Thermocouple calibrations

Until recently, calibrations of noble metal thermocouples involved measurements at the gold wire point (1064 °C), the copper fixed point (1084 °C), and the palladium wire point (1554 °C).

The wire point involves breaking the hot junction of the thermocouple, welding a short piece of Au or Pd wire to form a 'bridge' across the two elements, then melting it. This provides the high temperature calibration point of the thermocouple. Due to the large difference between the Au or Cu point and the Pd point, and because of the intrinsic uncertainty of the Pd wire point, thermocouple calibration uncertainties are generally quite high above 1084 °C.

This situation is set to change. NPL is involved in an extensive program of research and development with the goal of implementing high temperature fixed-points including Co-C and Pd-C into the thermocouple calibration chain. These will supplement and in the near future supplant current methods of thermocouple calibration. This should have the effect of reducing calibration uncertainties by at least a factor of two. This is already happening - a joint industrial project between NPL and two UK companies has demonstrated a real reduction in uncertainty in a process control application by a factor of three.

A summary of recent developments in high temperature contact thermometry is available here ( PDF 531 KB).

Platinum/palladium thermocouples

The ultra-precise platinum/palladium (Pt/Pd) thermocouple makes a natural partner for metal-carbon eutectic fixed points. Noble metal thermocouples (type S, R or B) performance is limited due to the alloyed thermocouple leg (made of Pt/Rh-alloy). However, before the advent of metal-carbon eutectic fixed points, the calibration of Pt/Pd thermocouples was limited to the copper fixed point (1084 °C) because the Pd wire point was not accessible for this couple. Since calibrations have begun with the Co-C fixed point, it has become clear that its high stability and high homogeneity makes it an ideal candidate for scale dissemination, with stability of better than 50 mK and homogeneity to match. Despite their superior characteristics in comparison with the platinum-rhodium/platinum (Pt-Rh/Pt) thermocouples, Pt/Pd thermocouples have until recently been a tool of high-level metrologists because of their fragility. Recent developments at NPL have overcome these developments and these thermocouples are now becoming a robust, high precision tool for industry.

NPL scientists are currently involved with several projects involving development and implementation of Pt/Pd thermocouples:

• Euromet 857 project involving NPL, LNE (France), PTB (Germany), and NMIJ (Japan)
• National Measurement System development work
• Joint Industrial Projects
• EMRP project 'Metrofission'

New services

While these applications are current the subject of vigorous research and development, NPL is developing a series of new services in this area to meet growing customer demands. We are now able to supply:

• Reliable ultra-low uncertainty Pt/Pd thermocouples calibrated to 1500 °C
• UKAS-accredited calibration of noble metal thermocouple types with uncertainties of 0 °C up to 1600  °C

In addition, we provide a free technical advice service and where necessary offer consultancy to help solve temperature measurement problems.