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Heatpipe Furnace Models 17702W, 17702P, 17702S (13
–
07/20)
CALIBRATION APPARATUS FOR INDIUM AND TIN FIXED POINTS
INTRODUCTION
Before calibrating a standard platinum resistance thermometer, it is usual to check its water triple point
resistance value and then to anneal the thermometer. The annealing temperature varies typically between
400 and 700°C.
DESCRIPTION
The new calibration apparatus developed by Isotech not only allows you to create and maintain the ITS 90
freezing points of indium and tin using a water heat-pipe, but also provides a second furnace for annealing
thermometers at temperatures up to 700°C.
Complementary features of the apparatus are a pre-heating tube, maintained at the cell temperature, and a
rack for storage of up to 4 thermometers at approximately ambient temperature.
METHOD OF OPERATION
1.
By adjusting the set-point to a few degrees above the appropriate melting point, the cell, (tin or
indium) is melted in the essentially temperature-gradient-free water-filled heat-pipe. When the cell's
contents are completely melted, the heat-pipe temperature is re-adjusted to a level 0.5°C below the
freeze temperature of the cell. When this set-point has been reached, a cold rod is introduced into
the re-entrant tube of the cell to initiate the freeze, giving a plateau that can be maintained for
between 12 and 24 hours. A modified technique is appropriate for tin, in the form of an applied
thermal shock to the whole cell to counteract the tendency for a slow, large undercool to occur.
2.
The thermometers are removed from their rack and, assuming they have already been annealed
and are stable, are placed successively in the pre-warming tube for 5 minutes and then transferred
to the cell for calibration. After 20 to 30 minutes the thermometer can be removed slowly from the
cell and, after cooling to room temperature, the R
TPW
-value can be checked.
