Supply of temperature fixed-points and triple-point cells for the calibration of standard platinum resistance thermometers and thermocouples
NPL supplies a range of fully certified sealed 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).
Freezing and Melting point cells
Sealed 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.
The approximate dimensions of the freezing point cells, in mm, are as shown in the table below:
| 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 freezing-point cells are available for zinc, silver and copper, and these are suitable for the calibration of thermocouples.
Cells are also available for the melting point of gallium (29.7646 °C) and the triple point of mercury (−38.8344 °C), the former being made from PTFE and the latter from stainless steel.
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.
Uncertainties
All the fixed-point cells are designed, manufactured and tested at NPL. Each cell is individually certified by comparison with NPL reference cells, ensuring direct traceability to National Standards, and is supplied with a calibration certificate. The uncertainties in realising the SPRT fixed-point cells are assessed individually and range from 0.2 mK for mercury and gallium to about 3 mK for silver
Contact
Customer Services tel: +44 20 8943 6315
E-mail: temperature_enquiries@npl.co.uk