Torque Calibration

Torque calibration compares a torque-applying or torque-measuring instrument against a reference with documented traceability. ISO 6789 applies to hand tools, and assessment differs by instrument type.

The aim is for bolted joints to be tightened to the correct preload. A deviation in torque leads either to the joint loosening or to the bolt's yield point being exceeded; either is a cause of structural failure.

• Torque wrenches (click-type, dial, digital)
• Torque screwdrivers
• Electric and pneumatic torque guns
• Torque-measuring instruments and transducers
• Torque testers and simulators

• Indication or set-value error
• Repeatability: the spread of repeated applications at the same setting
• Linearity across the measuring range
• Behaviour in the clockwise and counter-clockwise directions
• Assigning the type and class under ISO 6789

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
• ISO 6789-1 / ISO 6789-2 (hand torque tools)
• EURAMET cg-14 (static torque-measuring instruments)
• JCGM 100 (GUM — uncertainty)

• ISO/IEC 17025
• ISO 6789-1 / ISO 6789-2 (hand torque tools)
• EURAMET cg-14 (static torque-measuring instruments)
• JCGM 100 (GUM — uncertainty)

1. 1Receipt of the Instrument and Initial InspectionThe physical condition of the instrument is examined for wear and damage. On dimensional instruments the cleanliness of the measuring faces is also checked.
2. 2Conditioning to Ambient ConditionsThe instrument is brought to equilibrium at laboratory temperature. In dimensional measurement a temperature difference turns directly into measurement error through thermal expansion of the material.
3. 3Preparation of the Reference StandardsWeights, gauge blocks, a dead-weight tester or a reference transducer with documented traceability are prepared.
4. 4Preloading and Zero CheckIn force, torque and weighing calibrations the instrument is taken up to full load and back down several times; the zero point is then checked.
5. 5Readings at the Measurement PointsReadings are taken in increasing and decreasing directions at points representing the measuring range; hysteresis is determined in this way.
6. 6Repeatability MeasurementsThe same point is measured repeatedly; in weighing calibration an eccentricity (off-centre load) check is also performed.
7. 7Application of CorrectionsCorrections for local gravity, air buoyancy and temperature are taken into account; ignoring them produces significant deviation in precision measurements.
8. 8Uncertainty and CertificateThe uncertainty budget is established and the results are entered on the certificate; 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 torque transducer's uncertainty and drift
• Application speed and operator effect (for hand tools)
• The effect of grip point and lever length
• The temperature effect
• Repeatability
• Instrument resolution

• 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 instrument type and its ISO 6789 classification
• The calibrated torque values and direction
• The grip point and the fixtures used
• For adjustable wrenches, the results after setting

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.

• Pressure Calibration
• Mass Calibration
• Balance Calibration
• Force Calibration
• Dimensional Calibration