National Physical Laboratory

Calibration of standard long-stem platinum resistance thermometers (-196 °C to +962 °C)

PRT calibration

Calibration of standard long-stem platinum resistance thermometers

Standard platinum resistance thermometers (SPRTs) are the instruments specified in the International Temperature Scale of 1990 (the ITS-90) for performing measurements within the range -259.3467 °C (the triple point of hydrogen) to 961.78 °C (the freezing point of silver).

SPRTs of the long-stem construction can be calibrated over various temperature ranges, depending on the application and required accuracy, by measurement at the appropriate ITS-90 fixed points. NPL provides calibrations with uncertainties as indicated in the table below. All the calibration services are fully UKAS accredited (UKAS calibration accreditation number 0478).

Thermometers that are not suitable for measurement in fixed point apparatus (on account of length, immersion needs or for other reasons) may be calibrated by comparison with standard thermometers in appropriate baths. NPL provides this service, with UKAS accreditation, over the range -196 °C to
+100 °C.

Calibration ranges and uncertainties for SPRTs (95 % confidence level)

Point 1
t90/oC Uncertainty3/oC
Range 1 Range 2 Range 3 Range 4 Range 5
Liquid nitrogen -196
0.0017 0.0017 0.002
TP argon -189.3442 0.0005 0.0005 0.0007 0.001
TP mercury -38.8344 0.00035 0.00035 0.001 0.002
TP water 0.01 0.00016 0.0003 0.0007 0.002
MP gallium 29.7646 0.0003

FP indium 156.5985

FP tin 231.928
(0.0007)2 0.001 0.002 0.003
FP zinc 419.527

0.001 0.002 0.003
FP aluminium 660.323

0.0025 0.003
FP silver 961.78



  1. TP = triple point; MP = melting point; FP = freezing point
  2. Range 2 may extend either to the indium point or to the tin point, with the uncertainties as stated.
  3. The uncertainties in this table are larger than the uncertainty at a particular fixed point because they account for the propagation of uncertainty over the temperature range of calibration.


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Last Updated: 11 Dec 2017
Created: 11 Sep 2007


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