A matter of balance: calibration and use of weighing instruments

A matter of balance: calibration and use of weighing instruments
ARTICLE
BACK TO BASICS
A matter of balance:
calibration and use of
weighing instruments
In the modern laboratory, where commercially prepared consumables
now predominate, it is important to remember the importance of the
precision. The weighing pan(s) of a highaccuracy analytical balance may be found
inside a see-through enclosure with doors so
that dust does not collect and any air currents
in the room do not affect the delicate balance.
measurement of weight. Here, Peter Riddle concludes his back-to-basics
series with a look at laboratory balances.
Most, if not all, laboratories will have access
to balances, whether this is to weigh kilogram
or milligram quantities. Weighing raw
materials to prepare reagents in clinical
laboratories has now been replaced largely
by the purchase of ready-made reagents,
stains and culture media, but there remain
instances where the ability to weigh with
accuracy and precision is required in
analytical procedures.
The term ‘balance’ is derived from the
original beam balances. These worked by
comparing the unknown weight of a
substance with weights of known mass using
pans suspended from a beam, the pivot point
of which lay an equal distance from each pan.
The mass of the unknown was determined
when the beam was in balance. Beam
balances were either the two-pan variety in
which weights were added by hand, or the
single-pan form in which a system of weights
was added by a selection mechanism. Most
laboratories now use direct-reading electronic
balances where the weight is determined via
a pressure transducer located under the
single pan.
Laboratory staff should be aware that the
weighing operation may be a source of error
in the analytical process and that it may be
difficult to detect. The precision may be
excellent but the accuracy may be poor
because of a failure to detect an error due
to improper balance calibration or drift.
Balances are classified into several categories.
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THE BIOMEDICAL SCIENTIST
Top-loaders are balances with 0.001 g or
1 mg readability and above, where readability
is the lowest possible digit seen on the
display. Analytical balances use enclosed
weigh pans and come in two basic varieties:
micro balances, accurate to 1 µg, and semimicro balances (10 µg).
As the name implies, top-loading balances
accept samples on a pan that sits on top of
the instrument, directly above the weigh cell.
Top-loader capacities range from the low
hundreds of grams to several kilograms, with
specified accuracy of either 1 mg or 0.1 mg.
Top-loading balances are fast, easy to operate
and suitable for all but the most demanding
analytical applications. However, because they
are usually not enclosed, top-loaders are
susceptible to errors due to drafts.
Two-pan analytical balance
These balances consist of a symmetrical beam
and three knife-edges. The two terminal
knives support the pans and a central knifeedge acts as a pivot about which the beam
swings.
Single-pan mechanical balance
Single-pan instruments generally comprise a
beam with two knife-edges, one to support the
weighing pan and the other acting as a pivot.
A fixed counterweight balances the load on
the pan. The displays on these balances tend
to be of the optical variety and the user can
usually adjust the sensitivity of the balance.
ANALYTICAL BALANCE
This type of instrument is
used to measure mass to
a very high degree of
Modern weighing instruments have come
a long way from their balance-beam origins.
MAY 2013
ARTICLE
Use and maintenance
Single-pan electronic balances are easy to
use. Individual requirements will dictate the
degree of accuracy required but a few
general rules should be followed,
particularly when weighing small quantities.
r Do not disconnect the balance from
the power supply, and always leave it
switched on. This allows the balance
to reach thermal equilibrium. To switch
the balance off, use the display key (on
older models the tare key). The balance
is now in standby mode. The electronics
are still energised and no warm-up
period is necessary.
r Always check that the balance is correctly
levelled by checking the level indicator.
r Never weigh directly on the pan surface
but use an appropriate container or
BALANCE LOCATION
Balances must be sited on a firm, horizontal
surface. Many balances, especially those
measuring subgram quantities, will be fitted
with a type of spirit level and adjustable feet
to ensure a true level can be obtained.
A vibration-free balance stand not in contact
with both floor and wall may be required
when weighing extremely small quantities.
The environment must be free of draughts
and this is usually achieved by using an
instrument with a glass draught shield.
This is particularly important for balances
with 1 mg or 10 mg readability. Do not place
the balance next to a door, and avoid places
with high traffic. The effect of draught is
secondary for balances with readabilities
of ±0.1 g.
Temperature fluctuations will also have
a detrimental effect on the weighing process
and most electronic balances now have
some form of automatic temperature
compensation.
Balances should also be protected against
magnetism and static electricity, both of
which have an adverse effect on the accuracy
of the balance. Most manufacturers offer
balances with ‘antistatic’ weighing pans and
with draft shields coated with a substance
that conducts electricity.
Balance levelling and adjustment must
be performed to increase the sensitivity,
accuracy and efficiency of the device.
INSTRUMENT CALIBRATION
The standard for laboratory accreditation
is ISO 15189:2012, which specifies
requirements for quality and competence
in medical laboratories. At its most basic
level, calibration involves comparing a mass
reading against a standard weight. In the case
of a laboratory balance, standard weights are
graded according to class. There are four
ASTM weight classes (1–4), with Class 1
being the most accurate, and Class 4 being
MAY 2013
weighing boat. Always place the weighing
sample in the middle of the weighing pan,
as this will prevent corner load errors.
With micro and semi-micro balances,
the weighing pan should first be loaded
once briefly after a relatively long pause
(>30 min) to deactivate the ‘initial
weighing effect’.
r Always allow the substance to be weighed
to come to room temperature before
weighing.
r If the instrument has a protective shield,
always take the final reading with the
doors closed to avoid draughts.
r The balance should be checked for drift
each day using an external weight, and
a record should be kept in a logbook to
record the performance of the balance
the least accurate. The class of weight to be
used corresponds to the accuracy of the
balance. For laboratory balances, Classes 1
and 2 weights are most commonly used.
As analytical balances are used
infrequently, it is likely that calibration is
not carried out as often as ideally necessary.
Also, how many laboratories confirm the
calibration of a new balance? To calibrate,
you simply need to place a calibration weight
on the pan and record the result. If it is
within predetermined limits then no further
action is required. If the result is not within
limits then the balance should be adjusted to
the value of the calibration weight.
Balances need only be adjusted at a single
point close to the maximum capacity of the
instrument because modern instruments
have a very stable linearity characteristic.
Full calibration will require the balance to
be checked throughout its working range
using certified, traceable weights, although
scheduled checking at low, mid and high
levels should suffice to indicate any
significant drift from true values. These
procedures are usually carried out at service
visits. Analytical balances can be calibrated
externally or internally, as well as manually
or automatically, depending on the model.
Modern balances have internal weights
that can be used to make adjustments, but
the internal weight must be checked against
traceable weights when the instrument is
serviced.
The International Organisation of Legal
Metrology defines the maximal permissible
errors for calibration weights and National
Accreditation of Measurement and Sampling
(NAMAS) requires that weights used for
calibration should be traceable to UK
national standards. However, for the
purposes of good laboratory practice (GLP),
standard certification under BS EN ISO
9000–9002 will suffice. External weights
should be checked every four years.
over time (usually 100 g for a balance
with a readability to four decimal places,
and a 10 mg or 100 mg weight for a
micro balance).
r When handling calibration weights,
always use forceps to avoid contamination
from oils and salts from the hands.
Gravitational effects, due to altitude,
mean that precision balances must be
calibrated and adjusted at the place of use.
Calibration is also affected by changes in
ambient temperature and thus balances
should remain switched on to ensure
temperature equilibrium. If the instrument
has been switched off, or in the event of a
power failure for an extended period of time,
the balance should be switched on for a
minimum of one hour before a weighing is
made. Precision, micro and semi-micro
balances need an adequate time to warm up.
Precision balances need around three hours,
while semi-micro and micro balances need
six and 12 hours, respectively.
REPRODUCIBILITY
Reproducibility refers to the instrument’s
ability to deliver the same weight reading
repeatedly for a given object, and to return
to a zero reading after each weighing cycle.
This is usually stated as the standard
deviation that is determined from six to
10 measured values of a calibration weight.
LINEARITY
This test measures the ability of an
instrument to provide consistent sensitivity
throughout the weighing range. The test
requires several nominally equal weights,
each a fraction of the weighing capacity.
The group together should approximate
the weighing range of the instrument.
For example, a 160 g capacity analytical
balance might be tested with three 50 g
r
weights.
Other articles in this ‘Back to Basics’ series
by Peter Riddle (prriddle@hotmail.co.uk)
are on centrifugation (February, page 76),
liquid-handling devices (March, page 138)
and pH meters and hydrogen ion
concentration (April, page 202).
THE BIOMEDICAL SCIENTIST
273
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