Chemical Engineering June 2019 - 47

dividual requirements. Apart from providing traceable
calibration for the installed base, the calibration
partner also supplies precise and comprehensive
calibration certificates.
Traceability in accordance with international standards
is a requirement from ISO 9001, applicable in
the chemical industry. This is set out in section 7.1.5
(monitoring and measuring resources) of the standard.
Calibration of the reference instruments must
be carried out with traceability in mind (Figure 5).
If the service partner is ISO 17025 accredited, the
traceability of the calibration is guaranteed. Without
accreditation however, the calibration partner has
to separately demonstrate proof of traceability. That
means first and foremost being able to provide the
calibration certificates for the reference instruments
used. In addition, not only the reference device must
be traceable, but also comprise documentation of
measurement uncertainty, procedure, technical competence,
metrological traceability to the SI (Système
international or international standard) units, and calibration
intervals (see ILAC P-10:2002).
Time-saving methods
Calibration requires some amount of time. Calibration
runs that can be arranged as part of regular
maintenance works or scheduled downtimes will
not affect manufacturing, but experience shows that
this is a desired, but seldom achieved, scenario.
This is precisely the reason why calibration should
always be performed in a time-optimized manner.
On-site calibration (Figure 6), for example, reduces
the coordination effort as there is no need to remove
and send the instrument to a calibration facility. Of
course, traceability must be given even under these
conditions. Even necessary adjustments or repairs of
a device may under certain circumstances be carried
out on site.
Some instrument types can also be calibrated in
line. In this case, the instrument remains mounted
and installed in its working position. The difficulty in
this scenario lies in installing a reference meter in the
process line, up- or downstream of the instrument
to be calibrated, such as a flowmeter. This again will
usually interrupt production, simply because lines
have to be opened to install the reference instrument.
In addition, the flowmeter of our example is
usually calibrated with water, which might require
rinsing of the original media first.
Another time-saving method is the multiple calibration
of instruments such as thermometers, which
involves the use of an oil bath. Less common are
self-calibrating thermometers that ensure long-term
measurement consistency by means of a highly precise
and permanently stable internal fixed-point reference.
This requires neither instrument removal nor interruption
of the process. This type of calibration is particularly
advantageous in hygienic and aseptic applications.
Unique situations can arise with special types of
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Chemical Engineering June 2019

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