Chemical Engineering February 2016 - 35

Facts At Your Fingertips
Weighing Equipment Selection
Department Editor: Scott Jenkins
T
he number of options available
for scales and weighing equipment
can make it difficult to determine
which instruments will offer
the best value and which will meet the
application requirements (Table 1).
Focusing on the following ten areas
can help reduce the time needed to
research available models and help
ensure a good value.
1. Primary use. Identifying the instrument's
primary use is the first step in
selection. Will the instrument be used
for weighing solids or liquids? Will the
device be used at laboratory scale,
or larger? Is it needed to weigh large
quantities of uniformly sized objects,
such as capsules, tablets or small
parts? Will the instrument be used for
weighing moving items on a production
line? Do you need to control conditions
inside a weighing vessel, such
as heating, cooling or mixing?
2. Capacity. What is the largest possible
load that a scale would be required
to handle? Do you need overload protection?
What will the overall footprint
of the scale be and how will the items
being weighed fit within the weighing
area? Would a below-balance setup,
where weight is measured via tension
instead of compression, work for your
application? An unofficial guideline
recommends use of a balance for
samples from microgram levels to approximately
10 kg, and load cells for
those samples from 10 kg to several
metric tons. Try to have the weighed
quantities lie mostly in the middle of
the range of the unit's specific capacity
to minimize stress or damage to
Simple
scale
Counting
scale
Checkweigher
Batch
weighing
systems
sensitive
internal electronics, and also
to ensure greater accuracy.
3. Accuracy. In the context of weighing,
accuracy can be thought of as a
combination of several different factors,
including the quantifiable specifications
of resolution (the smallest
mass change that can be read on a
scale), reproducibility (ability to weigh
consistently over time and with different
operators), linearity (the variance in
accuracy over the weight values within
the scale's capacity) and uncertainty
of measurement (difference between
measured weight and true weight due
to environmental variances).
4. Materials of construction. Basic
materials include aluminum alloy, carbon
steel, aluminum-coated steel and
galvanized steel. For these, cleanliness
and corrosion-resistance are not critical.
When higher levels of cleanability
and chemical and environmental protection
are required, AISI-304 and 316
stainless steels are possibilities.
5. Environment. Environmental conditions
can affect weighing. Large
temperature fluctuations, vibration,
humidity, magnetic fields, air currents,
corrosive chemicals and electrical
interference can all influence weight
measurements, especially at higher
resolutions. Consider whether a particular
environment would require
specialized padding, protective covers,
or more frequent calibrations.
6. Industry regulations. Many industry-specific
regulations exist and may
be relevant for some sectors and not
others. Table 2 contains a list of websites
for several of the organizations
that set standards and regulations.
Table 1. insTRumenT caTegoRies
An appropriate vessel is placed on a weighing pan, the scale is tared accordingly, and a solid
or liquid sample is placed into the vessel
A small known quantity of pieces is placed on the weighing pan. The scale calculates the average
weight of these pieces and stores it in memory. Subsequent unknown quantities of the
object are instantly calculated using this average weight no matter how many are placed on
the pan. Results can be stored or printed for use in record-keeping and other documentation
A checkweigher is the best scale for weighing moving items on a production line, where
speed or the need for 100% inspection prohibits manual weighing. The device weighs each
item on the production line according to preset detection limits, ejecting non-compliant items
and even sorting items based on programmed user criteria. As each unit is inspected, overfilled
and underfilled packages are identified and can be quickly rejected
Scales designed for batch weighing incorporate a load cell, connections, valves, relay hardware
and process-control software into one integrated system. This system is connected
to and controls one or more feed systems that deliver different ingredients into a common
receiving vessel at user-defined quantities to blend or create a reaction
ChemiCal engineering www.Chemengonline.Com February 2016
Table 2. RegulaTions and sTandaRds
WebsiTes
FM Global www.fmglobal.com
ATEX
HACCP
IEC
ISO
NEMA
NTEP
UL/CUL
ETL
CSA
www.iec.ch
www.iso.org
www.nema.org
www.ncwm.net/ntep
www.ul.com
www.intertek-etlsemko.com
www.csa-international.org
7. Features. Additional features can
customize the scale for enhanced flexibility,
ease of use, functionality, protection
and others. Consider whether
your scale would need explosion protection,
internal calibration software,
interfacing ability with a computer
network, wireless connectivity, scale
readouts that are separated from the
weighing platform, multi-language
displays, backlit display for dimly lit
areas, or other needs.
8. Price. Choosing a scale should
never be based solely on price, but
the most expensive scale is not necessarily
the best choice.
9. Installation. When installing, it is recommended
to place scales in a permanent
location and connect to peripheral
equipment, including a remote display.
The resolution and readability should be
set, and an initial calibration should be
performed. For multiple load cells on a
large vessel, a corner load test should
be performed to ensure even weight
distribution. Scales should generally not
be moved from their point of use after
installation, if possible.
10. Calibration and service. Regularly
scheduled calibration of weighing
equipment is necessary, because with
use, normal stress can cause the accuracy
of a scale to drift slightly. A series
of certified test weights are placed
on the weighing platform and the results
recorded. When displayed results
do not correspond to the test weight,
manual or automatic adjustments can
be made to correct the drift.
n
Editor's note: This column is adapted from the following
article: Titmas, R. and Carey, S., Weighing Your Options:
The 10 Most Important Scale Considerations. Chem. Eng.,
December 2007, pp. 61-65.
35
ec.europa.eu/enterprise/atex
www.fda.gov/food/GuidanceRegulation/HACCP/default.htm

http://www.fmglobal.com http://ec.europa.eu/enterprise/atex http://www.fda.gov/food/GuidanceRegula http://www.iec.ch http://www.iso.org http://www.nema.org http://www.ncwm.net/ntep http://www.ul.com http://www.intertek-etlsemko.com http://www.csa-international.org http://www.Chemengonline.Com

Chemical Engineering February 2016

Table of Contents for the Digital Edition of Chemical Engineering February 2016