99
Operation
Notes on Analytical Weighing
with Weigh Cells
Handling of Samples and Containers
Samples should be acclimatized to the
temperature of the weigh cell.
This is the only way to avoid measurement
errors caused by air buoyancy and
fluctuations resulting from convection
currents across the surface of the sample.
These negative effects increase as the
volume and/or surface area of the sample
increases. For this reason, the size of the
tare container should be appropriate for
the sample.
Samples and containers should not be
touched by the operator‘s hands. This
is because the hygroscopic effect of
fingerprints and the effect of the hand’s
temperature can influence the measure-
ment results.
Samples must be applied very carefully,
whether manually (using a forceps) or
automatically (by a robot or filling system).
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 and Containers
Significant measuring errors can occur
when electrostatically charged objects
are weighed. This problem particularly
involves samples that have extremely poor
conductivity (glass, plastic, filters) since
they can discharge electrostatic – i.e.,
friction-induced – charges through the
weighing pan over a relatively long period
of time only.
The result is a force acting between the
charge on the sample and the permanently
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 sample), bombarding the
sample with ions of opposing polarity to
neutralize the surface charge is one of the
most effective methods for eliminating
static electricity. Sartorius can provide
ionization devices for installation in
weighing systems.
The area around the weigh cell, like
plastic parts, can also contain charges that
negatively affect the accuracy of weighing
results. Appropriate steps (grounding)
taken in the design of a draft shield device
can counteract such effects.
The weigh cell base plate and the
electronics base plate should be
grounded via the screw connections.
Weighing Magnetic or
Magnetizable Samples
It is technically impossible to avoid
using magnetizable materials for
the production of weigh cells. This
is primarily because the operating
principle of high-resolution weigh cells
is based on compensation of the load
through magnetic forces.
When weighing magnetic or
magnetizable samples or containers,
interaction between the sample or
container and the above-mentioned
parts inside the weigh cell may have
a distorting effect on 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
quadratically with the increase in
distance.
Magnetizable or magnetized samples
and the weigh cell itself interact with
magnetic fields and magnetizable
or magnetized parts in the area
surrounding the weighing system.
The system can be shielded from
external magnetic fields to some
extent using (soft magnetic) plates.
Effects of Drafts
Depending on the size of the load
receptor and the sample, the effects of
drafts may occur.
To minimize this effect, install a draft
shield for protection.
Protect the weigh cell from drafts.
Calibration/Adjustment
Calibration/adjustment can be
performed as follows:
– Using control commands sent by the
CAS-Suite configuration software from
Sartorius, installed on a computer
(see page 22 for the commands)
or
$ Using the optional YAC01ED
control unit
9
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