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 13.8033 K (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) will be calibrated by comparison with standard thermometers in appropriate baths.

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

Fixed
Point*		 t90/oC Uncertainty4/oC
Range 1 Range 2 Range 3 Range 4 Range 5
Liquid nitrogen2 -196
0.0017 0.0017 0.002
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
0.0007


FP tin 231.928
(0.0007)3 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



0.004


Notes:

  1. TP = triple point, MP = melting point, FP = freezing point
  2. Calibrations are not routinely carried out at the triple point of argon (- 189.3442 °C). Instead, comparison measurements are made in a bath of liquid nitrogen at a temperature close to - 196 °C, using NPL standards.
  3. Range 2 may extend either to the indium point or to the tin point, with the uncertainties as stated.
  4. 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.

Contact

Customer Services tel: +44 20 8943 6315
E-mail: temperature_enquiries@npl.co.uk

Last Updated: 7 Mar 2012
Created: 11 Sep 2007