Chemical Engineering July 2023 - 24

Facts At Your Fingertips
Standard Masses for Calibration of Weighing Instruments
Department Editor: Scott Jenkins
A
ccurately and efficiently weighing
raw materials and reagents
is a key part of many chemical
processes, research projects and
laboratory analysis. Poor weighing
accuracy can contribute to lost revenue,
increased labor costs and lower
product quality. Calibration of weighing
instruments with standardized
weights can establish a relationship
between a known value (standard)
and a measured value. This onepage
reference provides information
on the weights that are used to calibrate
weighing instruments.
Metrological traceability
Metrological traceability refers to a
property of a measurement whereby
the result can be related to a reference
through a documented unbroken
chain of calibrations. A metrologically
traceable calibration allows users to
know how accurately a weighing instrument
is measuring. Each country
has a designated National Metrology
Institute (NMI), which develops and
maintains national standards of measurement.
In the U.S., the NMI is the
National Institute of Standards and
Technology (NIST; Gaithersburg, Md.;
www.nist.gov).
According to NIST, its job in this
area is twofold: " to ensure U.S. national
standards are accurate realizations
of the units of the international
system (SI) of units, and to transfer
the values of those standards to the
U.S. measurement system through
calibrations, reference materials and
other measurement services. "
Direct users of NIST's measurement
services tie their internal measurement
standards to NIST standards
and hence, to the SI units. These users,
in turn, implement their standards
to provide measurement services to
their customers, to meet regulatory
requirements, and to provide quality
assurance in their manufacturing processes,
NIST says.
Test weights
Calibration weights are standardized
masses used to check the accuracy
of a weighing instrument. Reference
24
weights are ultimately
traceable to the consensus
value of the kilogram (CV).
In the past, the kilogram
has been defined by the
International Prototype Kilogram
(IPK), but in 2019
the kilogram was redefined
based on the fixed numerical value
of the Planck constant [1]. Doing so
gives (in principle) any NMI the potential
to realize the kilogram. The NIST
Mass and Force Group disseminates
the kilogram to the U.S. measurement
system through calibrations of
customer weights and weight sets.
According to NIST, the organization
maintains traceability to the CV using
its NIST-4 Kibble Balance, which
is used to transfer the CV to the NIST
Pool of Mass Standards. The pool
consists of platinum, iridium and
stainless-steel artifacts that are used
to create working standards [2].
Class
ASTM Class 1
(1 g weight)
ASTM Class 6
(1 g weight)
Tolerances
Test weights are classified based
on tolerances. Weight tolerances
can be thought of as the maximum
permissible error for a measurement
(Table 1). Weight tolerance indicates
how much deviation is allowed in the
mass of the weight, while still being
considered an accurate representation
of that mass [3].
Two common classification systems
for reference calibration weights
are maintained by ASTM International
(West Conshohocken, Pa., www.
astm.org) and the International Organization
for Legal Metrology (OIML;
Paris, France; www.oiml.org).
The OIML classes are designated
as E1, E2, F1, F2, M1, M2 and M3.
E1 has the tightest tolerance, while
M3 has the widest. Most laboratory
applications require OIML weights
of Class F2 or below. OIML class
weights are more commonly used in
Europe and Latin America.
ASTM International has developed
10 calibration weight classes, as
guided by document ASTM E 617 [4]:
ASTM Class 000 thru ASTM Class
7. Class 000 has the tightest tolerance
and Class 7 the widest. Most
TABLE 1. DIFFERING TOLERANCES FOR ASTM CLASS 1
REFERENCE WEIGHT VERSUS CLASS 6 WEIGHT.
Weight Tolerance " In-Tolerance " Range
0.034 mg
2.0 mg
0.998 g - 1.002 g
laboratory applications require ASTM
weights of Class 4 or below. ASTM
class weights are the most common
type of calibration weights used in
U.S. laboratories.
NIST field standard weights are
used primarily to test commercial
weighing devices for compliance
with commercial requirements. These
tests for calibration purposes can be
performed by in-house technicians,
maintenance workers, local weights
and measures officials, device installers
and service technicians [5].
Handling reference weights
Weights should be handled in ways
that avoid changing their metrological
characteristics. The weights should
not be placed on surfaces that will
cause scratches or on dirty surfaces.
When cleaning weights, special attention
should be paid to proper cleaning.
Smaller weights should be handled
with gloves (not bare hands) to
avoid any finger grease from getting
onto the weights and to avoid warming
the weights to a higher temperature
than the environment. When not
used, the weights should be stored in
their original storage boxes and only
authorized personnel should have access
to them. The temperature of the
weights should be stabilized to the
same temperature at which the calibration
is to be done [6].
n
References
1. Jenkins, S., Facts at your Fingertips: Redefining the Kilogram
Standard, Chem. Eng., September 2018, p. 30.
2. Abbott, P., Calibration of Mass Standards, National Institute of
Standards and Technology, www.nist.gov, accessed May 2023.
3. Troemner, Weights Reference Center, www.troemner.com,
accessed May 2023.
4. ASTM International, ASTM E617-18 Standard Specification for
Laboratory Weights and Precision Mass Standards,
5. Barton, J.W. and othersl., Specifications, Tolerances and other
Technical Requirements for Weighing and Measuring Instruments,
NIST Handbook 44, section 2, NIST, 2021.
6. Beamex Inc., Weighing Scale Calibration, white paper, Beamex,
www.beamex.com, accessed May 2023.
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
JULY 2023
0.999966 g - 1.000034 g
http://www.nist.gov http://www.astm.org http://www.astm.org http://www.oiml.org http://www.nist.gov http://www.troemner.com http://www.beamex.com http://WWW.CHEMENGONLINE.COM
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