pressure-temperature points for helium and hydrogen, the fixed
points for ITS-90 are freezing points, melting points or triple points.
triple point - The procedure for calibrating a thermometer at a triple point is:
1. completely freeze the sample (an appropriate mass of material) in a closed
container
2. experimentally determine the energy required to melt the sample
3. re-freeze the sample
4. Add energy and record the thermometer output at 10, 20 40, 60, 70 and 80%
melted. These readings should agree.
This procedure demonstrates that a container capable of preventing contamination of a
sample of material, while allowing the removal and addition of energy, is required to
establish the triple point for a material. Similar requirements are needed for
melting/freezing points, with the notable exception that containers must generally be
flexible, to accommodate thermal expansion. Representative values of measured
temperatures agree to 0.1 mK.
A sample which is 99.9999% pure will produce measured temperatures over a phase
change within 0.1 mK.
Weather and elevation should be eliminated through appropriate design of the
experimental apparatus.