Measurements to determine the EMF versus
temperature relation of Pt/Pd thermocouple in the
temperature range from 25 to 450 C were carried
out in stirred liquid and salt baths. The temperature
of the bath was determined with a 25 X
standard platinum resistance thermometer (SPRT)
calibrated according to ITS-90. A water bath
supplied by Hart Scientific was used from 25 to 95
C, an oil bath from 95 to 270 C, and a salt bath
from 220 to 450 C. All baths were manufactured
by Hart Scientific.
For mixing the bath fluid a stirring propeller
system is used to eliminate both horizontal and
vertical gradients. The baths consist of two stainless
steel tanks. The heater and propeller are fitted
at the outside stainless steel tank. The temperature
controller is connected to a platinum resistance
thermometer as the control sensor. The inside
stainless steel tank has a depth of 60 cm and is
used for thermometer calibrations. The horizontal
and vertical temperature stability of the bath is
±0.015 C.
During measurements in each bath, the thermocouple
was contained in a borosilicate glass tube (7
mm outer diameter and 5 mm inner diameter), 500
mm long closed at the bottom. The thermocouple
was positioned such that it’s measuring. Junction
was at the same immersion as the mid point of the
sensing element of the SPRT. The depth of
immersion was 30 cm below the liquid surface in
the bath. In all baths the temperature difference
between the sensing element of the SPRT and that
of the measuring junction of the thermocouple
differed by not more than 0.01 C, based on previous
measurements of the temperature uniformity
of each bath used.
For evaluation and calibration of the thermocouple
in the temperature range from 660 to 1064
C, the EMF of the thermocouple was measured at
the freezing point of Al and Ag and at the melting
point of gold by using the gold wire method. For
this purpose freezing point sealed cells containing
high purity Al and Ag were used.