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The Calo tester, also known as a ball craterer or coating thickness tester, is a quick, simple and inexpensive piece of equipment used to measure the thickness of coatings. Coatings with thicknesses typically between 0.1 to 50 micrometres, such as Physical Vapor Deposition (PVD) coatings or Chemical Vapor Deposition (CVD) coatings, are used in many industries to improve the surface properties of tools and components. The Calo tester is also used to measure the amount of coating wear after a wear test carried out using a Pin-on-Disc Tester.
The Calo tester consists of a holder for the surface to be tested and a steel sphere of known diameter that is rotated against the surface by a rotating shaft connected to a motor whilst diamond paste is applied to the contact area. The sphere is rotated for a short period of time (less than 20 seconds for a 0.1 to 5 micrometre thickness) but due to the abrasive nature of the diamond paste this is sufficient time to wear a crater through thin coatings.
Calculating coating thickness using the Calo tester
Where t = coating thickness, d = diameter of the sphere x = difference between the radius of the crater and radius of the part of the crater at the bottom of the coating x+y = diameter of the crater 
- "Coating thickness tester (Calo® test or ball crater test) - This website gives details of the equipment used to measure coating thickness". 28 June 2009. Archived from the original on 2009-06-28.