Electrical Calibration

Electrical calibration compares instruments that measure or generate electrical quantities — such as voltage, current, resistance, capacitance and frequency — against a reference with documented traceability. Multifunction calibrators and standard resistors are used as the reference.

The aim is for all decisions relying on electrical measurement to be reliable. Electrical safety tests, quality-control measurements and energy monitoring depend directly on the accuracy of these instruments.

• Digital and analogue multimeters
• Clamp meters
• Power supplies and current sources
• Oscilloscopes
• Insulation resistance testers (meggers)
• Earth resistance testers
• Power analysers and energy analysers
• Calibrators and process simulators
• Temperature indicators and transmitters (electrical input)

• DC and AC voltage
• DC and AC current
• Resistance (low and high value ranges)
• Capacitance and inductance
• Frequency
• Frequency dependence in AC measurements
• Interference and loading effect

The calibration interval is not fixed by any standard and there is no general rule such as "once a year". The interval is set by weighing together the manufacturer's recommendation, how often and under what conditions the instrument is used, how critical the measurement is, the drift shown by the instrument's previous calibration results, and any legal requirement. ILAC-G24 / OIML D10 describe the methods for setting it. The sound approach is to start with a reasonable interval for the first period and then lengthen or shorten it in the light of subsequent results: an instrument that proves stable can have its interval extended, one that drifts has it shortened.

• ISO/IEC 17025
• IEC 61010-1 (safety of measuring instruments)
• EURAMET cg-15 (multimeter calibration)
• IEC 61557 series (installation test instruments)
• JCGM 100 (GUM — uncertainty)

• ISO/IEC 17025
• IEC 61010-1 (safety of measuring instruments)
• EURAMET cg-15 (multimeter calibration)
• IEC 61557 series (installation test instruments)
• JCGM 100 (GUM — uncertainty)

1. 1Receipt of the Instrument and Initial InspectionThe instrument's functioning and the condition of connectors and probes are examined. The firmware version and any internal calibration status are recorded.
2. 2Warm-up TimeThe instrument is run for the period specified by the manufacturer so that it becomes thermally stable. A measurement made without warm-up does not show the instrument's true performance.
3. 3Preparation of the Reference SourceA multifunction calibrator or reference standard with documented traceability is put into service and its stability confirmed.
4. 4Selection of Function and RangeMeasurement points are defined for each function of the instrument (DC/AC voltage, current, resistance, frequency) and for the ranges in use.
5. 5Taking the MeasurementsAt each point the reference value is applied and the instrument reading recorded; cable, thermoelectric voltage and loading effects are taken into account.
6. 6Calculation of the DeviationThe difference between reading and reference is calculated both in absolute terms and as a percentage of reading.
7. 7Uncertainty BudgetThe uncertainty and drift of the reference calibrator, resolution, noise and repeatability are combined. In the electrical field the drift of the reference is usually the largest contribution.
8. 8Issuing the CertificateResults are reported per function and per point; where requested, a statement of conformity is added with a decision rule.

Every calibration result is given together with a measurement uncertainty; a result reported without its uncertainty is incomplete. The uncertainty is budgeted following the GUM (JCGM 100) approach: the uncertainty of the reference, repeatability, resolution, environmental effects and the stability of the instrument are combined. The certificate states the expanded uncertainty with the coverage factor (usually k = 2) and the approximate level of confidence. The smallest uncertainty a laboratory can report is limited by its calibration and measurement capability (CMC) in the accredited scope; no better uncertainty may be declared.

• The reference calibrator's uncertainty
• The reference's stability (drift) since its calibration — usually the largest item in this field
• Instrument resolution and noise
• Thermoelectric voltages (in low DC voltage measurements)
• Cable resistance and connection effects
• Temperature coefficient
• Repeatability

• Laboratory details and certificate number
• The customer and identification of the instrument calibrated: make, model, serial number
• Date of calibration and date of issue of the certificate
• The calibration method applied or reference to the procedure
• Reference standards used and traceability information
• Ambient conditions during calibration (temperature, humidity and, where relevant, pressure)
• Measurement points, reference values and instrument readings
• The deviations calculated
• Expanded measurement uncertainty and coverage factor
• The personnel who performed and who approved the calibration
• The calibrated functions and ranges
• The reference value, reading and deviation for each point
• The deviation expressed as a percentage of the reading
• Warm-up time

A calibration certificate is NOT a certificate of conformity. Calibration measures and reports the deviation of the instrument; it does not correct that deviation and does not say that the instrument is "correct". To decide whether an instrument is fit for use, a tolerance (acceptance criterion) must be defined — and it is the user who sets that tolerance, not the laboratory. If a statement of conformity is requested, ISO/IEC 17025 clause 7.8.6 requires a DECISION RULE to be agreed in advance, defining how measurement uncertainty is taken into account in the decision; a conformity statement made without regard to uncertainty is misleading. The certificate also shows the condition AT THE TIME of calibration: later use, transport or shock can change the instrument's condition, and the certificate does not cover that. Where a next calibration date appears on the certificate, it is not the laboratory's decision but a reminder entered according to the interval stated by the customer.

Calibration is carried out under ISO/IEC 17025, which requires the laboratory's technical competence, impartiality and metrological traceability. Accreditation is granted per METHOD and per MEASUREMENT RANGE: a quantity or range that does not appear in the laboratory's scope annex is not accredited, and no certificate bearing the accreditation mark may be issued for that measurement. The scope also states the smallest uncertainty the laboratory can offer (CMC); no better uncertainty may be declared. EUROLAB holds internationally recognised accreditation under ISO/IEC 17025, granted by accreditation bodies that are signatories to the ILAC mutual recognition arrangement; whether your instrument can be calibrated within accredited scope is confirmed in writing at the quotation stage.

• RF Calibration
• Time / Frequency Calibration