Realization of ITS-90 from 273.15 K through 1234.93 K:

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Realization of ITS-90 from 273.15 K through 1234.93 K: (291.87 KB)

All of the fixed points of the International Temperature Scale of 1990 (ITS-90) in the range from 273.15 K to 1234.93 K have been realized in specially-designed, permanently-sealed cells. The purity of all of the metals used was 99.9999% or greater. Many improvements in design and techniques used were made in order to get the highest possible accuracy. Since it is important that all of the fixed points be traceable to NIST (National Institute of Standards and Technology), these fixed points were compared with NIST data by using two NIST-calibrated standard platinum resistance thermometers (SPRTs). The differences between the fixed point values and those measured at NIST were within 0.5 mK for the freezing points (FP) of tin and zinc, and within 1 mK for the FP of aluminum. The expanded uncertainties (coverage factor K=2) at the fixed points were 0.4 mK for the melting point (MP) of gallium, 1.0 mK for the FP of indium, tin, and of zinc, 4.0 mK for the FP of aluminum and 10 mK for the FP of silver. Two SPRTs were used to interpolate between the fixed points. A 25-ohm SPRT was used up to 933.473 K and a 0.25-ohm SPRT up to 1234.93 K.