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For Rockwell hardness machines that have successfully passed both direct
and indirect verifications, there will continue to be some level of measurement
error or bias with respect to the reference standards. If this bias is felt to be
significant for the user’s application, then it may be advantageous to make
mathematical corrections based on the certified values of the reference test
blocks. Presently, ASTM and ISO test methods do not address making
mathematical corrections, although, in practice, a form of mathematical
correction is commonly made for newer Rockwell machines capable of being
electronically calibrated. These machines can determine correction curves by
comparing certified test block values with values measured during an indirect
verification. The curves are electronically stored in the Rockwell machine
and correct future measurement values based of this curve. In general,
mathematical corrections should only be made in cases where the Rockwell
machine has been successfully verified.
A practical method for applying corrections for the measurement biases is to
determine a linear calibration curve for the entire hardness scale. The linear
calibration curve provides a correction value to be applied to future hardness
measurements at any hardness level of that scale. The correction value is
dependent on the Rockwell scale and hardness level of the material under test.
A linear correction curve is chosen because indirect verifications are usually
made with test blocks at only three levels of hardness for each Rockwell scale
A curvilinear fit to only three data points often can produce impractical results
at hardness levels other than the three verified levels. A separate and distinct
correction curve is required for each Rockwell scale.
Figure 9 graphically illustrates how the biases can be corrected for a single
Rockwell scale. The left axis represents the difference between testing
machine measurements made on reference test block standards and the
certified values of the reference test blocks. This is the error or bias determined
as part of an indirect verification (average of machine measurements minus
certified reference value). The bottom axis represents the hardness level that is
measured. In this example, the round points indicate the bias values determined
by testing three reference test block standards. A linear fit is made to the three
bias values. The correction to be applied to future measurements is determined
from the value at each point along the linear fit line. For example, when a
future measurement is made at 25 HRC, the correction would be to add
approximately 0.25 HRC to the measured value. The square data points and
dashed line illustrate the result of applying these corrections to the bias values.
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