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. These instruments meet the requirements of ISO 1522, ASTM 4366, DIN 53157, and BS 3900: E8.

The Persoz test measures the time taken for the amplitude to decrease from 12° to 4°. The Koënig measures time taken from 6° to 3°.

The times taken in Persoz tests are approximately double those in Koënig tests and so the Persoz has a greater degree of discrimination. The Persoz does, however, tend to skid on harder surfaces.

The instrument is supported on a heavy cast iron base for stability. The apparatus must be leveled. The base is fitted with leveling screws, and a spirit level is mounted to the test panel support platform. The mechanism to support the test panels is common to both tests. A platform, onto which the test panel is placed, is lifted, by means of a remote lever. As it is lifted, the bearings of the pendulum come into contact with the test surface and the weight of the pendulum is taken by the sample.

In addition to the zero point, the scale is marked at the angles relevant to the test specifications, i.e. 12 and 4, or 6 and 3, or all four angles. At these limits, a photo-sensor is accurately positioned to register swings of the pendulum. The pendulum is cocked with a swinging arm. It is held in its start position by a lock pin, which is released by a flexible cable trigger.

The hardness of any given coating is given by the number of oscillations made by the pendulum within the specified limits of amplitude. The signals from the photo-sensor are registered by an automatic electronic counter, and the result of a test is shown on the digital display. The influence of surface smoothness, film thickness, temperature and relative humidity must all be carefully controlled, therefore, the instrument is housed in a perspex case.