che_march-2024 - 28

more detail.
There are several
sources
of uncertainty
for the error in
weighing instrument
calibration,
including the folShutterstock
FIGURE
7. Several factors can give rise to uncertainty when scales are used
for normal weighing
ditions during the calibration should
also be included in the certificate.
In summary, a calibration certificate
should include the following information:
*
Measurement results
* A unique certificate number
* User company name, address and
identification
* Name, signature and company of
the person who did the calibration
* Detailed information on the instrument
that was calibrated
* Identification of the reference
weights being used
* Identification of the calibration procedure
being used
* Calibration date
* Environmental conditions
* Measurement uncertainty and its
coverage factor
* Mention of the case when only a
partial calibration was done
* A graphical representation of the
calibration results if possible (useful
visual component)
If the calibration is accredited, then
the regulation will stipulate the contents
of the certificate.
Uncertainty
In weighing instrument calibration, the
uncertainty of the calibration should
be known. Knowing the error of the
scale indication at each calibration is
not sufficient. Users must also know
the uncertainty about the error found
at each point of calibration. This section
will discuss uncertainty at a more
general level, rather than providing
the details of how to carry out uncertainty
calculations. Refs. 5 and 6
cover the uncertainty calculations in
28
lowing (Figure 7):
* The masses of
the weights are
only known with
a particular uncertainty
*
Air
convection
causes extra
force on the load
receptor
* Air buoyancy around the weights
varies according to barometric pressure,
air temperature and humidity
* A substitute load is used in calibrating
the scale
* Digital scale indications are
rounded to the resolution in use
* Analog scales have limited readability
*
There are random variations in the
indications as can be seen in the repeatability
test
* The weights are not in the exact
center of the load receptor
The use of weighing instruments
during routine operation can differ
from use of the instrument during the
calibration process. This also gives
rise to additional sources of uncertainty.
Examples of these differences
include the following, for example:
* Routine weighing measurements involve
random loads, while calibration
is made at certain calibration points
* Routine weighing measurements
are not repeated, whereas indications
received through calibrations
may be averages of repeated weighing
measurements
* Finer resolution is often used in
calibration than normal weighing
* Loading and unloading cycles in
calibration and routine weighing may
be different
* A load may be situated eccentrically
in routine weighing
* A tare balancing device may be
used in routine weighing
* The temperature, barometric pressure
and relative humidity of the surrounding
air may vary
* The adjustment of the weighing instrument
may have changed
Standard and expanded uncertainties
of weighing results are calculated
using technical data of the
weighing instrument, its calibration
results, knowledge of its typical behavior
and knowledge of the conditions
of the location where the
instrument is used. Calculating the
uncertainty of the weighing results
assists in deciding whether or not the
accuracy of the weighing instrument
is sufficient and how often it should
be calibrated.
n
Edited by Scott Jenkins
References
1. EURAMET e.V. Technical Committee for Mass and Related
Quantities, EURAMET Calibration Guide No. 18, Version 4.0,
EURAMET e.V., Braunschweig, Germany, November 2015.
2. Butcher, T.G. and others, NIST Handbook 44 (2023 Edition)
- Specifications, Tolerances, and Other Technical Requirements
for Weighing and Measuring Devices, National Institute of Standards
and Technology (NIST) Office of Weights and Measures,
Gaithersburg, Md., 2022.
3. International Organization of Legal Metrology (OIML), OIML
Standard R76-1, Non-automatic weighing instruments Part 1:
Metrological and technical requirements - Tests, 2006 edition,
OIML, Paris, France, www.oiml.org, 2006.
4. BSI Group, EN 45501:2015, Metrological aspects of nonautomatic
weighing instruments, British Standards Institution, 2015.
5. European Co-operation for Accreditation (EA), EA-4/02, rev. 3,
Evaluation of the Uncertainty of Measurement in Calibration, EA,
Paris, France, www.european-accreditation.org, April 2022.
6. International Bureau of Weights and Measures (BIPM), JCGM
100:2008, Evaluation of measurement data: Guide to the expression
of uncertainty in measurement, BIPM, Paris, France,
2008.
Additional resources:
* OIML, OIML R111: Weights of classes E1, E2, F1, F2, M1,
M1-2, M2, M2-3 and M3, www.oiml.org.
* Council of the European Union, Directive 2009/23/EC,-Nonautomatic
weighing instruments, 2009.
Authors
Ned Espy, technical director, has
been promoting calibration management
with Beamex Inc. (2152
Northwest Parkway, Suite A, Marietta,
Ga.; Email: ned.espy@beamex.com;
Phone: 770-951-1927)
for over 20 years. Espy has helped
develop best practices for calibration,
with a focus on pressure, temperature
and multivariable instruments.
He is a consistent editorial contributor to leading
industry publications, and has received significant recognition
within the automation industry. Today, Espy teaches
calibration best practices and provides technical support
to end users and the Beamex sales team.
Roy Tomalino has been teaching
calibration management at Beamex
(same address as above; Email: roy.
tomalino@beamex.com) for over
20 years. He has taught on four different
continents to people from
over 40 countries. His previous
roles include Technical Marketing
Engineer and worldwide trainer for
Hewlett Packard and Application
Engineer
with
Roy is a Beamex Calibration Evangelist. His primary mission
is to demystify calibration and teach best practices.
Honeywell. Today,
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MARCH 2024
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