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perfectly controlled, cannot turn out identical products, so the
measurement process comes with errors of measurement that fluctuate
from one result to the next. That is why it is necessary to attempt to
monitor and control the measurement processes.
Principle
The objective of this method is to place and then maintain the
process under “statistical monitoring”: the dispersions of the results
that are observed are only due to the random fluctuations of the
instrument or of the environmental conditions, and sometimes to the
operator’s initiatives, but not to attributable causes that can be
controlled. From a statistical point of view, it can be said that the
samples represented by the series of measurements of the same object
are extracted from the same population and so have the “same mean”.
In order to monitor the measurement process, the different causes of
variability will be examined when the system of control charts is set
up. These causes come from:
– the medium, and the environment;
– the method of measurement and the measurement procedure;
– the measured quantity (measurand);
– the operator;
– the means (instruments, standards, etc.).
Check standard
Monitoring standards have to be used to implement these
techniques. A provisional definition of monitoring standards may be
measuring instrument, material measure, or product, whose function is
to generate or achieve the value of a quantity in a stable way in time.
These standards are used at regular intervals to ensure a statistical
control of the measurement processes. Several examples illustrate this
concept. A frequent measurement in a laboratory is the measurement
of direct current. In order to monitor the digital voltmeters, or the
automatic measurement sequences, a voltage reference can be used; it
can be introduced on different measurement benches to monitor them
(by the connection of the reference tension generator to a channel of a
channel scanner).
These techniques are applied in the field of dimensional
metrology. It is possible to associate one “check standard”, or even
several, to each measurement bench. These standards are of the same
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