Thermocouples made from platinum and platinum–
rhodium alloys have been used as a secondary
reference standard throughout the temperature
range from 0 to 1400 C [2]. The type-S thermocouple
(Pt–10%Rh/Pt) was formerly used in
defining the International Temperature Scale in
the range from 630 to 1064 C. It was found that
the Pt–10%Rh/Pt thermocouples are not stable.
This is due to the oxidation, evaporation and
migration of rhodium from the alloy arm of the
thermocouple [3].
In contrast thermocouples constructed from
pure elements do not suffer from such preferential
oxidation problem [4,5]. They are inherently more
thermoelectrically homogeneous and their thermoelectric
stability is not limited by changes in alloy
composition caused by preferential oxidation.
In a previous paper [6] we have carried out a
thorough investigation of Au/Pt thermocouple.
However this combination is limited to temperatures
below 1000 C. This was acceptable since
our main interest was in finding a practical alternative
to the high temperature platinum resistance
thermometer in the range up to 960 C.