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Page 57 of 97
Issue: 1.04
taken at slight different times. The system noise will again determine the
uncertainty of this unity ratio measurement.
6.6 Eliminating Self-Heating Effects
Although the sense currents used with SPRTs are small, they can still generate
self-heating ‘errors’ of several mK. The most accurate SPRTs typically have very
lightly supported elements so the self-heating effect is ironically worst in those
SPRTs designed for the most accurate measurements.
The microK includes individual “keep-warm” current sources for each of the
three input channels. These replace the sense current when a channel is not being
measured and ensure that the power dissipated in an SPRT remains constant.
The most accurate measurements with an SPRT involve measuring its resistance
at two sense currents and then extrapolating back to the zero-power value. The
microK’s user interface provides a simple feature to allow the sense current to be
scaled by a factor of root-two (a factor of 2 in power) to make this technique easy
for the user to implement (see section 3.6).
