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Dual Furnace Models M17705, N17706 & 17707 Iss.09 – 12/14
NEXT GENERATION CALIBRATION APPARATUS FOR ZINC, ALUMINIUM
AND SILVER FIXED POINTS
INTRODUCTION
Using high temperature thermometers presents many problems due to strain and contamination, which can very
easily be introduced into the thermometer during thermal cycling.
Limited understanding of the relevant mechanisms has resulted in a dearth of published information and of available
apparatus for the safe treatment of these sophisticated devices.
DESCRIPTION
Sufficient information now exists at Isotech to allow introduction of a new apparatus, specifically designed not only to
realise and maintain the ITS 90 fixed points of Zinc, Aluminium and Silver, but also to pre- and post-condition the
thermometers to be calibrated. To this end a second furnace has been incorporated, which, because of its design,
will permit conditioning to be carried out safely and without introducing contaminants into thermometers.
Complementary features of the apparatus are a pre-calibration tube (held at the temperature of the fixed point)
made of a unique and gas tight material, together with a rack for storage of up to 4 thermometers at approximately
ambient temperature.
METHOD OF OPERATION
1. By adjusting set-point to a few degrees above the appropriate melting point, the cell (zinc, aluminium or
silver) is melted in an essentially temperature gradient free potassium-filled heat-pipe. When melting is
complete, the heat-pipe temperature is readjusted 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.
2. The thermometers are removed from their storage rack and placed in the pre-conditioning furnace. The
furnace is slowly heated to 400°C (if the cell is Zinc), 650°C (for Aluminium) or 900°C (for Silver).
During conditioning, thermometers are protected from contamination by a slow air flux around them.
CAUTION: it is essential, when thermometers are inserted into furnaces, to keep their heads cool. This can
be accomplished by blowing air, at ambient temperature, across them by means of a free-standing fan.
3. Thermometers are transferred individually to the cell for 20 to 30 minutes, for calibration, and then returned
to the post-conditioning furnace.
4. When all the thermometers have been calibrated, the post-annealing furnace is slowly cooled to 450°C after
which they can safely be exposed to room temperature and, thereafter, measured at the triple point of
water.
Throughout the time the thermometers are above 450°C the temperature changes are slow enough to
prevent strain, and the slow flow of air prevents contamination at high temperatures.
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