Capacitance

Calibration and standards for capacitance

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 R_K = 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.

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
• Calibration of AH2500A, AH2550A and AH2700A Andeen-Hagerling capacitance bridges at various frequencies and nominal values

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

Capacitance standards

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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