ITS-90 temperature scale

An empirical temperature scale is based on some arbitrary physical property (such as density, electrical resistance, magnetic susceptibility, etc.) that changes in a way that is continuous and single valued. The ITS-90 temperature scale described in Appendix 2 is an empirical scale that is designed to closely approximate the absolute (ideal gas) temperature scale. 

This volume also contains four appendices. The appendices give the mathematical foundation for the thermodynamic derivations (Appendix 1), describe the ITS-90 temperature scale (Appendix 2), describe equations of state for gases (Appendix 3), and summarize the relationships and data needed for calculating thermodynamic properties from statistical mechanics (Appendix 4). We believe that they will prove useful to students and practicing scientists alike. 

For a themometer used in the field, its reliability can be achieved by the traceability requirement. Traceability is a term describing the link between the accuracy of the thermometer s readings to the ITS-90 temperature scale maintained in a national laboratory. In a calibration or testing laboratory, traceability of results (be it thermometer calibrations or temperature measurements) can be achieved through a laboratory accreditation program. This will be described in more detail later. 

The values of the vapor (sublimation) pressure of ice in this table were calculated from the equation recommended by the International Association forthe Properties of Steam (lAPS) in 1993. Temperature values correspond to the ITS-90 temperature scale. The uncertainty in the pressure is estimated to be 0.1% for t > -25°C and 0.5% for t < -25°C. The first entry in the table is the triple point of water. 

The data in this table have been adjusted to the ITS-90 temperature scale. The uncertainty in density values is 0.0003 g/mL between-20 and-10°C 0.0001 or less between -10 and 200°C and 0.0002 between 200 and 300°C. 

This value has an estimated relative imcertainty of 0.17%. This is based on the value of 1.0019 mPa s reported by Swindells in 1952 (Swindells et al, 1952), which was also the basis of ISO/TR 3666 1977. The small difference in value is due to the difference between the ITSA8 and ITS-90 temperature scales. The temperature dependence of the viscosity of water at atmospheric pressiue in the temperature range 0.01-100 °C, is given by the following recommended correlation (Kestin et al, 1978b)... 

Major update of Reference Points on the ITS-90 Temperature Scale... 

SECONDARY REFERENCE POINTS ON THE ITS-90 TEMPERATURE SCALE (continued)... 

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