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Dual Furnace Models M17705, N17706 & 17707 Iss.09 – 12/14
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.
There are, unfortunately, no convenient metal freeze points or triple points at the cryogenic end of the Scale. The
defining point applicable to long stem thermometers at the low end of their useful range is the triple point of argon.
In practice, the difficulties of setting up conditions to facilitate this measurement can conveniently be circumvented by
carrying out the alternative procedure of comparison calibration, in which the thermometer is compared, in an
environment of boiling nitrogen, to a similar thermometer which possesses a calibration traceable to national
standards. A separate manual describes the nitrogen boiling point apparatus.
The temperature at which the change of phase occurs at atmospheric pressure is specific to the upper, exposed,
surface of the metal. However, it is not feasible (because of the temperature gradient in this locality of the
thermometer well) to obtain an accurate measurement under this condition. The closest approach to temperature
uniformity demands insertion of the thermometer to the foot of the well with the consequence that the change-of-
phase temperature measured is influenced by the static pressure head of the column of metal above the effective
level of the thermometer sensing element.
Corrections that are used to enable measured phase-change temperatures to be converted to values that would
pertain at 1 standard atmosphere pressure, for the various metals (and for mercury and water at their triple points),
are given in the table. For a given column height (of the order of 200mm for Isotech sealed freeze point cells), the
correction will be proportional to metal density and to a coefficient expressing the sensitivity to pressure of the
phase-change temperature. The sign of this coefficient will depend on whether the metal contracts or expands on
freezing.
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