National Physical Laboratory


Capacitance bridges

The unit of capacitance (farad) is measured in terms of the world-wide consensus value of the von Klitzing constant RK-90 = 25 812.807 Ω which is derived from the DC Quantum Hall Effect (QHE).

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.

Calibration Service

The complete range of measurement capabilities is summarised below:

  • Calibration of capacitance standards in the range 0.001 pF to 10 mF
  • Calibration of General Radio Type 1417 Four-terminal capacitance standards having the range 1 µF to 1 F
  • Calibration of Dissipation Factor
  • Experience in the calibration of at least 80 different types of capacitors, from more than 20 leading manufacturers
  • Capacitors may be measured in the frequency range 20 Hz - 1 MHz depending on the type
  • Measurements are normally carried out at an ambient air temperature of 20 °C or 23 °C
  • Non sealed capacitors are preconditioned at NPL in a humidity chamber, maintained within the range 50 ± 5 % RH prior to calibration
  • Capacitance calibrations are operated in batch cycles twice a year, commencing in April and November
  • Out-of-cycle calibrations are available on request and may incur a surcharge

The best measurement capability, expressed as an expanded uncertainty (k =2) is for fused silica dielectric capacitors:

  • ±0.7 ppm of nominal value for 10 pF capacitors calibrated at 1 kHz or 1.592 kHz
  • ±0.9 ppm of nominal value for 100 pF capacitors calibrated at 1 kHz or 1.592 kHz

More information on Standard Capacitors


Customer Service tel: +44 20 8943 7166

Last Updated: 18 Aug 2015
Created: 17 Oct 2007


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