Calibration & Standards

When relevant measurements are taken on components, the standard-compliant functionality and accuracy of the hardness testing machine and all measurement-related components must be ensured. The hardness testing standards (ISO, ASTM) define clear rules for how this must be demonstrated and monitored. This requirement for monitoring applies to every test method used and every degree of loading.

As an initial check of the accuracy of a hardness tester, for example for new machines or after changes have been made, calibration is required.

The precision of the hardness testing machine must also be checked regularly as part of a periodic daily or weekly test​​​​​​​, even after calibration. How these tests are performed and the permissible deviations of the results are precisely defined for each test method and series of standards (ASTM, ISO).

All standard-compliant tests performed for monitoring the hardness tester must be documented.

• Calibration means that a measurement is performed in which the measured object (test specimen or calibration object) is compared with the values of a better and traceable standard according to a documented method.
• The result of the calibration is expressed with deviation and measurement uncertainty so that the calibration result is also traceable.
• The calibration is intended to ensure that the hardness tester, which is also relevant for the product quality, produces the “right” results.

• Growing demands on materials and manufacturing technologies demand the use of precision testing machines.
• Inaccurate measurements can have costly economic consequences.
• In order to be able to guarantee process reliability, hardness testing machines must also be inspected at regular intervals. This requirement is also described in ISO 9001.

• In principle, any company can carry out calibrations themselves.
• The precondition, however, is that the measuring equipment used is traceable to international or national measurement standards and that the tests/calibrations are performed according to the requirements of the standards.
• The international standards therefore recommend/propose that these tests are performed exclusively by accredited  calibration laboratories (in accordance with ISO/IEC 17025).
   

• An accredited calibration laboratory (measured variable hardness) has the formal approval for calibrating hardness testing machines according to precisely defined parameters and measuring conditions.
• The binding quality specifications of ISO/IEC 17025 apply.
• The quality management system is constantly monitored by experts, in Austria these are e.g.  appointed by the Federal Ministry of Science, Research and Economy.

The calibration process is defined in the standard as follows:

• Assessment of the general conditions
• Direct testing
• Indirect testing
• Drawing up of the calibration certificates

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A direct verification of the hardness testing machine directly checks all measurement variables used for determining the hardness value. The measurement equipment used for this must be properly traceable to national standards. In practice, direct verification is performed as part of calibration by an accredited calibration lab (service provider.)

Depending on the method, the following measurement variables of the testing device are checked:

• Test force
• Indenter geometry
• Measurement system (depth measurement system or measurement system for determining the indentation diameter)
• Test cycle

Direct verification must be performed prior to initial commissioning of a hardness testing machine, after repairs and modifications, as well as whenever indirect verification fails. Direct verification is also required if the most recent indirect verification was performed outside of the cycle defined in the standard.