Compared with dead weight-type hardness testers, closed-loop tester systems offer unmatched performance reliability, stability and flexibility.
For more than 75 years, the most common hardness-testing instruments in industry used dead weights to apply the test forces. Benefits of using dead weights include their low cost and the relative ease of manufacturing them to the degree of accuracy required by commonly used test methods. However, their use also presents a problem; that is, the force must be applied to the test piece through some type of small indenter, such as a diamond or ball indenter. The transfer of a dead weight force to the tip of a small indenter is difficult to accomplish, especially a force as great as the 150 kgf, or 1471 N (330 lbf), used for a Rockwell C scale (HRC) hardness test, for example. Because of the large size of a 150-kg weight, it is necessary to design the testing instrument using smaller weights and levers to magnify the force to the required levels. However, the levers require pivots, guides and other friction-producing elements that induce errors. Instrument manufacturers have done an excellent job trying to control these sources of error, but any friction point in the system has a negative effect, which increases during use.