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ITS-90 Isothermal Towers Iss.07 – 05/14
LEARNING MORE ABOUT YOUR ISOTOWER
1.
Try adjusting the controller temperature between melts by 0.1 or 0.2°C. See the melts get longer and shorter.
Select the setting that gives you the length of plateau that suits you (I like about 15 hours so that I can
have a new plateau each day).
2.
Check your furnace profile each time you use your Isotower. After the Isotower settles down 2°C below its
melt, raise the monitor 100mm (4”) and check the temperature is the same or increases slightly. This takes
only 10 minutes and checks for any drift on the Immersion Compensation Controller.
3.
When calibrating, leave the Unit Under Test (UUT) in the Isotower long enough for the system to fully
restabilise – see notes below.
We recommend pre-warming the UUT in all cases.
Note: How the thermal profile along the cell can affect or even destroy it
The easiest cell to describe is the Water Triple Point because the container, being made of glass, is transparent, so the
effects can be seen.
As cold rods are inserted into the cell to create an ice mantle an ice bridge forms at the surface of the water in the
cell. If this is not immediately re-melted then further cooling and ice formation expands the volume beneath the ice
bridge and the cell will implode.
This is because ice is 8% less dense than water, for example in a Isotech B11/50 cell the water level increases 12mm
as the ice increases from 0 to 50%.
Although we cannot see inside other fixed point cells, during melting, a similar process is occurring.
For example the metal inside a Slim Aluminium Cell will expand 7.2% and the level of aluminium will increase by
12.6mm as the cell melts.
The cells are heated in furnaces which will be either 3 Zone or Heatpipe furnaces.
It should be obvious from the above that the cell should either have zero temperature gradient along its length or the
top of the cell should be hotter than the rest of the cell.
If the cell melts from the bottom upwards the expanding liquid metal will rupture the graphite crucible.
