NPL provides calibration services for resistance standards ranging from 0.1 mΩ to 1 GΩ. Measurement capability exists for low power dc measurements, high power dc measurements of current shunts and ac impedance measurements.
All measurements of resistance at NPL are referred to the quantised Hall effect using the conventional value of the von-Klitzing constant RK-90 = 25 812.807 Ω exactly. The uncertainty of the value of RK-90 in the SI system is not included in the uncertainty assessments.
Two 100 Ω room temperature resistors are measured in terms of the quantised Hall effect on a regular basis using a resistance bridge based on a cryogenic current comparator. These resistors are used as day-to-day working standards, and all other resistance standards at NPL are related to these resistors using a variety of measurement techniques.
Low power resistors
Resistors designed for low power dissipation are normally measured at a power of 1 mW or less. Standards are measured in either an oil bath or an air bath at measurement temperatures of 20 °C or 23 °C (other temperatures are available on request). Resistors at decade values and at 25 Ω are calibrated as standard with the uncertainties given in the table below. Other values can be calibrated on request.
(95 % confidence level)
High power resistors
Calibration of high power resistors is offered at powers up to 100 W and currents of up to 100 A.
Temperature coefficient of resistance
Resistors submitted for temperature coefficient determination are measured at several temperatures in the range from 17 °C to 25 °C and the measurements are fitted to a second order polynomial.
AC impedance of resistors
Resistors in the range 1 Ω to 10 kΩ can be measured at frequencies from 40 Hz to 20 kHz. The uncertainty quoted varies with resistor value and frequency. The best uncertainty currently available is 0.5 ppm for the real part of the impedance and 10 ns for the time constant.
Booking a calibration
Resistance calibrations are carried in batch cycles according to an annual timetable, which specifies the measurement dates for each resistance value. It may be possible to carry out calibrations out-of-cycle, but this may incur an extra charge.
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