Fluke 9116A User Manual page 72

9116A Furnace
User's Guide
70
3) Initiating the Freeze and Making Measurements: The freezing curve usu-
ally lasts more than 4 hours and the temperature in the first half of the
freezing curve is usually stable within 0.2 mK or 0.3 mK. If the tempera-
ture of the furnace is closer to the freezing point, a longer freezing point
can be obtained. A freezing curve longer than 10 hours or more is not
difficult to obtain if the temperature of the furnace is carefully
controlled.
The first thermometer to be calibrated should not be preheated. The cold
thermometer enhances the rate of freezing at the beginning of freezing,
i.e. "induces" the freezing.
Take the average of several thermometer resistance or thermocouple volt-
age readings over a period of about 10 minutes. This average is the resis-
tance or voltage at the freezing point of copper R
be calibrated during one freezing curve.
Since a cold thermometer absorbs a large amount of heat which shortens
the freezing curve greatly, subsequent thermometers to be calibrated
should be preheated to a temperature very near the freezing point before
inserting each into the copper cell. Another advantage of preheating is
that the equilibrium time in the cell may be shortened by nearly one-half,
i.e. from about 20 minutes to 10 minutes.
Preheat the thermometers for 20 minutes or so near the freezing point.
Preheating the thermometers for too long is unnecessary and should not
be done. The thermometer sensors could possibly be contaminated if
they remain in metal wells for a long period of time.
4) SPRT Annealing: The rapid cooling from the freezing point of copper to
room temperature introduces extra crystal defects - vacancies in the plati-
num wire of the thermometer - resulting in a noticeable increase in resis-
tance at the triple point of water (R
the equivalent of 30 mK can be observed. An appropriate annealing gets
rid of these defects and returns the R
the thermometer at 700°C for 2 hours in a clean furnace and then cool it
from 700°C to 450°C over 3 hours. An alternative annealing procedure is
to anneal at 970°C for 30 minutes and then cool at a constant rate to
500°C over a period of 4 hours. After annealing the thermometer, take it
out of the furnace and cool it to room temperature in air. Measure the R
and calculate the resistance ratio W
R
=
Cu
W
Cu
R
tp
A thermocouple does not need to be annealed after calibration at the freezing
point of copper.
Several probes can
Cu.
). Sometimes a change larger than
tp
to the equilibrium value. Anneal
tp
:
Cu
tp