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Dual Furnace Models M17705, N17706 & 17707 Iss.09 – 12/14
NEXT GENERATION CALIBRATION APPARATUS FOR ALUMINIUM, SILVER
AND COPPER FIXED POINTS
INTRODUCTION
Using high temperature thermometers presents many problems due to strain and contamination which can very
easily be induced into the thermometer during temperature 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 Aluminium, Silver and Copper, but (housed in the same facility)
incorporating a second furnace which, because of its design, can be used safely and without contamination, to pre-
heat and anneal the thermometers for calibration.
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
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.
1.
By adjusting the set-point to a few degrees above the appropriate melting point, the cell (aluminium, silver or
copper) is melted in an essentially temperature-gradient-free sodium-filled heat-pipe. When melting is
complete, the heat-pipe temperature is re-adjusted to a level 0.5°C to 1°C below the freeze temperature of
the cell. When this temperature 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-heating furnace. The furnace
is slowly heated to 600°C (if the cell is Aluminium) and 900 or 950°C (if the cell is Silver or Copper).
During conditioning, thermometers are protected from contamination by a slow air flux around them.
3.
Thermometers are transferred individually to the cell for 20 to 30 minutes, for calibration and then returned
to the annealing furnace.
4.
When all the thermometers have been calibrated and have been returned to the annealing furnace, they are
slowly cooled to 450°C at which stage they can safely be exposed to room temperature and, when cool,
measured at the triple point of water. Throughout the time the thermometers are above 450°C
temperature changes are slow, which prevents strain and the flow of air prevents contamination at high
temperatures.
Содержание M17705
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