Temperature and Humidity Standards
kelvin, K: The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.
In principle temperature is based on a thermodynamic concept that characterises the average kinetic energy of small particles in thermal equilibrium. The magnitude of the unit is equivalent to a definition of Boltzmann's constant k, and this constant turns up in all of the physical formulae that describe the physical effects caused by temperature. One such relationship is Boyle’s law:
which relates the pressure and volume of n atoms of an ideal gas to its temperature. Unfortunately gas thermometers, which exploit Boyle’s Law, and other thermodynamic thermometers are neither small, convenient nor accurate enough to be of any practical use. The best gas thermometer experiments take several months of work and a million dollars worth of equipment to measure temperatures to a couple of hundredths of a degree. This is hardly good enough when some industrial measurements require accuracy ten times better than this.
Instead of a true metric temperature scale the CGPM has defined a more practical scale based on defined temperatures for the melting, freezing and triple points of pure substances, and approved interpolating thermometers that define the temperature at intermediate temperatures. This scale, which is reproducible to about 1 mK for temperatures below a few hundred degrees Celsius, is updated approximately every 20 years. The current version is called the International Temperature Scale of 1990, or ITS-90.