Evaluation of the uncertainty component due to resistance measurement in the calibration of PRTs at the Al freezing point

The uncertainty component due to resistance measurement in determining the resistance ratio W(=R_tal/R_tpw) at Al freezing point is evaluated. The sources of uncertainty dealt are, (1) the accuracy of the bridge, (2) the non-linearity of the current applied in the measuring bridge, (3) the non-linearity of the bridge reading value, (4) the difference among different measuring bridges and (5) the short-term stability of the standard resistor. The uncertainty value is found to depend on the structures of thermometers.