Freeze Point Realization; General; How To Realize The Freezing Point Of Copper - Fluke 9116A User Manual

9

Freeze Point Realization

9.1

General

This discussion assumes SPRT calibrations at the copper point. Other freeze
points are similar.
Successful copper point realization requires a cell of the following
specifications:
• The purity of copper: 99.9999%
• The reproducibility: 5 mK
• The expanded uncertainty: 30 mK
• The outer diameter of the cell: 48 mm
†
The expanded uncertainty was evaluated at the level of two standard deviations
(95% confidence)
9.2

How to Realize the Freezing Point of Copper

1) Melting the Cell: Switch on the power to the furnace from the front
panel. The ramp rate of the furnace should not be set too high. Set the
ramp rate of the instrument should be set such that the furnace heats
from ambient (approximately 25°C) to 550°C in no less than 2.5 hours
and an additional 1-1.5 hours to reach 1084°C. The final step from
1084°C to 1090°C should take approximately 20 minutes. The ramp
rates are programmable from the controller. Use a working high temper-
ature platinum resistance thermometer or a Type S (Type R) thermocou-
ple to monitor the temperature in the cell. When the copper sample
begins to melt, the temperature stops rising and remains almost constant
during the melting process. Write down the resistance or EMF indicated
by the working thermometer at the melting point for future reference.
2) As soon as the copper sample is melted completely, set the furnace at a
temperature of about 3°C higher than the freezing point. Maintain a sta-
ble temperature for 20 minutes. Then let the temperature of the furnace
decrease at a rate of 0.2 to 0.3°C per minute until the temperature indi-
cated by the working thermometer stops decreasing and starts to rise.
This indicates that freezing has started. Usually the copper may super
cool by an amount approximately 1°C or more before the start of freez-
ing. Take the working thermometer out of the furnace and put the ther-
mometer or thermocouple to be calibrated into the furnace. Meanwhile,
maintain the temperature of the furnace at a temperature between 0.5 and
1.0°C lower than the freezing point.
9 Freeze Point Realization
†
General
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