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September 30, 2019
Your tool is working fine, but how do you know it is measuring accurately? That’s where calibration comes in.
Calibration is determining and documenting the difference in readings given by a tool compared with a reading given by a measuring standard (fixed device that is known to be accurate). It is generally followed by the adjustment of the tool.
The process checks whether the values are within an acceptable range (tolerance level) in relation to the specific application. If not, then the equipment must be adjusted to make sure that the deviation between the actual and the required value is minimum, and that the tool will have its original performance accuracy restored, giving more precision. A calibration check can be done after adjustment of the tool.
The performance of tools and instruments tends to drift over time and every tool loses accuracy to some extent. The following example shows how the torque of a BlueTork nutrunner, which is used to tighten the bolts on truck wheels, can vary over a few years under normal operating conditions.
Although the torque value is the same, the actual measured value is different. In the first couple of years that value is still within the tolerance interval, but at year three, it falls outside the tolerance interval and the nutrunner must be adjusted to ensure it continues to give an accurate result.
Calibration is essential to maintain tools’ accuracy and repeatability over time.
At the end of the calibration process, a calibration certificate must be delivered by a laboratory.
Note: a test certificate is different from a calibration certificate. Test certificates only show the performances of the tool at a given point of time.
Only laboratories complying with ISO/IEC 17025 can deliver a valid calibration certificate. You can contact your local accreditation organism to get the list of all accredited laboratories. As an example, please find below some existing lists for the American market:
| ISO Standard… | … defines |
| ISO 9001 | Quality management systems accreditation given to companies that prove they follow and adhere to processes including managing traceability and calibration. |
| ISO 5393 | Performance test method of rotary tools for threaded fasteners for the measurement of torque repeatability (torque scatter) |
| ISO 6789-2 | The calibration procedure for manual torque wrenches |
| ISO/IEC 17025 | General requirements for the competence, impartiality and consistent operation of laboratories performing calibration |
The quality standard ISO 9001 does not specify the calibration period and frequency depends on several factors: for example, the use of the tool and the environment. Calibration may need to be carried out as often as daily, or as little as yearly (for example, manual torque wrench calibration is recommended once a year or every 5000 cycles – whichever comes first).
So, ultimately, it is up to the maintenance manager to specify a reasonable calibration interval as they understand their job and the tools’ application better than anyone else.
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