On the characterization of an AHF cavity radiometer and its traceability to WRR/SI

In a complementary way to the comparison to WSG to get traceability to WRR (and consequently, to SI), a solar-type cavity radiometer can also be characterized, determining the deviations of the instrument from the ideal realization of the principle of electrical substitution and obtaining its total measurement uncertainty. This work summarizes different techniques and procedures applied for the characterization of an Eppley AHF radiometer. The approach for characterization is based on the analysis of the measurement model function of the instrument. Some results obtained from calibration and testing (voltmeter, area of the precision aperture, resistance of the leads, non equivalence factor), and from numerical simulation (effective absorptance, scattering) are presented. According to these results, current value of standard uncertainty for this instrument is about 0.28% but it is expected that further improvements in the equipment and tests can reduce this figure below 0.1% (1000 ppm) in the near future.