National Physical Laboratory

Calibration and supply of platinum-rhodium thermocouples (0 °C to 1800 °C)

Thermocouple calibration and purchase

Platinum-rhodium thermocouples

NPL offers a UKAS-accredited thermocouple calibration service (laboratory number 0478) for platinum-rhodium thermocouples using freezing points of zinc, silver and copper (up to 1100 °C) and the melting points of gold and palladium (up to 1600 °C). Furthermore, calibrations including the melting point of platinum can be provided with an NPL certificate (up to 1800 °C).

NPL also supplies calibrated platinum-rhodium thermocouples, which are constructed in-house and may be supplied with any of the calibration options below. These are made from wires assembled into insulators. The wires are 0.5 mm in diameter and 2 m long, and the thermocouples can be purchased with or without cold-junction attachments to copper extension leads.

Thermocouples are widely used in industry to provide temperature measurement and control for numerous applications. They are popular because they are small and rugged, suitable for use to very high temperatures and are relatively low-cost. They also provide a convenient electrical output for data logging and control. Platinum-rhodium thermocouples (Types R, S and B) are suitable for use as standards for the calibration of other base-metal thermocouples, and for high-temperature applications.

Calibration using new metal-carbon fixed points

As a result of extensive research and development of high temperature metal-carbon eutectic fixed points, NPL now offers a service supplying and/or calibrating Type R, S and B thermocouples with UKAS certificates, with significantly reduced uncertainties of measurement.

These uncertainties are achieved by employing the cobalt-carbon eutectic fixed point (1324°C) or palladium-carbon eutectic fixed point (1492 °C) as the highest temperature fixed point in the calibration. The Co-C and Pd-C fixed points have an inherently low uncertainty in comparison with the wire bridge fixed points.

See the page on eutectic fixed-points for more information about these state-of-the-art fixed points which have recently been introduced to NPL's calibration schedule.

Ranges and uncertainties

Calibration uncertainties (coverage probability of approximately 95 %) for Type R, S and B thermocouples:

Calibration range Calibration uncertainty (k = 2)
Types R and S Type B* Calibration points
0 °C to 1100 °C* ± 0.3 °C falling from ± 1 °C at 400 °C to ± 0.5 °C at 800 °C and above Zn, Ag, Cu
0 °C to 1330 °C† ± 0.3 °C from 0 °C to 1100 °C, rising linearly to ± 0.55 °C at 1330 °C ± 0.3 °C from 0 °C to 1100 °C, rising linearly to ± 0.60 °C at 1330 °C Zn, Ag, Co-C
0 °C to 1500 °C ± 0.3 °C from 0 °C to 1100 °C, rising linearly to ± 0.72 °C at 1500 °C ± 0.3 °C from 0 °C to 1100 °C, rising linearly to ± 0.76 °C at 1500 °C Zn, Ag, Pd-C
0 °C to 1600 °C* ± 1 °C up to 1100 °C
± 1.5 °C from 1100 °C to 1600 °C
± 1.5 °C up to 1600 °C Au wb, Pd wb
0 °C to 1800 °C ± 0.3 °C up to 1100 °C rising linearly to ± 1.2 °C at 1800 °C Zn, Ag and Pt wb

* Type B thermocouples are calibrated from 400 °C
† Calibration points used are melting or freezing fixed-point cells, unless otherwise stated (wb = wire bridge method)

Download Noble metal thermocouple calibration options flyer Adobe Acrobat PDF file


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Last Updated: 27 Aug 2015
Created: 30 Nov 2010


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