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1: Because of the shape of the indent, you can get close to the surface, yet still retain a higher testing load (like 500g), which increases accuracy of your measurements. This is especially important when measuring the hardness of stuff 50HRC and above and you are taking measurements manually (unless you have a very expensive tester, unfortunately we don't). With case hardened steels, 3 thousands is a comfortable starting point for 500g, otherwise you can take it to 2 thousands from the surface with 200g and still retain a measurement that satisfies ASTM specs. 2: I'm lazy and would much rather make 1 measurement than 2 when I have to do dozens of impressions, haha! But you are correct in that Vickers is more of a global scale, and Knoop maybe pertains to more Americans than anyone else.
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