Rice Lake Weigh Modules/Mount Assemblies - Manual (Page 68)

4-5
INSTALLATION & SERVICE TIPS Load Cell Trimming and Calibration
Load Cell Trimming
It may be necessary to trim the load cell outputs as a first step before starting the calibration process. Trimming is
performed at the junction box to equalize the weight reading from all cells in a system. This ensures that the scale
weighs correctly regardless of where the load is applied to the scale.
Trimming is necessary if:
1. It is a legal-for-trade weighing application.
2. The location of the center of gravity of the contents is not fixed, e.g., powder material which may accumulate on
one side.
3. A high-accuracy weighing system is required.
Trimming is not necessary if:
1. Matched output load cells are used (as in the Paramounts).
2. Weighing self-leveling materials (liquids).
3. The vessel is partially supported on flexures.
2 and 3 above assume that the vessel’s center of gravity rises along the same vertical line as the vessel is filled. Each
load cell is always subjected to the same percentage of the weight.
Trimming involves placing the same weight over each load cell in turn, and adjusting the corresponding trim pot in
the junction box until the indicator reads the same for all cells. To further illustrate load cell trimming, please review
the following examples of signal trim and excitation trimming procedures.
Understanding Load Cell Trimming
Many weighing systems use multiple load cells and therefore require a summing junction box to tie or “sum” the load cell signals together,
allowing a digital weight indicator to read a single “system” signal. The summing process actually wires multiple load cells so that all
their signal lines and excitation lines are in parallel, providing instantaneous electronic summing of the signals.
Load cell summing is necessary because:
■ Weight distribution in multiple load cell systems is not equal at each load cell. The vessel loading process, presence of agitators, and
the characteristics of the material and many other factors affect weight distribution on the load cells.
■ It is virtually impossible to make each load cell exactly alike. Load cell manufacturing process tolerances allow for some variance
in individual cell specifications. This variance, if unchecked, would not allow for the kinds of accuracy required in modern process
applications.
There are two summing methods; Excitation trim and Signal trim.