The SI derived unit of capacitance (farad) is realised by comparing the impedance of a known resistance, obtained using the Quantum Hall Effect (QHE) and the value of the von Klitzing constant RK = 25 812.807 459 3045 Ω.
The NPL 10 pF primary capacitance standard is determined from the QHE using a cryogenic current comparator bridge and a series of AC coaxial bridges. Customer standards are then calibrated in terms of the 10 pF primary capacitance standard using coaxial bridge systems.
The complete range of measurement capabilities is summarised below:
The best measurement capability, expressed as an expanded uncertainty (k =2) is for fused silica dielectric capacitors:
NPL has developed highly stable four terminal-pair capacitance standards of compact design of selected values between values between 50 pF and 1 μF available. Their stability derives from using a ceramic dielectric and by keeping the standards at constant temperature using a two-stage oven.
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