Inductance

The unit of inductance

At SP the unit of inductance, the henry (H), is realised in the range 10 mH - 1 H depending on the measurement frequency with traceability from the units of capacitance, frequency and resistance. The realisation is performed with a resonance bridge. The measurement uncertainty is on the order of 6 μH/H for the realisation of the inductance unit at 1 kHz.

The realised inductance unit is transferred with bridge measurements to a group of primary standards in the range 1 µH – 10 H maintaining the inductance unit. Below 100 nH calculable standards are used with traceability from the unit of length and the numerical value of the permeability µ0. 

Bridge measurements of impedance (inductance, capacitance and ac-resistance) at frequencies ≤100 kHz are based on ac voltage ratio determined by inductive voltage dividers (IVD) with known ac voltage ratio. At SP, ac voltage ratio errors (in-phase and quadrature) of IVD ac ratios are determined by a method based on permutation of capacitors.  

In order to further secure traceability, SP also participates regularly in international comparisons.

Calibration service

SP offers calibration of inductance standards and different types of instruments such as bridges and LCR meters in the range 10 pH – 100 H. Inductance standards are calibrated with the so-called substitution measurement in the range 1 µH – 10 H for measurement frequencies up to 10 kHz. The measurements are performed with a LCR-meter.The best measurement capability is 100 μH/H at 1 kHz.

Calibration of inductance standards up to 10 kHz is also offered with inductance bridge measurements for lower measurement uncertainty. The best measurement capability at 1 kHz is 40 μH/H.

Inductance bridges and LCR-meters are calibrated by measuring on primary or secondary inductance standards at frequencies up to 10 kHz (in some cases above 10 kHz).

IVDs are calibrated with an IVD comparison bridge in the range 50 Hz – 10 kHz. The best measurement capabilities at 1 kHz are 1,2  x 10-7 for the in-phase correction och 3,0 x 10-7 for the quadrature correction. 

Other types of measurements are also performed, for example, determination of temperature coefficients.

The final uncertainty given at calibration depends not only on the uncertainty contribution from the calibration equipment but also from the device under test (stability, temperature coefficient, voltage dependence, etc.). A preliminary measurement uncertainty can often be estimated from experience prior to calibration.