This section shows the basic strain gauge load cell faults and the various tests that can be performed and 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
- 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
- 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. This test is done when there is a sudden change in the zero balance value which is always due to the overloading of the load cell. The zero balance is the output signal of the load cell in the no-load condition, when measuring for this value, all weights including dead load have to be removed from the load cell.
- 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 testsThis 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
Check the bridge integrityThis 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
Shock resistance testsThis 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