How to measure the inner hardness in a curved surface?

Measuring the inner hardness in a curved surface it is not as simple as it may seems. It has to be measured with a penetration hardness tester but under certain conditions.
When the surface is curved, we have to apply to the obtained measure a correction of the hardness obtained in curved surfaces according to the attached table.
When the penetration hardness tester is used in curved surfaces the tip of the penetrator has a less effective contact which produces a hardness measure lower than the one that the piece really has.
Therefore, to the obtained results with the hardness tester we have to add the value of the attached table depending on the diameter and the result.
Additionally, it is very important to verify that the puncture has been made totally perpendicular to the curved surface.
Evidently, with the use, all the hardness tester lose its calibration, that is the main reason why they have to be recalibrated annually.