Abstract :
The performance of a simple, new design Au/Pt thermocouple developed
by NMIA is assessed. This thermocouple is proposed as a more accurate replacement,
over the temperature range from 0 to 1,000◦C, for the commonly used Type R and S
industrial transfer standards, in a robust form familiar to industrial calibration laboratories.
Due to the significantly different thermal expansions of the Au and Pt thermoelements,
reported designs of the Au/Pt thermocouple incorporate a strain-relieving coil
or bridge at the thermocouple junction. As the strain relieving coil ismechanically delicate,
these thermocouples are usually mouInted in a protective quartz tube assembly,
like a standard platinum resistance thermometer (SPRT). Although providing uncertainties
at the mK level, they are more delicate than the commonly used Type R and S
thermocouples. A new and simple design of the Au/Pt thermocouple was developed in
which the differential thermal expansion between Au and Pt is accommodated in the
thermocouple leads, facilitated by a special head design. The resulting thermocouple
has the appearance and robustness of the traditional Type R and S thermocouples,
while retaining stability better than 10mK up to 961◦C. Three thermocouples of this
design were calibrated at fixed points and by comparison to SPRTs in a stirred salt
bath. In order to assess possible impurity migration, strain effects, and mechanical
robustness, sequences of heat treatment up to a total of 500 h together with over 50
thermal cycles from 900◦C to ambient were performed. The effect of these treatments
on the calibration was assessed, demonstrating the sensors to be robust and stable to
better than 10mK. The effects on the measured inhomogeneity of the thermocouple
were assessed using the NMIA thermocouple scanning bath
