EN - 21
5
Preparation
Thorough preparation of the
sample‘s surface ensures high accuracy. The sample must not be magnetic. The
surface temperature must be below 120 °C.
5.1
Weight requirements
To get correct measurement values, the sample should be as thick, heavy and solid as possible. The hardness of
the surface to be hit by the impact body should be consistent.
A solid sample over 5 kg can be tested without any special preparation or base.
A sample between 3 and 5 kg should be fixed to a rack or support of more than 5 kg to avoid the sample from
bending, deforming or moving during the hardness measurement.
If the weight of the sample is below 2 kg, it should be fixed to a workbench or a stable support. The surface between
the sample and the support should be hard, clean and even. To attach the sample safely, apply petroleum jelly or
yellow grease to the surfaces of the sample and support, press the sample firmly onto the support and move it back
and forth to make sure there is no more air between the sample and the support.
5.2
Coupling
For coupling, some experience is necessary. When samples are coupled insufficinetly, there are often substantial
differences between the individual measurement results, low HL values and a rattling noise when the measuring
tip hits the surface.
The surface of the sample and that of the base / support should be flat, even and polished.
The impact direction should be vertical to the surface of the coupled sample.
The minimum thickness of a sample for proper coupling is 3 mm.
5.3
Roughness requirements
To eliminate measurement errors which are due to excessive roughness of the tested surface, the surface should
be polished, so that it has a metallic gloss. The roughness should not exceed 2 µm. The rougher the surface of the
sample, the lower the result of the hardness test will be.
5.4
Cleanliness requirements
To ensure high accuracy, the sample must be clean and free from oil stains, rust and residues of electroplating and
paint.
5.5
Stability requirements
To prevent the sample from moving during the measurement, the sample should rest on the base firmly enough and
its surface should be vertical to the impact direction. The test surface can deform or vibrate due to the impact of the
impact body. This may even apply to samples with the right weight and a suitable thickness.
The measured hardness might be lower than it would normally be. This applies in particular to samples like big
plates, long bars or samples with curved surfaces. Please consider the following recommensations for these types
of samples.
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