The unit of temperature at present is defined as a fraction 1/273.16 of the thermodynamic temperature of triple point of water (TPW). The primary standards of temperature based on TPW and other fixed points called the thermal equilibrium states are well established at NPL as per International Temperature Scale of 1990 (ITS-90) in the form of sealed cells of high purity materials. The temperature scale at NPL from -189 °C to 962 °C, (FPs Ar, Hg, Sn, Zn, Al, Ag) is realized by SPRT with uncertainty varying from ±0.17 m°C at TPW (0.01 °C) to maximum ± 6 m °C at Ag point (961.78 °C). In the thermocouple thermometry, the range from 0 °C to 1554 °C is realized by standard Type-S & R TCs with uncertainty varying from ±0.32 °C at indium point (In,156.5985 °C) to ± 0.5 °C at Palladium point (Pd,1554 °C). The radiation temperature scale has been established by installing photoelectric linear pyrometer, LP4 and realizing the silver point (Ag, 961.78 °C) and copper point (Cu, 1084.62 °C) in the form of metal-in-graphite blackbodies with an uncertainty better than ± 0.5 °C. The high temperature range from 800 °C to 2200 °C is thus maintained by transfer standard lamps using LP4 pyrometer with uncertainty varying from ± 0.9 °C to ± 2.5 °C while the total radiation infrared thermometry is available in the range from 50 ºC to 3000 ºC. The apex level calibration in temperature metrology is available in the laboratory in the range from -200 °C to 3000 °C.

Realization of Triple Point of Water (TPW) & Set-up for Primary Standard Fixed Points by using SPRT