Physical Influences
If the weight display does not stabilize, the result slowly drifts in one direction,
or quite simply if wrong values are displayed, this is often due to
undesired physical influences.
In the next section we shall explain these influences and what causes
them in more detail, and describe corrective measures.
The most frequent causes are:
- Influences of the weighing sample
- Influences on the balance from the surrounding area
- Moisture gain or loss by the weighing sample
- Electrostatically charged weighing samples or vessels
- Magnetic weighing samples or vessels
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Temperature
 Problem
The weight display of a weighing sample drifts in one
direction.
Possible reasons
The balance has not been connected to the power supply long
enough.
There is a temperature gradient between the weighing sample and the
surroundings that leads to air currents along the weighing vessel. The
air flowing along the side of the vessel generates a force in an upward
or downward direction that falsifies the weighing result.
This effect is called dynamic buoyancy. The effect does not die away
until a temperature equilibrium is established. The following applies:
A cold object appears heavier, a warm object lighter. This effect can
lead to problems, especially in differential weighings with semi-micro,
micro and ultra-micro balances.
Example
 You can test
the dynamic buoyancy with the following experiment: Weigh a conical or
similar flask and record the weight. Hold the flask in your hand for
about one minute and repeat the weighing. Because of its higher
temperature and the temperature gradient that developed, the flask
appears lighter.
(The perspiration on your hand plays no role in this effect.
Otherwise the sample would have become heavier).
Corrective measures
- Never weigh samples taken directly from a dryer or
refrigerator
- Acclimatize weighing sample to the temperature of the lab or
weighing chamber
- Hold sample containers with tweezers
- Never put your hand in the weighing chamber
- Choose sample vessels with a small surface area
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Moisture gain/evaporation
Problem
 The weight
display of a weighing sample permanently drifts in one direction.
Possible reasons
You are measuring the weight loss of volatile substances (e.g. the
evaporation of water) or weight increase of hygroscopic weighing
samples (atmospheric
moisture gain).
Example
You can reproduce this effect with alcohol or silica gel.
Corrective measures
 Use clean and
dry weighing vessels and keep the weighing pan free from dirt and water
drops. Use vessels with narrow necks and mount covers or stoppers. Do
not use cork or cardboard supports for flasks with a circular base.
Both can gain or lose a considerable amount of water. Metal triangular
holders or the “ErgoClips” for the Excellence and Excellence Plus
family of balances are neutral in this regard.
Using a weighing vessel with a larger opening increases the
risk of measuring errors through evaporation or condensation.
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Electrostatics
 Problem
Each weighing shows a different result. The weight display is unstable.
The repeatability of the result is poor.
Possible reasons
Your weighing vessel or the sample has become electrostatically
charged. Materials with low electrical conductivity such as glass,
plastics, powder or granulates cannot or can only very slowly (over
hours) drain away electrostatic charges. This charging occurs primarily
through stirring or rubbing during the handling or transport of
containers or materials. Dry air with less than 40 % relative humidity
increases the risk of this effect.
The weighing errors arise through the electrostatic forces that act
between the weighing sample and the surroundings. These electrostatic
charges can be measured by micro, semi-micro and analytical balances
and lead to the weighing errors described.
Example
A clean glass or plastic vessel that has been gently rubbed with a
woolen cloth shows this effect quite clearly.
Corrective measures
 Increase the
atmospheric moisture Electrostatic charging is particularly a problem
in winter in heated rooms. In air conditioned rooms, setting the air
conditioning to increase the humidity (45-60 % relative humidity) can
help.- Screen electrostatic forces Place the weighing vessel in a metal
container.
- Use other weighing vessels. Plastic and glass charge quickly and
are therefore unsuitable. Metal is a better material.
- Use antistatic guns. The commercially available products, however,
are not effective in all situations.
- Use external or internal antistatic kits from METTLER TOLEDO.
Note: The balance, and hence the weighing pan, should always be
grounded. All METTLER TOLEDO balances with three-pin power plugs are
automatically grounded.
 TIP: The “ErgoClip
Basket” taring container holder does an excellent job of eliminating
electrostatics, thereby effectively preventing the described problems
with tubes and test glasses.
Magnetism
 Problem
The weight of a weighing sample depends on its position on the weighing
pan. The repeatability of the result is poor. But the display remains
stable.
Possible reasons
You are weighing a magnetic material. Magnetic and magnetically
permeable objects exert a mutual attraction. The additional forces that
arise are wrongly interpreted as a load.
Practically all objects made of iron (steel) are highly permeable to
magnetic forces (ferromagnetic).
Corrective measures
 If possible,
demagnetize the magnetic forces by placing the weighing sample in a
vessel made of Mu Metal film, for example. Since the magnetic force
decreases with increasing distance, the sample can be distanced further
from the weighing pan by using a non-magnetic support (e.g. beaker,
aluminum stands). The same effect can be achieved by means of a hanger.
This “below-the-balance” setup is built in as standard
with most METTLER TOLEDO micro, semi-micro, analytical and precision
balances. Wherever possible, METTLER TOLEDO uses non-magnetic materials
to keep this effect to a minimum.
TIP: To weigh average- and
large-sized magnets with precision balances we recommend an “MPS
Weighing Pan” (Magnetic Protection System). For analytical balances, we
recommend using a triangular holder, which increases the distance
between the magnets and the weighing pan. For balances in the
Excellence and Excellence Plus lines, we offer special “ErgoClips” for
this purpose.
“ErgoClip Flask” taring container holder for balances in the
Excellence and Excellence Plus lines.
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Static buoyancy
Effect
 A weighing sample weighed in air and in a
vacuum does not have the same weight.
Reason:
«A body experiences a loss in weight equal to the weight of the
medium it displaces» (Archimedes’ principle). This principle provides
an explanation of why ships float, a balloon rises, or why the weight
of a sample is affected by atmospheric pressure.
The medium that surrounds our weighing sample is air. The density of
the air is approximately 1.2 kg/m³ (depending on the temperature and
atmospheric pressure). The buoyancy of the weighing sample (body) is
thus 1.2 kg per cubic meter of its volume.
Example
If we place a 100 g calibration weight in a beaker on a beam balance
and then add water to an identical beaker on the other weighing pan
until the weighing beam is in equilibrium, the two weighing samples,
weighed in air, have the same weight.
If we then enclose the beam balance with a bell jar and generate a
vacuum in it, the weighing beam will tilt to the side with the water,
since the water displaces more air owing to the larger volume, and has
hence experienced a greater buoyancy. In vacuum there is no buoyancy.
Therefore, in the vacuum, there is more than 100 g of water on the
right side.
 Corrective
measures
The sensitivity of the balance is adjusted with reference weights of
density 8.0 g/cm³. In the weighing of samples of different density, an
air buoyancy error arises. In weighings with high measurement accuracy,
the displayed weight should be corrected accordingly. In weighings on
different days (differential weighings, comparative weighings), check
atmospheric pressure, atmospheric humidity and temperature, and
calculate the air buoyancy correction as follows:
Procedure to determine the mass of a weighing sample:
- Calculate air density
- Determine the mass of the weighing sample (correct air buoyancy)
Example
Balance display 200.000 g
Atmospheric pressure 1018 hPa
Relative atmospheric moisture 70 %
Temperature 20 °C
Density of weighing sample 2600 kg/m³
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Gravitation
 Effect
The weighing values are different when the weighing height changes.
For example, the weight display changes when the weighing is performed
10 m higher (e.g. moving from the first floor to the fourth floor of a
building).
Reason
To determine the weight of a body, the balance measures the weight
force, i.e. the force of attraction (gravitational force), between the
earth and the weighing sample. This force depends essentially on the
latitude of the location and its height above sea level (distance from
the center of the earth).
 The following holds:
- The further a weight is from the center of the earth, the smaller
the gravitational force acting on it. It decreases with the square of
the distance.
- The nearer a location is to the equator, the greater the
centrifugal acceleration due to the rotation of the earth. The
centrifugal acceleration counteracts the force of attraction
(gravitational force). The poles are the greatest distance from the
equator and closest to the earth’s center. The force acting on a mass
is therefore greatest at the poles.
Example
In the case of a 200 g weight that shows exactly 200.00000 g on the
first floor, the following weight results on the fourth floor (10 m
higher):
Corrective measures
Level and adjust the balance whenever it is moved or before using it
for the first time.
TIP: Balances with
built-in “FACT” (fully automatic
motorized selfcalibration) perform this calibration automatically.
METTLER TOLEDO balances of the Excellence and Excellence Plus line come
standard with “FACT”. |
