HIGH TEMPERATURE
FURNACE HANDBOOK
ISSUE 8 - 05/05
Page 15 of 37
Depending upon factors such as furnace control and the number of thermometers
successively loaded into the cell, plateau durations between an hour and many hours may
be achieved. Thermometers may be preheated prior to transfer to the cell. A pocket is
provided in the furnace for this purpose. It is wise to ascertain from time to time that the
plateau is still in existence, by checking the cell temperature with the monitoring
thermometer at intervals within the measurement sequence. Let us suppose that Th
m
is the
monitoring thermometer and Th
1
...
n
are thermometers to be calibrated.
If n=2 a suitable sequence might be:-
Th
m
, Th
1
, Th
m
, Th
2
, Th
m
and if n=4
Th
m
, Th
1
, Th
2
, Th
m
, Th
3
, Th
4
, Th
m
The Th
m
measured last should be equal in indication to that of the Th
m
measured first to
ensure that the plateau has been present for Th
1
, etc.
At the temperatures of Indium, Lead and Zinc it is generally permissible to withdraw a
thermometer, of type Isotech 909, directly into room temperature. After exposure to
Aluminium and Silver Point temperatures, slow cooling is vital to ensure that thermometer
characteristics remain unaltered.
A USEFUL HINT
When first creating freezes use large under settings - typically 3 to 5°C BELOW the freeze
plateau. The result will be a shorter freeze time than ideal, but will engender confidence in
establishing a plateau. Once familiar with the procedure using coarse settings, on
subsequent exercises bring the setting of the controller closer to the known freeze
temperature to increase the plateau length.
There follows later a tutorial about Fixed Points.
