uTrsh: uncertainty component due for self-heating of the reference standard. The measurement of resistance involves passing a current through the resistor and, therefore, heating of the resistor. For the highest accuracy measurements, corrections are applied by measuring at two currents, I1 and I2, and extrapolating to zero current. For the determination of the electrical resistance, an electrical measurement must be carried out for which a measurement current must be fed through the sensor. The measurement current leads to the sensor being heated (self-heating) and thus to the measurement result being falsified. This effect is dependent not only on the magnitude of the measurement current but also on the measurement conditions themselves. In the calibration, the self-heating mechanism is to be investigated or a measurement current is to be chosen at which this effect is negligible.
Example: The calibration certificate states that a measurement current of 1 mA in a water triple point cell has led to a heating of 2.1 mK. This contribution is neglected in the following as the thermometer is both calibrated and used now at a measurement current of 1 mA.
