HIGH TEMPERATURE
FURNACE HANDBOOK
ISSUE 8 - 05/05
Page 25 of 37
Figure 1 shows the design of a cell for realising the liquid-solid equilibrium of pure metal.
The metal is contained in a crucible of purified graphite, with a graphite cover and a graphite
re-entrant sleeve. The crucible is enclosed in an envelope of fused quartz, which extends
into the sleeve interior to form the thermometer well. The cell is charged with a pure metal,
purged and filled with sufficient argon (or another inert gas) to give a pressure of 101kPa (1
standard atmosphere) at the freezing temperature and then sealed. Thus it is at once
protected from contamination and supplied with an inert atmosphere at the required
pressure at the equilibrium temperature. A correction for the effect of change in ambient
pressure on freezing point need not be made. Sealed cells of this type have shown no
measurable change after 15 years of use.
In general, sealed fixed-point cells are used in vertical-tube furnaces which provide good
temperature control and sufficient cell immersion to make axial temperature gradients, in the
measurement zone, negligible. With the cell in the furnace, the controller is first set about
5°C above the anticipated value corresponding to th e melting temperature of the metal in the
cell. The onset of melting is indicated by a cessation of temperature rise because of the
latent heat required to produce the phase change. This melt plateau can last for a
considerable period of time. When melting is complete, the cell temperature will rise to the
furnace temperature.
The furnace temperature is then reduced to a value slightly below the melt temperature. The
temperature falls until the first solid nucleus of metal is formed, at which stage the
temperature drop is arrested. With both liquid and solid metal present in the cell, a constant
temperature is maintained by the latent heat released by the freezing metal. The controller
temperature setting will cause the rate of heat egress from the cell to be relatively low, thus
generating a freeze plateau that can usually be maintained for a number of hours, during
which time thermometers may be calibrated.
A variation on this is the establishment of the triple point of mercury. Since this temperature
is below normal ambient, the apparatus in which the point is realised must provide
refrigeration as well as controlled heat. A separate manual describes the use of this
apparatus.
Another variation is the realisation of the melting point of gallium. This metal is used on the
melt plateau rather than on the freeze plateau. A separate manual describes the use of the
apparatus for realising this fixed point.
A TUTORIAL
There are, unfortunately, no convenient metal freeze points or triple points at the cryogenic
