Rice Lake Weigh Modules/Mount Assemblies - Manual (Page 70)

4-7
INSTALLATION & SERVICE TIPS Load Cell Trimming and Calibration
Traditional Approach
The conventional approach adjusts the short circuit current (mV/
V/ohm) of each load cell to a standard value, within a close toler-
ance. This does, indeed, ensure that multiple load cell systems will
be “corner adjusted” without further trimming, providing there are
no mechanical load introduction asymmetries. It also ensures that
the system corner adjustment is preserved, even when a load cell is
subsequently replaced. It does not, however, preserve the system
calibration. That will change!
Let’s look at this using a simple two-load cell example. Extension to
the “n” load cell case is straightforward. In the figure below, two
identical load cells are assumed and the conventional equations for
What is mV/V/Ohm Calibration?
The Paramounts Vessel Weighing System utilizes a unique system of mV/V/ohm calibration to ensure that all their load cell outputs match
precisely. While there are other manufacturers who offer a similar calibration concept, there are important technical differences provided
with Flintab products. To understand these differences, let’s first review the “traditional” method of matching load cell outputs.
Paramounts
Given the same set of circumstances regarding the replacement
load cell (source impedance 2% higher), the short circuit current
is set to the standard value, as before, but the open circuit voltage
is adjusted to a standard value by loading the output terminals with
resistance that drops the output voltage of the replacement load cell
to the standard value. In this example, a resistance of 51R is placed
across the output terminals of the replacement load cell and that
additional resistor is shown added to the paralleled source resis-
tances in the figure below.
I I
R 51R 1.02R
V
0
= +
V V
R R
x
(1.02) (51) R 3
(1.02) (51) R 2 + 1.02 R 2 + 51 R 2
=
2 V
R
R
2
x = V
V
0
R
1
R
2
V
1
V
2 V
0
+ R
1
R
2
R
1
R
2
V
1
V
2
R
1
R
2
V
1
= V
2
= V
R
1
= R
2
= R
V
1
V
2
R
1
R
2
x
+
R
1
R
2
R
1
R
2
V
0
= +
= +
Where V
1
and V
2
are voltage sources, R
1
and R
2
are resistances. It is
easier to understand the concepts by using the Norton equivalent
circuit. Here, we have two current sources driving currents
through the parallel combination of the load cell source imped-
ances. The currents are the short circuit currents (I) of the
respective load cell (mV/V/ohm) and they are set equal to some
standard value. Note that the mV/V output is the same as in the
arrangement above.
In either case, the system is “cornered.” That is, the system output
is the same whether the load cells are equally loaded or all the load
is on one or the other load cell. Now let’s replace the right hand load
cell with a unit which has a source resistance that is 2% higher than
the load cell it replaced. Since it must have the same short circuit
current (mV/V/ohm), its open circuit output voltage will be set 2%
+ R R +
I I
V
0
= +
V V
R R
x
+
= V
I =
V
R
V
0
RR
R R
1.02 V
1.02 R
=
V
R
V V
R R
V
0
= + =
1.02 RR
2.02 R
= 1.01 V I =
Now we have the two current generators driving their currents
through the parallel combination of their source impedances as
before. The system is still “cornered” but the system output is 1%
higher, because the parallel combination of the two source imped-
ances is now 1% higher, or the open circuit output voltage of the
replacement load cell is 2% higher. So, the system must be
recalibrated! As you know, this can be a difficult task, especially
with high-capacity vessel scales. Unfortunately, the conventional
approach does nothing to avoid the need for recalibration after load
cell replacement.
Now the standardized current sources are driving their short
circuit currents through the paralleled source resistances; the
third resistance, the paralleled combination of the three resis-
tances, is now equaling the original value of R/2. Hence the output
voltage with the replacement load cell in place is the same as it was
before the replacement. Not only is the system still “cornered,” but
the system calibration has been maintained. There is no need for
system recalibration after load cell replacement. All Flintab SB4
and UB1 load cells are factory-calibrated in the above manner.