Highlights ...
The international measurement community, through the International Committee
for Weights and Measures, is considering updating the International System of
Units (SI). This update, which will probably occur in 2011, will redefine the
kilogram, the ampere and the kelvin in terms of fundamental physical constants.
The kelvin, instead of being defined by the triple point of water as it is
currently, will be defined [for thermodynamic temperature] by assigning an
exact numerical value to Boltzmann’s constant. The change would generalise
the definition, making it independent of any material substance, measurement
technique, and temperature range, to ensure the long-term stability of the unit.
For almost all users of temperature measurements, the redefinition will pass
unnoticed; water will still freeze at 0 °C, and thermometers calibrated before
the change will continue to indicate the correct temperature [because the
International Temperature Scale of 1990 will not change]. The immediate benefits
of the redefinition will be to encourage the use of direct measurements of
thermodynamic temperatures in parallel with the methods described in the
International Temperature Scale.
In the longer term, the new definition will allow the accuracy of temperature
measurements to gradually improve without the limitations associated with the
manufacture and use of triple point of water cells. For some temperature ranges
at least, true thermodynamic methods are expected to eventually replace the
International Temperature Scale as the primary standard of temperature.
For further information contact your NMI or see
http://www.bipm.org/wg/CCT/TG-SI/Allowed/Documents/Report_to_CIPM_2.pdf.