8
Possible Cause
No return to zero
Mechanical binding or debris in seals or under load cells; may have lost system
calibration
Non-linearity
Thermal expansion or deflection under load causing binding or side load
Non-repeatability
Loose load cell mount; drifting caused by moisture, load cell overload or shock
damage; mechanical binding
Lost calibration
Out of level or plumb; moisture problem; mechanical binding
Drifting readout
Moisture in junction box, cables, or load cell; mechanical binding
Symptom
5. Troubleshooting
If the system powers up and gives some type of stable digital readout that varies with the
load on the system, any system problems are probably caused by factors other than the
load cells. The load cells are often blamed for a malfunctioning system, but 90% of the
time, the problem lies elsewhere. Look for mechanical causes for your problem first.
If the system can be calibrated but doesn’t return to zero, loses calibration, or demon-
strates non-linearity or non-repeatability, see the following chart for possible causes and
do the following checks.
1. Check load cell mount for debris restricting load cell movement or debris
between scale and structure.
2. Check that tank/vessel and mounts are plumb, level, and square at critical areas.
3. Check all piping and conduit for connections which restrict vessel movement.
4. If check rods are used, loosen all connections to finger tight only for testing.
5. Check load cell cables for physical or water damage.
6. Check all electrical connections, especially in the junction box.
If the problem still is not found:
7. Check possible indicator malfunction by using a load cell simulator to input a known
good signal into the indicator.
8. Disconnect each load cell’s signal leads at the junction box and check individual load
cell outputs with a multimeter. Then check input/output impedances for comparison
with load cell manufacturer’s specifications.
If after all these checks the problem still cannot be isolated, reconnect all but one load cell.
Replace load cell with a load cell simulator. Alternate so that each load cell is individually
disconnected and replaced with a simulator. If there is a problem with a particular load
cell, the symptom should disappear when that load cell is disconnected and replaced with
simulator.
Note: The RL71000 HE Series utilizes a unique method of compensation for temperature,
hysterisis, and linearity. Because of this, power must be applied to the excitation
loads to “close” the circuit and excite the bridge. Then the load cell bridge
resistance can be checked.
