Rice Lake Weigh Modules/Mount Assemblies - Manual (Page 10)

SYSTEM DESIGN
1-8
Design Elements
High-accuracy systems are generally considered to have system errors of
±
.25% or less; lower accuracy systems will have system errors
of
±
.50% or greater. Most weight indicators typically have an error of
±
.01%, hence, the main source of error will be the load cells and,
more importantly, the mechanical arrangement of the scale itself. In vessel weighing, each installation is unique in terms of the
mechanical arrangement, site conditions and environmental factors. Therefore, it is impossible to be specific in this publication about
the system accuracy that can be achieved. The first requirement is to determine what the customer’s accuracy expectations/requirements
are, then design the system accordingly. Grouped under various subheadings below are various recommendations that contribute to high
accuracy. It will not be possible to comply with all these recommendations; however, they should be kept in mind when designing a system.
Maximizing System Accuracy
Mechanical/Structural
■ Support the load cell mounts on a rigid structure; this will
ensure a high natural frequency and reduce the amount of
bounce and instability. All support points must be equally
rigid to avoid tipping of the vessel as load is applied. Minimize
interaction between adjacent weigh vessels mounted on the
same structure. Vehicular traffic must not cause deflection of
the vessel’s support structure.
■ Ladders, pipes and check rods, etc. should not be allowed to
shunt the weight that should rest on the load cells.
■ Where piping or conduit must be attached to the vessel, use
the smallest diameter acceptable for the application. Use the
longest unsupported horizontal length of pipe possible to
connect to the vessel.
■ Use an indicator that is EMI/RFI protected. Provide ground-
ing and transient protection in accordance with the
manufacturer’s recommendations. In general, take measures
to reduce electrical interference.
■ Use a good-quality junction box which remains stable with
changing temperatures. Look for a junction board which has
a solder mask at a minimum and which preferably is
conformally coated also. Ensure that the enclosure is suited
to the environment.
Calibration
■ Design in a convenient means of hanging weight from the
corners of the vessel to trim the load cell outputs and for
calibration. Use weights as described above, or known weight
of material to perform the calibration. See Calibrating Vessel
Weighing Systems in Section 4.
Operational Considerations
■ Maintain an even and consistent flow of material.
■ Avoid simultaneous fill/discharge of weigh vessel.
■ Slow down the filling cycle as much as possible and/or use a
2-speed fill cycle.
■ Reduce to a minimum the amount of “in flight” material.
■ Use preact learning to predict the optimum cutoff point based
on past performance.
■ Use Auto Jog to top off contents after fill.
■ If possible, switch off any vibrating or mixing equipment
while the weight is being determined.
■ Reduce to a minimum the surging of liquids while a weight
reading is being taken.
Environmental
■ Install the vessel in a controlled environment where seasonal
temperature fluctuations are minimized. If this is not fea-
sible, use load cells with temperature compensation specifica-
tions that will allow satisfactory performance over the ex-
pected temperature range.
■ Use a metal shield to protect the load cells from radiant heat
sources. Use an insulating pad between the vessel and load cell
mount if heat is being conducted.
■ If thermal expansion/contraction of the vessel is expected,
choose a mount which will allow unhindered lateral move-
ment. If stay rods are required, position them so that ther-
mally-induced movement is minimized. See Vessel Restraint
Systems in Section 3 for more information.
■ Place the vessel indoors, if possible, where it will be protected
from wind and drafts.
■ Do not place the vessel in an environment where its support
structure is subject to vibration. Ensure that vibrations are
not transmitted via attached piping or stay rods.
■ Select load cells and mounts that will give the degree of
corrosion protection required.
■ Use load cells that have the degree of environmental protec-
tion required for the application. For example, avoid possible
drifting problems with standard load cells in washdown appli-
cations by specifying hermetically-sealed cells.
Load Cell and Mount
■ Choose load cells whose accuracy is consistent with the
desired system accuracy.
■ Do not grossly oversize the load cells; see Load Cell Capacity
on page 1-9. Best accuracy will be achieved when weighing
loads close to the vessel capacity. As a general rule, do not
attempt to weigh a load of less than 20 graduations.
■ If it is not possible to trim the corners, use load cells with
matched outputs, particularly if the vessel is not symmetrical
and/or the material is not self-leveling. Otherwise, use a
pretrimmed junction box.
■ Support the vessel entirely on load cells; do not use dummy
cells or flexures that would hinder a good calibration. See
Partial Mounting on Flexures on page 3-11.
■ Use proven load cell mounts that will provide optimal loading
conditions.
■ Orient the mounts as recommended in the installation manual.