Comparison of PD Calibration Capabilities in Four National Metrology Institutes Down to 0.1 pC

Modern commercial PD calibrators often have their lowest charge level as 0.1 pC. However, now National Metrology Institutes (NMIs) provide traceable calibration services for apparent charge starting from 0.5 pC upwards and the relative uncertainty at this level is typically 20 %.
The main problem with the widely used numerical calibration method proposed in the standard is its low signal to noise ratio, which limits the accuracy on low PD levels below 10 pC. In practice, the lower limit of meaningful calibration work using this method is c. 1 pC. Charge sensitive amplifiers have been recently introduced for low PD calibration in the participating NMIs, and significant improvement in low-level calibration uncertainty has been achieved using this method.
This paper describes the results of a comparison between four NMIs. The aim was both to confirm their existing claims for measurement uncertainty from 1 pC to 50 nC, and to provide support for their plans to improve calibration services the low PD levels down to 0.1 pC.
The comparison was performed by transporting a reference calibrator between the four participating NMIs. Each NMI calibrated the 18 fixed settings of the PD calibrator, ranging from 0.1 pC to 50 nC. Values for measured charge, rise time, time to steady state and step voltage duration, as defined in current version of IEC 60270, were compared.