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Cable runs from summing box to
Instrument
Cells are typically 4
wire connections (+/- Excitation, +/- signal) and
terminate in a summing box.
An interface cable is
then run
from the summing box to the instrument location with 4 or 6 wires.
Understanding load cell
outputs.
Load cells are
specified in Mv/V normally 1,2 or 3 at full cell capacity.
Lets assume a 2
Mv/V cell is
loaded to its full capacity of 20, 000 Lbs.
At an excitation of 10
VDC the output signal will be (2 Mv/V x 10 VDC) = 20 Mv.
If the
excitation changes up or down so will the output signal for the
fixed 20,000 Lb weight applied.
It is important to keep the excitation at
the summing box constant which is why we use SENSE wires.
The sense wires measure
and compensate for voltage
drops or resistive changes in the interface cable run.
4-Wire Cable
runs do not sense excitation at the
summing box location.
Your display can drift due to cable resistance
changes such as daytime temperature swings.
4-Wire runs are not recommended
unless the distance from summing box to instrument is very short.
6-Wire Cable IS
recommend between the instrument and summing box.
Sense lines measure excitation at the summing box
compensating for any voltage drop in the run.
Since Excitation
and Signal are measured at the summing box the
Mv/V specifications are maintained.
Cable specifications:
A 100% overall shield is
recommended since we are dealing with low DC signals.
Twisted pairs for
Excitation, Sense and Signal helps common mode rejection.
Minimum 20 gauge is suggested for
distances up to 250 feet,
18 gauge up to 500 feet.
The wire gauge will
affect voltage drops on long runs, contact us for cable runs up to 1000 feet.
Cutting or extending Load Cell Cable
Load cells are
calibrated with a fixed cable length to set the
Mv/V rating.
The Mv/V will change if
this cable is cut or extended without sensing compensation..
Manufacturers tell you not to cut the supplied cable but
they are seldom in the field trying to shove 30 extra feet of
cable into a compact summing box.
To cut or not to cut may
be determined by the accuracy expectations of the system.
Mechanical influences on your scale
will exceed any error
associated with changing cable lengths.
For scales where each
cell is adjusted on the sum card such as a floor scale, cables can
be cut.
The following suggestions
are for most process applications.
1: For optimum accuracy cut the same length of cable from each load cell so all
are affected equally.
This may
not be
practical as the distance to the summing box varies.
2: When cutting
different lengths a cell output changes 0.0004 MV/V for each meter of cable cut or
added.
Example:
A 1000 Lb cell
with 20 feet of
cable is cut to 15 feet, difference of 0.2 lbs at 1000 Lbs
load.
A 50,000 Lb cell with 50 feet
of cable cut to 35 feet, difference of 30.5 lbs at capacity.
In the larger capacity
the cell may need to be adjusted at the summing card depending on the application.
For a truck scale the
error would exceed the allowable tolerance but not on a storage
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Industrial Weighing
Systems
