For accurate measurements it is necessary to determine the area function of the indenter and to check its validity in short intervals because also diamond is wearing.
The standard DIN EN ISO 14577-2 annex C distinguishes between direct and indirect calibration methods.
In the direct method the indenter is scanned with another instrument, mostly with an Atomic Force Microscope (AFM), and the projected area is calculated in dependence on the outermost tip.
This method is not often used because another accurate instrument is necessary.
In the indirect method the projected area is calculated from load-displacement measurements of a reference material with well-known modulus and Poisson's ratio.
The reference material further should have a very small roughness, high homogeneity and should not show pile-up or sink-in effects. Mostly fused silica is used.
The result for the area function must not be material dependent. The only possibility to check this is the measurement of another reference material with distinctly different modulus.
For the ZHN sapphire single crystal is used for that. For both materials the area functions must agree.
The software of the ZHN is the only one which is prepared for the use of two reference materials and their comparison.
The comparison further enables the calibration of the instrument stiffness or compliance in the same procedure.
Further radial displacement correction and variable epsilon factor are used by default.