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The Rockwell hardness test is usually thought of as a method that measures
the hardness of a material. A more accurate description might be that the
Rockwell hardness test only provides an estimate of the absolute “true"
hardness value. It is only an estimate since, like essentially all measurement
systems; there is always some level of error in a Rockwell hardness
measurement. For a hardness measurement to be useful, the level of error
must be small enough to meet the user’s needs.
The total error in a measurement is often the result of a combination of
errors from multiple sources. In the case of a Rockwell hardness machine,
errors associated with machine components, testing cycle variations, and
environmental conditions, as well as other sources, contribute in varying
degrees to the overall measurement error. When it is practical, the
measurement result should be corrected for these errors. However, in many
cases, the errors may occur randomly and cannot be corrected. In other
cases, the errors may be systematic, but there may be valid reasons for not
correcting these errors. Even when corrections are made to compensate for
the errors, there will be an additional error associated with the correction.
These uncorrected errors then account for an “uncertainty" in the accuracy
of the measurement result. To have confidence that the result of a hardness
measurement is appropriate for a particular application, some understanding
of the level of uncertainty in the measurement must be known.
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