This part of ISO 16859 specifies methods for direct and indirect verification of test instruments used
for determining Leeb hardness in accordance with ISO 16859-1, and also describes when these two
types of verification are to be performed.
The direct verification involves checking that individual instrument performance parameters fall
within specified limits, whereas the indirect verification utilizes hardness measurements of reference
test blocks, calibrated in accordance with ISO 16859-3, to check the overall performance of the
instrument for testing in the direction of gravity. The indirect method can be used on its own for the
periodic performance checking in service.