ISO 16859-2:2015 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.