A Calibration Certificate issued under ISO 6789-2:2017 is not simply a confirmation that a torque wrench is “within tolerance”.
It is a structured technical document that must:
- Quantify measurement error
- Calculate measurement uncertainty
- Demonstrate reproducibility
- Assess geometric influence
- Prove traceability
Part 2 of ISO 6789 sets defined requirements for how calibration must be performed and how results must be reported.
If those elements are missing, the document may look official, but it may not meet the standard.
Below are the core features that should appear on an ISO 6789 compliant Calibration Certificate.
Clear Identification of the Torque Tool
A compliant certificate must clearly identify:
- Tool type
- Manufacturer
- Model
- Serial number or asset ID
- Rated torque capacity
- Units of measurement
Calibration is meaningless without traceability. The tool must be uniquely identifiable.
Explicit Reference to ISO 6789-2:2017
The certificate must state that calibration was conducted in accordance with ISO 6789-2:2017.
The edition year matters. The 2017 revision introduced defined uncertainty requirements and structured evaluation methods that were not present in earlier versions.
Controlled Environmental Conditions
ISO 6789-2 refers to environmental requirements aligned with Part 1 Section 6.3.
Calibration must be conducted:
- Between 18°C and 28°C
- With temperature stability within ±1 K
- With relative humidity below 90%
These conditions should be recorded.
Torque mechanisms are temperature sensitive. Without environmental control, measurement reliability cannot be defended.
Reference Equipment and the 1/4 Rule
ISO 6789-2 Section 4.3 requires that:
- The calibration system’s relative uncertainty interval does not exceed one quarter of the expected maximum permissible deviation of the torque tool.
- The reference equipment is traceable to an ISO/IEC 17025 accredited laboratory.
A compliant certificate should state that this requirement has been satisfied.
Without this, uncertainty reporting is incomplete.
Defined Test Points
Calibration must be performed at approximately:
- 20%
- 60%
- 100%
of the wrench’s rated capacity.
These points represent the critical operating range.
Testing at only one point does not satisfy ISO 6789-2.
Five Measurements Per Test Point
At each calibration point, five independent measurements must be recorded.
This is mandatory under the standard.
These repeated measurements allow calculation of repeatability and form part of the uncertainty budget.
Reproducibility Assessment
ISO 6789-2 requires evaluation of reproducibility.
This involves removing and reattaching the torque wrench between measurements to quantify variation introduced by remounting.
A compliant calibration certificate should indicate that reproducibility has been assessed.
This is a key distinction between a basic Declaration of Conformance and a full calibration.
Geometric Variation Assessment
The standard requires assessment of geometric effects.
This includes evaluating variations caused by:
- Output drive geometry
- Interface alignment
Typically, this involves rotating the drive in defined increments and recording variation.
This contributes to the overall uncertainty calculation.
Force Loading Point Assessment
Calibration must evaluate the influence of applying torque at slightly offset positions on the handle.
This quantifies how variation in loading position affects measurement.
It forms another component of the uncertainty budget.
Relative Measurement Error
For each test point, the certificate must report:
- Target torque
- Mean measured value
- Calculated relative measurement error
This shows the actual performance of the tool at each point.
A calibration certificate does not simply declare compliance. It quantifies performance numerically.
Uncertainty Budget
This is the defining feature of ISO 6789-2 calibration.
The certificate must demonstrate evaluation of uncertainty components, which typically include:
- Display or scale resolution
- Repeatability
- Reproducibility
- Geometric effects
- Loading point influence
These components are combined to calculate:
- Combined standard uncertainty
- Expanded uncertainty (typically using k = 2 for 95% confidence)
- Relative uncertainty interval
Without an uncertainty budget, it is not a true ISO 6789-2 calibration certificate.
Expanded Uncertainty Statement
The certificate should clearly state:
- The expanded uncertainty value
- The coverage factor used (typically k = 2)
- The associated confidence level
This defines the interval within which the true value is expected to lie.
Calibration without uncertainty is incomplete.
Clear Compliance Statement
Even after all calculations, the certificate must still clearly state whether the maximum relative measurement error remains within the permissible deviation defined for the tool classification.
Numbers alone are not enough. The conclusion must be explicit.
What Should Raise Questions
A calibration certificate should raise concern if it:
- Does not reference ISO 6789-2:2017
- Omits uncertainty evaluation
- Lacks reproducibility testing
- Does not assess geometric effects
- Fails to state expanded uncertainty
- Provides only a single reading per point
Those omissions suggest the process may align more closely with basic conformance testing rather than full calibration.
Final Thoughts
An ISO 6789 compliant Calibration Certificate should demonstrate structure, measurement discipline and quantified uncertainty.
It should not simply say a tool is accurate.
It should show:
- How it was measured
- Under what conditions
- With what uncertainty
- And whether it meets the defined limits
That is the difference between a basic verification and a defensible calibration under ISO 6789-2:2017.
Contact us today if you would like to discuss calibration of your torque wrenches.