webster hardness tester model

W-20: the most popular model, used to test normal aluminum profiles.
W-20a: used to test aluminum profiles with thickness within 13 mm.
W-20b: used to test aluminum tubings with inner diameter over 6 mm.
W-B75: used to test brass tubings and brass sheets.
W-BB75: used to test copper tubings and copper sheets.
W-B92: used to test soft stainless steel sheets, cold-rolled steel, etc

detail diference as below :