Thermodynamic Temperature Scale Research

Research on the Thermodynamic Temperature Scale

In collaboration with NIST Division 838 and Division 844, the NIST Thermometry Group is engaged in a project to improve the measurement of thermodynamic temperature by a factor of five for all temperatures above 500 K.

A temperature scale that is consistent with all the known laws of thermodynamics is termed a thermodynamic temperature scale. In practice, a well-understood thermodynamic system, such as an ideal gas, is prepared in the laboratory such that its thermodynamic temperature can be predicted from other known properties. Laboratory thermometers can then be calibrated at this thermodynamic temperature. The International Temperature Scale of 1990 is an approximation to the thermodynamic temperature scale. Further refinements in our ability to measure thermodynamic temperature will lead to an improved International Temperature Scale that more closely obeys the laws of thermodynamics.

Up to 700 K, the most accurate measurements of thermodynamic temperature are the NBS/NIST results for Constant Volume Gas Thermometry (CVGT) by Guildner and Edsinger and Edsinger and Schooley. Above 700 K, spectral radiometry is used to measure the ratio of radiances from a reference blackbody and from a blackbody at unknown thermodynamic temperature. Existing determinations of the thermodynamic temperature scale are limited by the accuracy of the CVGT measurements. Discrepancies between the two CVGT measurements indicate that the uncertainty of these measurements is both larger than expected and also not well understood.

To improve on the CVGT measurements, speed-of-sound measurements will be used to reduce the uncertainty in determinations of thermodynamic temperature by a factor of five in the temperature range 500 K to 750 K. The speed of sound of low pressure argon will be measured in a spherical acoustic resonator. This technique has been proven to have high accuracy. Thermal radiometry, referenced to the acoustic results, will be improved by the same factor at all temperatures.

For more information, contact Dean Ripple at dean.ripple@nist.gov

Home

Last Updated on: 2/25/04