Current calibration
Current is usually measured as the voltage across a known resistance, so its accuracy follows the shunt, the voltage measurement and heat. Clamp meters and coils bring extra traps.
Instruments and typical accuracy
| Instrument | Typical range | Typical accuracy | Where it is used |
|---|---|---|---|
| Bench multimeter, current ranges | 10 µA to 10 A | ±(0.03 to 0.1 % of reading + range term) | Calibration labs, production test |
| Shunt resistor with voltmeter | 1 mA to 1000 A and more | 0.01 to 0.1 %, set by the shunt | High-current reference and power measurement |
| Current calibrator | Microamps to 20 A | ±(0.01 to 0.05 %) | Calibrates multimeters and panel meters |
| Clamp meter (Hall or transformer) | 1 A to several thousand A | ±1 to ±3 % of reading | Electricians, HVAC, power systems |
| Transconductance amplifier with coil | To hundreds of A | 0.05 to 0.5 % | Calibrates clamp meters |
AC current adds frequency and waveform: a true RMS meter is rated for a crest factor. The accuracy figures are typical ranges to help you compare instruments. Always use the specification of the instrument in front of you.
Reference standards and traceability
- Traceability. Current follows from voltage and resistance: I = V / R, so the standards are a voltage reference and a standard resistor or shunt.
- Shunts. Four-terminal shunts with low power coefficient are used as transfer standards for DC and low-frequency AC.
- Calibrators. A multifunction calibrator is the usual working standard for meter ranges below 20 A.
- Quality system. ISO/IEC 17025, scope listing DC and AC current with frequency and range.
How it is calibrated
Typical procedure
- Check safety first: fuses, rated leads and lockout for the high-current ranges.
- Connect the meter in series with the source (or the shunt to a voltmeter for high currents).
- Check each range at zero, about 10 %, 50 % and 90 to 100 % of full scale in both polarities.
- For AC, check at the frequencies of use, for example 50 or 60 Hz and 400 Hz.
- For clamp meters, wind several turns through the jaw and divide by the number of turns, centring the conductor.
- Compare with the reference, calculate the error and report with uncertainty.
Where it goes wrong
- Burden voltage. A meter on the 10 A range can drop a volt or more, changing the circuit current.
- Shunt heating. Resistance drifts with power, so let the shunt settle after each change.
- Blown fuses. Leaving a lead in the current jack and measuring voltage is the classic.
- Clamp position. Off-centre conductors or neighbouring cables bias the reading.
- Lead resistance. At 10 A, 10 mΩ of leads is 0.1 V.
What goes in the uncertainty budget
- Reference standard uncertainty, including the shunt or resistor and the voltmeter.
- Shunt temperature rise and power coefficient.
- Burden voltage and source stability.
- Frequency response and waveform for AC.
- Resolution and repeatability of the unit under test.
Combine the contributions as in the uncertainty budget calculator, then decide pass or fail with a guard band as explained on the calculators page.
Calculators
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