Rice Lake Z6 Single-Ended Beam, SS Welded-seal, IP67, OIML C3 - Manual (Page 16)

12 Load Cell and Weigh Module Handbook
8.0 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:
4. Matched output load cells are used (as in the Paramounts).
5. Weighing self-leveling materials (liquids).
6. The vessel is partially supported on flexures.
*Assume that the vessel’s center of gravity (see 2 and 3
above) 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.
8.1 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
p r o c e s s , p r e s e n c e o f a g i t a t o r s , a n d t h e
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.
8.2 Excitation Trim
This is the oldest method of trimming the output from a strain
gauge load cell. Excitation trimming adds series resistance to
the excitation circuit of the load cell, thereby reducing the
excitation voltage at the cell. The load cell with the lowest mV/
V output receives the full excitation voltage. All other load cells
in the system with a higher mV/V output receive proportionally
smaller excitation voltages. This results in matched full load
outputs for all load cells in the system.
Figure 8-1 is a functional diagram of an excitation trim J-box.
Note that a variable resistor or potentiometer (pot), is inserted
in the + excitation lead of each load cell. If the pot is opened
so that resistance is zero, the full excitation voltage is applied
to the load cell. As resistance is increased, excitation voltage
decreases.
Excitation Trimming Procedure
The simplest method of trimming with excitation is to set up
your system, turn all trim pots to the “open” or full excitation
setting, and test each corner of the system with a calibrated
test weight or any dead weight. Once the lowest output
corner is located, the other cells are trimmed to match by
physically loading with the same weights and adjusting the
pots. This procedure can be practical if used in field
replacement of load cells in light-capacity floor scales. It is not
typically used in heavy-capacity scales where application of
test weights to corners in such a manner is not practical.
Another method is “pretrimming.” Here, the load cells are
trimmed by mathematically calculating the excitation voltage
for the load cell, then measuring the excitation voltage with a
voltmeter, while adjusting the pot to the required voltage. The
following five steps walk you through this procedure.
Figure 8-1. Excitation Trimming Load Cells
1. Determine how much excitation voltage your
electronic digital weight indicator is supplying to the
load cells. This is found by measuring, with a
voltmeter, the actual excitation voltage present at the
reference cell’s excitation leads. For this example, we
will use 10 volts DC.
NOTE: The reference cell is the cell with the lowest mV/V
rating, as shown on its calibration certificate.
2. Determine the exact mV/V rating of each load cell
and locate the cell with the lowest rating. The exact
mV/V rating is found on the calibration certificate
supplied with the load cell or on the label itself. Just
because a cell is rated at 3 mV/V, don’t assume it’s
exactly 3 mV/V.
#1 = 2.997 mV/V #3 = 2.999 mV/V
#2 = 3.003 mV/V #4 = 3.002 mV/V
Cell number 1 has the lowest rating at 2.997 mV/V.
3. Calculate the trimming factor by multiplying the
lowest mV/V by the excitation voltage.
2.997 mV/V x 10V = 29.970 mV
4. Calculate the adjusted excitation voltage for the
remaining load cells and adjust each respective trim
pot to the appropriate voltage level.
#1 = leave alone, lowest mV/V
#2 = 29.97 mV ÷ 3.003 mV/V = 9.980 volts
#3 = 29.97 mV ÷ 2.999 mV/V = 9.993 volts
#4 = 29.97 mV ÷ 3.002 mV/V = 9.983 volts
The scale is now trimmed.
5. Verify your results with certified test weights or a
known amount of material.
TRIM POT
EXCITATION
SIGNAL
+
−
+
−