Dual Chamber Metrology Furnaces
ITL-M-17707: Metrology Furnace for Indium and Tin Fixed-Point Cells
ITL-M-17706: Metrology Furnace for Zinc, Aluminum and Silver Fixed-Point Cells
ITL-M-17705: Metrology Furnace for Aluminum, Silver and Copper Fixed-Point Cells
Using standard platinum resistance thermometers presents many problems due to strain and contamination, which can too easily be introduced into the thermometer during thermal cycling. 156.5985°C. Because such problems have been only partially understood, little published information or equipment is available for the safe-handling of these sophisticated devices.

To complete the apparatus,
a further pre-warming tube (with a temperature
approximately equal to that of the heatpipe)
made of a unique and gas-tight material,
is provided, together with a storage rack
for 4 thermometers.

We feel that enough information exists for us to propose a new apparatus specifically designed, not only to create and maintain the Fixed Points of ITS-90, but also to pre- and post-condition the thermometers to be calibrated. Thus, in our dual-metrology furnaces, we have incorporated a second furnace which, because of its unique design, will safely (and without contamination) pre- and post-condition the thermometers.

Method of operation

The fixed-point cell is melted in an essentially gradient-free heat-pipe.
When melting is complete, the heat-pipe temperature is readjusted to be 0.5°C below the freeze temperature of the cell. 
A cold rod introduced into the cell’s reentrant tube initiates the freeze, to give a plateau that will last between 12 and 24 hours.
The thermometers are removed from their storage rack and placed in the pre-conditioning furnace. 
The furnace is slowly heated to the Cell temperature.
The thermometers are protected from contamination by a slow air flux around them.
One by one the thermometers are transferred into the cell for 20 to 30 minutes for calibration and then back to the conditioning furnace.
When all the thermometers have been calibrated, the conditioning furnace is slowly
cooled back to 400°C where the thermometers can safely be removed into room temperature.