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ITS-90 Isothermal Towers Iss.07 – 05/14
If the top of the cell is colder than the rest of the cell then on melting then as the metal expands from the bottom
upwards there will not be space for the expansion and the graphite crucible will rupture.
See illustration below.
It is vitally important that the top of the cell is at the same temperature or slightly hotter than the rest of the cell to
stop any pressure building up within the cell
The most common cause for the top of the cell to be colder is due to thermometers being placed in the Isotower
conducting heat out of the cell.
Test thermometers with larger thermal mass such as Metal Clad Thermometers or a continuous throughput of
thermometers being calibrated will cool the top of the cell and freeze the metal.
To avoid this it is important to pre-warm the thermometers being inserted close to, or ideally above, the cell
temperature before inserting it into the Isotower
Melting from the bottom upwards blocks
room for expansion and can rupture cell
Keeping the top at or above the temperature at
the bottom allow for expansion without pressure
