Operation; Information On Using Weigh Cells - Sartorius WZG Series Operating Instructions Manual

Operation

Information on Using
Weigh Cells
Handling of Samples and Containers
Samples should be acclimatized to the
temperature of the weigh cell to avoid
negative effects on results, such as
measurement errors and fluctuations
caused by air buoyancy resulting from
convection currents across the surface
of the sample.
These negative effects increase as the
volume and/or surface area of the sam-
ple increases. For this reason, the size
of the container should be appropriate
for the sample.
When designing a draft shield device,
steps must be taken to keep the increase
in temperature within the weighing
chamber to a minimum (e.g., using
a bypass).
Weighing Electrostatically Charged
Samples or Containers
If a sample or container is electrostati-
cally charged, significant errors may
result during weighing. Materials with
low conductivity, such as glass, plastic
or filters, are particularly susceptible to
static electricity (resulting, e.g., from
friction) because the weighing pan can
discharge the static electricity only very
slowly.
The result is a force action between the
charge on the sample and the perma-
nently installed parts of the weigh cell.
This causes the readout to fluctuate
constantly.
Ionization can be applied to make the
air around the sample conductive. This
allows the charge to be compensated
through the air or discharged through
the ground (grounded).
Aside from purely mechanical solutions
(e.g., using a special weighing pan to
shield the system), bombarding the
sample with ions of opposing polarity
to neutralize the surface charge is one
of the most effective methods for elimi-
nating static electricity. Sartorius can
provide ionization devices for installa-
tion in weighing systems.
The area around the weigh cell can also
contain charges that negatively affect
the accuracy of weighing results.
Appropriate steps taken in the design of
a draft shield device can counteract
such effects.
Weighing Magnetic or Magnetizable
Samples
For technical reasons, the use of mag-
netizable materials in the manufacture
of weigh cells is unavoidable, primarily
because the operating principle of high-
resolution weigh cells is based on com-
pensation of the load through magnetic
forces.
When weighing magnetic or magnetiza-
ble samples or containers, interaction
between the sample or container and
certain parts inside the weigh cell may
distort the weighing results.
To keep such effects to a minimum, we
recommend increasing the distance
between the sample/container and the
weighing system using a non-magnetic
material. The force is reduced quadrati-
cally with the increase in distance.
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