This section shows the basic strain gauge load cell faults and the various ways to test load cells along with how to troubleshoot these faults.
The following faults are common to a typical strain gauge load cell
- A sudden change in the zero balance value of the load cell
- Unstable readings and random changes in the zero balance point of the load cell
- The weighing scale reads overload, incorrect readings for known weight or does not give any reading at all
- It gives erratic output display when load is applied or after load is removed
It is also important to verify the integrity of the weighing system before troubleshooting load cells when they becomes faulty:
- Check the mechanical supports, the load cell orientation and mounting surfaces for cleanliness, flatness and alignment
- Check the interconnecting cables at the summing box
- Check the connection of the summing box to the digital indicator panel
After verifying the weighing system’s integrity, then you can proceed to troubleshoot the load cell for faults. To troubleshoot faults, here are the test equipment’s required:
- A high quality digital voltmeter and ohmmeter capable of reading from 0 – 50mV and 4 – 20mA with accuracy of ±0.5? and ±0.1mV.
- A screw driver set
- A mega-ohmmeter capable of reading 5000M with accuracy of 500M at 50V. It should be noted that in using the mega-ohmmeter, do not use the ones that can supply more than 50volts to the load cell, as these can result in permanent damage
The fault evaluations are then carried out as following:
- Check the zero balance.
- In carrying out the procedure
- Connect the load cell input terminals to an excitation/input voltage
- Measure the voltage across the load cell output terminals with a millivolt meter (mV) and divide the result by the excitation voltage to obtain the zero balance in mV/V
- This value for the zero balance is then compared with the value in the datasheet
Insulation resistance tests
This load cell test is carried out when there are unstable readings and random changes in the zero balance point of the load cell. It is done by connecting all the conductors together and then the resistance between all the connected wires and the load cell body is measured
- Firstly, disconnect the load cell from the summing box and indicator panel
- Connect all the input, output and sense (if the load cell has) wires together
- Measure the insulator resistance between the connected wires and the load cell body with a mega-ohmmeter
- Measure the insulation resistance between the connected wires and the cable shield
- Measure the insulation resistance between the load cell body and the cable shield
The insulation resistance should match the value in the load cell datasheet. A value lower than that shows an electrical leakage caused by moisture which causes short circuits and hence results in unstable load cell outputs.
Check the bridge integrity
This is done when the weighing scale reads overload, incorrect readings for known weight or does not give any reading at all. It involves measuring the output resistance, input resistance and the bridge balance.
- Disconnect the load cell from the junction box or measuring devices
- Measure the input and output resistance across input and output terminals with an ohmmeter
- The values are then compared with those in the datasheet
- The bridge balance is obtained by comparing the resistance from the negative output lead to the negative input lead with the resistance from the negative output lead to the positive input. The difference between the readings is meant to be small or at least equal to 1
Changes in these measured values are caused by electrical component failures or an internal short circuit which results from lightning strikes (overvoltage), excess temperature or physical damages.
It should be noted as a warning to never use a mega-ohmmeter to measure the input and output resistance of the load cell, as it operates at a voltage level (50V) far greater than the normal input and output voltage, hence it can damage the load cell.
Shock resistance tests
This is carried out if the load cell gives erratic output display when load is applied or after load is removed. The procedures are:
- Connect the load to a stable voltage source
- Connect a voltmeter to the output terminal
- Gently tap on the load cell probably with a small mallet to mildly shock it. Care should be taken however to not overload the load cell
When checked, the readings should not become erratic, they should be stable and return to the zero point. If erratic readings are observed, then there might be a failed electrical connection or maybe a glue layer is between the strain gauge sensor and element as a result of electrical transient.
If after all the following tests for faults and the load cell is still defective, provided it is still under a valid warranty the load cell can be returned to the OEM for repairs or replacement. If it is not under a valid warranty and it turns out that the problem cannot be fixed, then the load cell should be replaced.
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