Rice Lake Z6 Single-Ended Beam, SS Welded-seal, IP67, OIML C3 - Manual (Page 19)

Maximizing System Accuracy 15
10.0 Maximizing System Accuracy
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.
10.1 Environment
• Install the vessel in a controlled environment where
seasonal temperature fluctuations are minimized. If
this is not feasible, use load cells with temperature
c o m p e n s a t i o n s p e c i f i c a t i o n s t h a t w i l l a l l o w
satisf ac tory perform anc e over the expect ed
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
e x p e c t e d , c h o o s e a m o u n t w h i c h w i l l a l l o w
unhindered lateral movement. If stay rods are
required, position them so that thermally induced
movement is minimized. See Section 20.3 on
page 42 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
protection required for the application. For example,
avoid possible drifting problems with standard load
cells in washdown applications by specifying
hermetically sealed cells.
10.2 Load Cell and Mount
• Choose load cells with accuracy that is consistent
with the desired system accuracy.
• Do not grossly oversize the load cells; see
Section 4.2 on page 6. The 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 Section 19.0 on page 36.
• Use proven load cell mounts that will provide optimal
loading conditions.
• Orient the mounts as recommended in the
installation manual.
10.3 Mechanical/Structural
Considerations
• 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
grounding and transient protection in accordance
with the manufacturer’s recommendations. In
g e n e r a l , t a k e m e a s u re s t o re d u c e e l e c t ri c a l
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.
10.4 Calibration
• Design in a convenient means of hanging weight
from the corners of the vessel to trim the load cell
o ut p u ts a nd f or ca l i b ra t i on . Us e w e i g ht s a s
described above, or known weight of material to
perform the calibration.
10.5 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.