The ITS-90 stipulates the triple point of Argon as defining fixed point in the range below 273 K. In theory this fixed point is very simple to establish, but to realize it in the laboratory implies the equipment necessary, consisting of one argon sealed cell and a cryostat. This equipment is expensive and it requires a long period of time to establish the triple point temperature. An alternative used by many calibration laboratories is the boiling point of nitrogen. An investigation on the use of the boiling point of nitrogen and the accuracy of the calibration was performed with the help of a commercially available equipment and instrumentation. A serial of Standard Platinum Resistance Thermometers were calibrated by fixed point method and the interpolation function of ITS-90. The same thermometers were then calibrated in the same temperature range by using the boiling point of nitrogen and liquid calibration bath. Different methods were adopted and the results were compared, including an investigation of the measurement uncertainties.
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