che_march-2024 - 27

fore, the gravity difference needs to
be considered when use the reference
weights to calibrate and adjust
your weighing instruments.
Air buoyancy. When weights are
being used, air buoyancy results in
a small force that reduces the force
of the mass. So, in effect, air buoyancy
lifts the weights up very slightly.
The effect of air buoyancy depends
on environmental conditions and on
the differences of the density of the
weights compared to air density. Because
the effect of air buoyancy is
relatively small, and does not change
that much from day to day, it is not
always considered.
Effect of convection. If the temperature
of the weight differs from the
environmental temperature, there
will be an air convection around the
weight. The wider the temperature
differential, the larger the air convection
will be. This convection will have
a small effect on the indication of the
weighing instrument.
For example, if the weight is colder
than environment, the air convection
around the weight will move downward
facing the weighing instrument
and cause small additional weight to
the weighing instrument. The effect
of convection is more relevant with
high-accuracy instruments, although
it is still relatively small.
Substitution loads
Substitution loads are items that
can be used for help in calibration
if the available weights are insufficient
for the calibration. An example
is the case where the calibration of
a 1,000-kg weigh scale is required,
but only one weight of 100 kg and
two weights of 200 kg (total 500
kg) are available. To address this,
the procedure would be to calibrate
the first points up to 500 kg using
the weights you have, then read the
exact indication of the weighing instrument
with the 500-kg load. The
weights would then be removed
and a substitution load would be
placed on the weighing instrument
until the indication matches readout
when the 500-kg weights were
on the scale. When the substitution
load has the same measured weight
value as the available accurate reference
weights, you can then continue
by adding the existing reference
weights to the load surface that is
still holding the substitution load to
reach 1,000 kg target.
Calibration certificate
An essential part of calibrations is
to document the results with a calibration
certificate. The certificate
includes the measurement results
of the calibration (that is, the mass
of the weights placed on instrument
and the indication of the instrument).
The certificate should also include
the total uncertainty of the calibration.
Measurement values without
related uncertainty do not tell very
much about the measurement. The
certificate should also include a clear
description of the instrument being
calibrated and the weights being
used as reference. The calibration
procedure that was followed should
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