NIST reduces uncertainty for NIST capacitance calibrations.

Recent work at NIST has resulted in a factor of three decrease in the uncertainty for certain of the highest-level calibrations of fused-silica capacitance standards. At NIST, capacitance calibrations are based on the SI farad as realized by the NIST Calculable Capacitor. Until recently, measurements on the Calculable Capacitor had been performed only at a specific frequency, 1592 Hz. As a result, the uncertainty budget for NIST capacitance calibrations included a component to account for the uncharacterized frequency dependence of NIST standards between 1592 Hz and the frequency at which calibration measurements were performed (100 Hz, 400 Hz, or 1 kHz, for fused-silica capacitance standards). NIST staff have recently completed a detailed characterization of the transfer of the farad unit from the Calculable Capacitor to Calibration Laboratory reference standards at a frequency of 1 kHz. As a result, the relative uncertainty (95 % confidence) for the calibration of 10 pF and 100 pF fused-silica capacitance s tandards has decreased from 1.5 X [10.sup.-6] to 0.5 X [10.sup.-6].

CONTACT: Gerard Stenbakken, [phone]; [e-mail].