CALIBRATION OF HARDNESS TESTERS

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

What is meant by calibration?

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.

What is a direct verification?

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.

What is an indirect verification?

For indirect verification, the accuracy of the testing machine is determined using a series of measurements on calibrated test blocks.

The number of test blocks required, the test points to be set, and the requirement of whether the reference indentation of the test block must also be evaluated are all regulated differently based on the standard and the testing method.

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