[2] The United States Army Air Corps required a hand-held method of checking the hardness of rivets due to concerns that aircraft could be sabotaged by replacing normal rivets with soft lead or wooden ones which would fail during flight.
[3] The hardness of the material determines how far the tip indents and this is transferred by a tension spring and lever to be read on a dial.
[3] The Barcol hardness test is generally used on soft materials such as rigid plastics.
It measures hardness based on indentation of a sharp point with a flat tip.
[4][6] As defined in ASTM D 2583 the scale divisions from 0-100 should each indicate a depth of 0.0076 mm or the equivalent 0.0003 inches.