**GMP 10**

**Good Measurement Practice**

**for**

**Understanding Factors Affecting Weighing Operations**

Good laboratory techniques and practices, as well as good standards and
balances, are required to obtain good mass measurements. Errors can
arise from a variety of sources, such as balance drift, air currents,
lack of thermal equilibrium, magnetic or electrostatic fields, and to
manipulative skills using the balance. The magnitude and significance of
these errors depen­ds upon the laboratory environment, the balance, and
the accuracy required for the measurement. Different methods of weighing
are used to eliminate or reduce the effects of sources of error.
Particular sources of error and ways to eliminate the errors are
discussed with the weighing proce­dures. The accuracy required for a
measurement and the criticality of the measurement often influence the
choice of the weighing method used in the measurement. Regardless of the
procedure used, several practices should be followed to promote good
measurements.

[Environment and Preparation]{.underline}

First, the balance should be installed in a laboratory having good
temperature and humidity control. The requirements for temperature
stability increase as more precision is needed in the measurement. For
high precision measurement, the temperature should not vary by more than
± 1 °C in a 24 h period throughout the year and should vary by less than
0.5 °C during the course of any one measurement series (typically less
than 1 h). General laboratory measurements can probably withstand
temperature variations of ± 2 °C per day. Varying temperatures result in
balance drift and in unequal temperatures for the air, balance, and
objects to be weighed. The relative humidity should be maintained
between 40 % and 60 %. If the relative humidity falls significantly
below 40 %, electrostatic discharge may be generated both in and near
the balance. This can result in erratic readings and make precision
weighing impossible. If precision mass standards are cleaned, they
should stabilize in the laboratory environment for seven to ten days.

Thermal and environmental equilibrium are critical for mass
calibrations. This is why each mass SOP will provide instruction that
the environment must be stable and the weights set in or near the
balance for 24 hours prior to a calibration. The test objects, balances,
reference standards, and air should all be in thermal equilibrium. When
possible, the objects to be weighed and the refer­ence standards should
be placed either in the balance chamber or adjacent to the balance so
all can come to the same thermal equilibrium. If there is a lack of
thermal equilibrium, convection currents will be generated when an
object is on the balance pan and an erroneous reading may result. These
types of errors are likely to go unnoticed when check standards are
handled the same way, so care must be taken to reduce the likelihood of
their occurrence. Tests have shown that these types of errors depend
upon the temperature differences among the objects and the balance, and
on the geometry of the objects being weighed. On 20 g standards of
greatly differing geome­try, studies have shown that errors as large as
0.2 mg can occur for a 4 °C temperature difference among the standards
and the balance. Errors as large as 3 mg have also been observed at 1 kg
loads when standards were stored in a cabinet, and unknown test items
near the balance where a temperature gradient of 2 °C was present (when
the uncertainty was less than 0.1 mg).

The balance must be installed in an area free from air currents.
Balances should be installed away from heating/cooling vents. It is not
adequate to merely close vents when weighing because disrupting the
temperature control system may result in temperature changes near the
balances.

[Handling Weights]{.underline}

The masses of standard weights or objects weighed can be affected
significantly by the manner in which they are handled. Human contact can
leave grease or oily films that affect the mass at the time of
measurement and can even cause permanent damage due to corrosion.

Small weights should never be touched by hand, but handled using
forceps, clean gloves, or swatches of cloth. In the latter case, the
cloth must be lint free. Instructions for cleaning weights and for
removing adhering foreign material are described in GMP 5.

Large weights of lower tolerance classes (NIST Class F) may be handled
by bare hands. Large weights are a source of special problems. Fork
lifts, portable cranes, hoists, or even overhead cranes may have to be
used to move the weights within the laboratory and on or off the
balances. Laboratory personnel must become expert in using such
equipment, as necessary, to avoid damage to the laboratory facilities,
to the balances used, and even to the weights them­selves. Special
hoist/crane hydraulics or multi-speed systems are available to gently
set large weights in place on large comparators to avoid damage. The
problem of temperature equilibrium for large weights is qualita­tively
the same as for small weights and needs consideration on an individual
basis.

Large weights must be clean at the time of use, but cleaning may be a
problem. Minimally, they should be inspected to ensure that foreign
material is not present. Cleaning is addressed further in GMP 5.

[Operation]{.underline}

Analytical balances are high precision instruments and should be
operated carefully. Excessive shock can damage a balance. Avoid shock
loading the balance. Gently place all weights in the center of the
weighing pan/platform. The dials on mechanical balances should be turned
slowly and gently. Careful balance operation will improve the
repeatability of measurements.

Mechanical analytical balances are provided with partial and full
release positions. The partial release position is used when determining
if an unknown load will provide an on-scale reading. The balance beam
has limited movement in this position. The partial release position
provides some protection to the balance when the dialed-in weights are
not close to the actual mass placed on the pan. It is good practice to
arrest the pan each time a dial is being changed to protect the balance
from shock loading. It is acceptable to change the dial representing the
smallest built-in weights when in the partial release position because
the small weight changes should not result in shock loading of the
balance.

When releasing the pan to either the full or partial release position,
the action should be done slowly and carefully. The objective is to
minimize disturbances to the balance as much as possible.

Similarly, all loads should be placed on the balance pan carefully and
centered on the pan.

When a mechanical balance is released, the beam goes through a series of
oscillations. The frequency of these oscillations diminishes as time
passes until they are almost imperceptible to the naked eye. At this
point, optimal stabilization is achieved. This stabilization of the
balance typically lasts for a very short period of time, after which the
balance reading will usually drift. A similar situation occurs when a
mass is placed on the pan of an electronic balance. Therefore readings
should be taken at the precise moment of achieving balance stability.
This interval between the releasing of a pan on a mechanical balance, or
the placing of a mass on an electronic balance, and the reading of the
observation, varies from balance to balance. Stabilization time differs
for all balances, even among those of the same type and model. Knowledge
of the instrument is critical to correctly establish this time interval.
Although manufacturers will usually state this value, it is necessary
for the metrologist to verify its reliability. Many electronic balances
have a stability signal incorporated into the design, but this also must
be verified. All measurements in a calibration should be performed at
the same time interval, and within the shortest time possible.

All balances should be exercised before taking readings. A load equal to
the load to be measured should be placed on the balance, a reading
taken, and the pan arrested, if appropriate, or the weight removed from
electronic balances. This operation should be repeated several times
before readings are taken for measurement purposes. Once the balance has
been \"warmed-up\", better repeatability will be achieved. Balances can
be very accurate even when used without being exercised first, but
improved results can be obtained by going through a \"warm-up\"
procedure. If the larger variation present in initial weighings on a
balance that has not been exercised is not significant to the
measurement, the warm-up procedure may be minimized.

To determine the repeatability of measurements when a balance has not
been exercised versus its performance after being exercised, and to
determine how much warm-up time is required, begin measurements starting
with the unexercised balance and record the readings. Repeat a series of
measurements until you have obtained several measurements after the
balance has been exercised. This test can be repeated over several days
using the same objects and the results compared. The readings obtained
when using an unexercised balance are likely to show a slightly larger
variation than those obtained after the balance has been exercised.
Balance drift is likely to be larger initially and then reach a steady
state when the balance has been \"warmed-up\".

[Comparison Weighing - Mechanical Balance]{.underline}

For mass calibrations, the unknown object must be compared to a known
mass standard. Comparison weighing eliminates the errors of the built-in
weights, reduces disturbances during the measurement because dial
settings are not changed during the measurement, and can cancel the
effect of drift by selecting the appropriate weighing design. Comparing
the unknown, *X*, to a standard, *S*, eliminates the built-in weights
from the measurement: thus, the built-in weights act only as
counterweights; they do not affect the difference measured between *X*
and *S*. Consequently, the dial settings must not be changed during a
comparison measurement; otherwise the built-in weights would be part of
the measurement.

When comparison measurements are made on a single pan mechanical
balance, all readings are taken from the optical scale. The unknown and
the standard must have nearly the same mass so that the difference
between them can be measured on the optical scale. If the masses of the
unknown and the standard are significantly different, small mass
standards are used as tare weights with either the unknown or the
reference standard or both to obtain an observed mass difference that is
significantly less than one-fourth the range of the optical scale.

As part of a comparison measurement, the mass value of a scale division
is determined by performing a sensitivity determination. The small mass
standard used as part of the weighing design to calibrate the optical
scale is called the sensitivity weight and should have a mass between
1/5 and 1/2 the range of the optical scale, inclusive. Additionally, the
mass of the sensitivity weight should have a mass that is at least twice
the observed mass difference between the standard and the unknown. Since
the maximum size of the sensitivity is limited to one-half the range of
the optical scale, it may be necessary to carry tare weights with the
standard and the unknown to ensure that the observed difference between
them is less than one-half the mass of the sensitivity weight. The value
of the sensitivity weight should include the correction plus its nominal
value. For high precision calibrations, the inclusion of the air
buoyancy correction for the sensitivity weight is critical. See GMP 14
for detailed guidance on the selection of sensitivity weights.

The first readings for the standard and the unknown in a comparison on a
single pan balance should fall in the first quarter of the optical
scale, but well ahead of zero, so the balance drift will not result in
negative values for any readings. Although negative numbers may be used
in calculations, they are avoided to simplify calculations and reduce
calculation errors. Because the sensitivity weight may have a mass as
large as one-half the range of the optical scale and the measured
difference between the standard and the unknown may be as large as
one-fourth the range of the optical scale, it is necessary to obtain the
first two readings in the first quarter of the optical scale so all
readings will remain on-scale for the measurement. In this way it is not
necessary to change the dial settings to measure the difference between
the standard and the unknown.

[Comparison Weighing - Electronic Balance]{.underline}

Measurements made on a full electronic balance are simplified because
there are no built-in weights to consider. Although many electronic
balances are equipped with a built-in calibration weight, the weight is
not involved in the comparison weighing.

The principles for comparison weighing on a full electronic balance are
the same as when using a single pan mechanical balance. The balance
indica­tions are used to measure the mass difference between the standard
and the unknown, and a sensitivity weight is used to establish the mass
value for a digital division on the balance. Since there are no built-in
weights in the full electronic balance, the entire range of the digital
indications can be considered for \"optical scale\" of the balance.

For comparison weighing the standard and the unknown should be \"nearly
the same mass.\" Since a full electronic balance has a much larger range
for indicating mass values, the masses do not have to be as close
together as when a mechanical balance is being used. When using an
electronic balance, the difference in mass between the standard and
unknown should be less than 0.05 % of the balance capacity. Tare weights
that are known standards should be used if the masses are not
sufficiently close together. The sensitivity weight used to determine
the mass per digital division should have a mass that is at least 10 to
20 times the mass difference between the standard and the unknown but
not exceeding 1 % of the balance capacity. For high precision weighing,
air buoyancy corrections must be made for all objects used in the
intercomparison.

[Magnetic and Electrostatic Fields]{.underline}

Care must be taken when weighing magnets or objects having electrostatic
charges. A magnetic field will likely affect results of a mass
measurement. A magnet is attracted to ferromagnetic materials in the
balance and balance chamber. The magnetic field may also affect the
magnetic field generated in an electronic balance that utilizes the
principle of magnetic force restoration as its method of measurement.

Weights made of ferromagnetic material can become magnetized during
manufacture or during use if they are placed in a strong magnetic field.
Magnetized weights can result in measurement errors that may go
undetected. If a measurement problem is suspected, the weights should be
checked for magnetism and may have to be rejected if excessively
magnetized.

If magnets or magnetized material must be weighed, the material should
be placed in a magnetically shielded container to prevent the magnetic
field from generating measurement errors. If balance design and
conditions permit, an alternative is to position the magnetized material
a relatively large distance from the balance pan using a
non-ferromagnetic object on the pan to serve as a platform for the
magnetic. Since the strength of the magnetic field drops off at a rate
of the cube of the distance from the magnetic, it may be possible to
effectively isolate the magnet from other ferromagnetic material in the
balance.

Electrostatic fields can also cause errors in measurements. If there is
a static charge in a mechanical balance, the balance may give erratic
readings and lack repeatability. If the object being weighed has a
significant electrostatic charge, it may result in measurement errors
and may leave a static charge on the balance. Electrostatic charges are
of particular concern when plastic containers are placed on the balance.

Care should be taken to remove electrostatic charges from objects being
weighed by grounding the objects, if necessary, before placing them on
the balance. To prevent the build-up of static electricity in a balance,
the relative humidity in the laboratory should be maintained between
40 % and 60 %. The water vapor in the air will serve to drain off
electrostatic charges from the balance.

Balances utilizing the magnetic force restoration principle for weighing
should be checked to verify that the magnetic field generated by the
magnetic cell in the balance does not exist around the balance pan. If
the shielding of the magnetic cell is inadequate, measurement errors may
occur when weighing ferromagnetic objects or when the balance is placed
on a surface comprised of ferromagnetic material.