National Physical Laboratory

Supply of temperature fixed-point cells for the calibration of standard platinum resistance thermometers and thermocouples

Inspection of water triple point cell
Inspection of water triple point cell

NPL manufactures in-house, and can supply a range of fully certified fixed-point cells for the calibration of standard platinum resistance thermometers (SPRTs) and thermocouples to meet the requirements of the International Temperature Scale of 1990 (the ITS-90). Furthermore, open cells for high temperature calibrations of thermocouples are also available using state-of-the-art eutectic alloy melting temperatures.

Melting, freezing and triple point cells

Sealed or open cells are available for the freezing points of indium (156.5985 °C), tin (231.928 °C), zinc (419.527 °C), aluminium (660.323 °C), silver (961.78 °C) and copper (1084.62 °C).

These consist of a high-purity graphite crucible containing an ingot of at least 99.9999% pure metal. The crucible is sealed in argon within a silica (quartz) envelope. The cells are supplied with an Inconel® holder, thermometer guide tube and insulation as necessary for operation.

The approximate dimensions of the freezing point cells, in mm, are:

External length of crucible 240 mm
Depth of graphite re-entrant well 200 mm
Inner diameter of thermometer well    8 mm
Outer diameter of Inconel® holder  52 mm
Length of Inconel® holder 460 mm

Smaller open cells are available for zinc, silver and copper freezing points: these are suitable for the calibration of thermocouples.

Cells are also available for the triple point of water (0.01 °C), the melting point of gallium (29.7646 °C) and the triple point of mercury (-38.8344 °C), the first being made from quartz, the second PTFE and the third from stainless steel.

Components of freezing-point cell
Components of sealed freezing-point cell

Melting and freezing plateaux of not less than six hours duration are achievable.

The cells may be supplied separately or as part of a complete package, including furnace or bath for establishing the melting or freezing plateaux, all ancillary components, and full assembly and operating instructions. A furnace for preheating and annealing SPRTs, complete with silica tubing, insulation bricks and monitoring thermocouple, can also be supplied.

Open, eutectic melting point cells

For details of our open, eutectic melting point cells at the temperature fixed points of Co-C (1324 °C) and Pd-C (1492 °C), please see here

Download here details of the high-temperature fixed point (eutectic) cells available to purchase from NPL, for the calibration and validation of thermocouples.

Uncertainties and certification

Each cell is individually certified by comparison with NPL reference cells, ensuring direct traceability to National Standards and the lowest possible uncertainties, and is supplied with a calibration certificate.

Calibration of temperature fixed points at NPL are accredited under ISO 17025 (UKAS) and the CIPM (Mutual Recognition Agreement, MRA) arrangement - see table below for availability. In some cases, lower uncertainties are available with NPL calibration certificates, which are internationally accepted. Please contact us for further details.

Fixed point cell


Best measurement capability (with UKAS accreditation)


For PRT measurements

For thermocouple measurements

Covered under the CIPM Mutual Recognition Agreement (MRA)?

Argon (T)

-189.3442 °C

0.0005 °C



Mercury (T)

-38.8344 °C

0.0002 °C



Water (T)

0.01 °C

0.00007 °C



Gallium (M)

29.7646 °C

0.0002 °C



Indium (F)

156.5985 °C

0.0007 °C



Tin (F)

231.928 °C

0.0006 °C



Zinc (F)

419.527 °C

0.0009 °C



Aluminium  (F)

660.323 °C

0.0025 °C



Silver (F)

961.78 °C

0.004 °C



Copper (F)

1084 °C


0.031 °C


Co-C (M)

1324 °C


0.44 °C


Pd-C (M)

1492 °C


0.65 °C


Note: T = triple point, F = freezing point, M = melting point

The uncertainties are assessed individually and depend on performance of the cell/s at time of test.


Customer Services tel: +44 20 8943 6315

Last Updated: 1 Sep 2015
Created: 11 Sep 2007


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