Rice Lake Z6 Single-Ended Beam, SS Welded-seal, IP67, OIML C3 - Manual (Page 20)

16 Load Cell and Weigh Module Handbook
11.0 Load Cell Troubleshooting
Here are some easy-to-follow steps to help you troubleshoot
potential load cell problems. Before you begin you will need a
good quality digital multimeter, at least a 4½ digit ohm meter.
The tests are: physical inspection, zero balance, bridge
resistance and resistance to ground.
11.1 Physical Inspection
How does it look? If it is covered with rust, corroded or badly
oxidized, chances are the corrosion has worked its way into
the strain gauge area as well. If the general and physical
condition appear good, then you need to look at specifics:
sealing areas, the element itself, and the cable.
In most load cells, areas of the load cell are sealed to protect
the contents from contamination by water and chemicals. To
see if any seals have been degraded, get right up close to the
cell and look at the strain gauge seals (Figure 11-1 points A).
Is rust concentrated on a part of the cover weld? If there is no
cover, do you see any tiny holes in the potting? These are
indications that there has been contamination to the gauge
area. Check the load cell cable entrance (Figure 11-1 point B)
for signs of contamination.
Figure 11-1.
Other items to look for: metal distortion or cracks, metal
rippling, cracks in the weld, or abrasions in the metal. It may
be necessary to remove the load cell and check it for physical
distortion against a straight edge.
No inspection would be complete without thoroughly
inspecting the cable. Cable should be free of cuts, crimps and
abrasions.
If your cable is cut and in a wet environment, water or
chemicals can “wick” up the cable into the strain gauge area,
causing load cell failure.
If your physical inspection fails to uncover any identifiable
damage, a more detailed evaluation is required.
11.2 Zero Balance
This test is effective in determining if the load cell has been
subjected to a physical distortion, possibly caused by
overload, shock load or metal fatigue. Before beginning the
test, the load cell must be in a “no load” condition. That is, the
cell should be removed from the scale or the dead load must
be counterbalanced.
Now that the cell is not under any load, disconnect the signal
leads and measure the voltage across the negative signal and
positive signal. The color code for determining negative- and
positive-signal leads is provided on the calibration certification
with each load cell. The output should be within the
manufacturer’s specifications for zero balance, usually ± 1% of
full scale output. During the test, the excitation leads should
remain connected with the excitation voltage supplied by the
digital weight indicator. Be certain to use exactly the same
indicator that is used in the cell’s daily operation to get a
reading accurate to the application.
The usual value for a 1% shift in zero balance is 0.3mV,
assuming 10 volts excitation on a 3 mV/V output load cell. To
determine your application’s zero shift, multiply the excitation
volts supplied by your indicator by the mV/V rating of your
load cell. When performing your field test, remember that load
cells can shift up to 10% of full scale and still function
correctly. If your test cell displays a shift under 10%, you may
have another problem with your suspect cell, and further
testing is required. If the test cell displays a shift greater than
10%, it has probably been physically distorted and should be
replaced.
11.3 Bridge Resistance
Before testing bridge resistance, disconnect the load cell from
the digital weight indicator. Find the positive and negative
excitation leads and measure across them with a multimeter
to find the input resistance. Don’t be alarmed if the reading
exceeds the rated output for the load cell. It is not uncommon
for readings as high as 375Ω for a 350Ω load cell. The
difference is caused by compensating resistors built into the
input lines to balance out differences caused by temperature
or manufacturing imperfections. However, if the multimeter
shows an input resistance greater than 110% of the stated
output value (385Ω for a 350Ω cell or 770Ω for a 700Ω cell),
the cell may have been damaged and should be inspected
further. **
If the excitation resistance check is within specs, test the
output resistance across the positive and negative Signal
leads.
This is a more
delicate reading,
and you should
get 350Ω ±1%
(350Ω cell).
Readings outside
the 1% tolerance
usually indicate a
damaged cell.
N o w c o m e s t h e
tricky part. Even if
the overall output
r e s i s t a n c e t e s t
was within normal
specifications, you
could still have a
damaged load cell.
Often when a load
cell is damaged by overload or shock load, opposite pairs of
resistors will be deformed by the stress—equally, but in
opposite directions. The only way to determine this is to test
each individual leg of the bridge. The Wheatstone Bridge
diagram, in Figure 11-2, illustrates a load cell resistance
bridge and shows the test procedure and results of a sample
cell damaged in such a manner. We’ll call the legs that are in
t e n s i o n u n d e r l o a d T
1
a n d T
2
, a n d t h e l e g s u n d e r
compression C
1
and C
2
.
A
B
282W
– Sig + Sig
– Exc
+ Exc
278W
C
1
T
2
T
1
C
2
278W
282W
Figure 11-2.