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Low Temp Furnace Model ITL-M-17701 13-07/20
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 Defining Fixed Points and Related Data 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.
