The instrument consists of a pendulum which is free to swing on two balls resting on a coated test panel.
The pendulum hardness test is based on the principle that the amplitude of the pendulum's oscillation will decrease more quickly when supported on a softer surface. The hardness of any given coating is given by the number of oscillations made by the pendulum within the specified limits of amplitude determined by accurately positioned photo sensors. An electronic counter records the number of swings made by the pendulum. A transparent acrylic case excludes draughts.
Standard hardness tests relate oscillation damping to surface hardness. The Konig test for hard coatings measures the time taken for the amplitude to decrease from 6° to 3°. The Konig pendulum is triangular with an adjustable counterpoise and swings on two ball bearings of 5mm diameter which rest on the test surface. The counterpoise is used to adjust the period of oscillation to the specified 1.4 seconds.
The Persoz test for soft coatings measures the time for the amplitude to decrease from 12° to 4°. The Persoz pendulum is a square frame with a pointer. It is supported on ball bearings of 8mm diameter and the overall weight is greater than the Konig pendulum.
The times taken in Persoz tests are approximately double those in Konig tests and so the Persoz has a greater degree of discrimination. The Persoz does, however tend to skid on harder surfaces.
The influence of surface smoothness, film thickness, temperature and relative humidity must all be carefully controlled.