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instruments. In the case of equipment such as heavy machinery
(traction, compression, hardness, etc.), scales, air conditioning
chambers, etc., the verification can only be done on-site. It is not
necessary for the provider to have their own laboratory since the
whole intervention is carried out onsite. However, the provider must
use working standards which are related to the calibration chains.

Calibration requirements
Several problems come to mind when thinking of calibration.
First of all, how can a particular measuring instrument be calibrated?
If it is a calliper, you will think about using gauge blocks. Has anyone
even considered measuring rods for a micrometer? What is to be done
with dynamometric spanners, balances, etc.? If you go into physical
chemistry, etc. it gets even more complex! Some methods of
measurement demand equivalent methods of calibration. Fortunately,
some manufacturers of materials provide tips. When you look deeper
into the matter, you realize that quite often you talk about calibration,
but what you actually need is a verification, perhaps even a
metrological confirmation (see ISO 10012 standard). Therefore, it
might be necessary to proceed to an internal checking between two
interventions, which is just a simplified examination of good working
order. Calibration must be done intelligently, which means doing just
what is necessary; it is not only a means to avoid auditor’s critical
views. How many firms, which work in mechanical engineering and
have their sets of gauge blocks calibrated in an accredited calibration
laboratory simply open their calibration certificate? Never must it be
forgotten that the major purpose of calibration is to verify the
measuring instrument and calculate the uncertainties that go with the
results of the measurements taken with that instrument. The question
of the interval of the calibrations inevitably arises quickly. The answer,
which should make everybody happy, is that it depends. Some
methods of measurement meet a few demands, particularly in the field
of physical chemistry. In any event, measuring instruments should be
calibrated reasonably frequently, so as to detect and prevent any
possible drift, but not too often because of the overall cost involved.
The reader should wary of any person who claims that they can
tell which intervals are the right ones. As a matter of fact, you always
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