Secondary/transfer standards used to capture a calibrated quantity (e.g. resistance) need to be stable, as this is something that can easily limit the reliability of a calibration (rather than the source uncertainty for instance). However, what exactly limits the stability, e.g. aging or thermal/mechanical stresses is a complicated question. The Ultrastable Low-Noise Current Amplifier (ULCA) can measure or source current accurate at part per million levels under lab conditions…but do they ‘travel’ well?

Commercial current to voltage converters, in either modular amplifiers or integrated electrometer systems typically have 10-100 part per million variations in their gain on time scales of hours-days, or after power cycling. As such, they cannot be reliably used as transfer standards at the levels required for top level, small current/high resistance metrology.

High value standard resistors perform slightly better, but need to be tightly temperature controlled and recalibrated on time scales weeks to months. They also suffer ‘transport effects’ when used in round trip intercomparisons, shown by sudden deviations from long term drift lines.

Destinations of ULCAs for round trip comparisons in the e-Si-Amp project.

The ULCA, purpose-built for stability at the sub part per million level and calibrated against quantum resistance and voltage standards, should outperform all of these other standards.

In previous intercomparison exercises, transport discrepancies in transfer standards were so large that this limited the ability of participants to spot weaknesses in their measurement systems. It also limited the scientific conclusions that could be drawn about novel electrical standards such as electron pumps.

The e-SI-Amp project has enabled the distribution of ULCAs across partner institutes (see figure) as a means of testing partners small current systems, and to probe the robustness of the ULCA as a travelling standard. Sending three ULCAs over more than a dozen journeys revealed changes in the calibration factor that were typically much less than 0.5 part per million, even when shipped as air cargo!*

If you would like to know more about the ULCA, please contact or the licensed manufacturer Magnicon where further technical details can be found.


*One unusually cold (-4° C) trip to KRISS, Korea perturbed the calibration factor by 1.8 ppm, still better than almost all NMI capabilities in this current range!