The International Temperature Scale of 1990 (ITS-90) defines temperature from 0.65 K up to the highest temperatures radiometrically measured. Standard Platinum Resistance Thermometers (SPRTs) and secondary standard PRTs are the defining interpolating instruments from 13.8033 K to 1234.93 K (the freezing point of silver).
1. ITS-90 Reference Function and Resistance Ratios (W)
Temperature measurement with an SPRT relies on the resistance ratio W(T90) = R(T90) / R(273.16 K), where R(273.16 K) is the resistance at the Triple Point of Water (TPW). High purity sensors like the DB Metron Series are manufactured with strain-free platinum wire to ensure W(Ga) ≥ 1.11807 or W(Hg) ≤ 0.844235.
2. Minimizing Self-Heating and Thermal EMF
SPRT measurements must account for self-heating by measuring at two excitation currents (e.g., 1.0 mA and 1.414 mA) and extrapolating resistance to zero power (0 mA). True 4-wire DC polarity reversal or AC bridge measurement eliminates parasitic thermoelectric voltages.