How to Test for Faults in Load Cells
Load cells may, from time to time, encounter faults that disrupt their performance. This article highlights common strain gauge load cell faults and how to troubleshoot them.
Common Load Cell 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,
- Incorrect or overload readings for a known weight
- No reading at all
- Erratic output display when a load is applied or after load is removed.
A different test can be done to troubleshoot each of these situations; these tests are described in later sections.
Basic Troubleshooting Steps
When these or other faults are encountered, it is important to verify the integrity of the weighing system before troubleshooting the load cell. This includes the following steps:
- Check the mechanical supports, load cell orientation and mounting surfaces for cleanliness, levelness and alignment.
- Check the interconnecting cables at the summing box.
- Check the connection of the summing box to the digital indicator panel.
Once these steps are taken to verify the weighing system’s integrity, troubleshooting the load cell for faults can proceed. This requires the following test equipment:
- A high quality digital voltmeter and ohmmeter capable of reading from 0 – 50mV and 4 – 20mA with accuracy of ±0.1mV and ±0.5mA,
- A screw driver set,
- A mega-ohmmeter capable of reading 5000 ohms with accuracy of 500 ohms at 50V. It should be noted that the chosen mega-ohmmeter should not supply more than 50V to the load cell, since voltages exceeding this can cause permanent damage to the load cell.
Using these tools, the fault evaluations are then carried out as follows:
- Check the zero balance reading.
- Next, 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.
- Compare this value for the zero balance with the value in the product’s datasheet.
Insulation Resistance Tests
An insulation resistance 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 measuring the resistance between the load cell body and all its connected wires, as follows:
- First, disconnect the load cell from the summing box and indicator panel.
- Connect all the input, output and sense (if equipped) 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 product’s load cell datasheet. A lower value shows an electrical leakage caused by moisture; this causes short circuits, giving unstable load cell outputs.
Bridge Integrity Tests
A bridge integrity test is done when the weighing scale indicates overload, gives incorrect readings for a known weight, or does not give any reading at all. It is done by measuring the output resistance, input resistance and the bridge balance, through the following steps:
- Disconnect the load cell from the junction box or measuring devices.
- Measure the input and output resistance across input and output terminals of the load cell with an ohmmeter.
- Compare the resistance values with those in the product datasheet.
- Compare the resistance from the negative output lead to the negative input lead with the resistance from the negative output lead to the positive input to obtain the bridge balance. The difference between the readings is meant to be small or equal to 1.
Changes in these measured values are caused by electrical component failures or an internal short circuit. Often these problems are caused by lightning strikes (overvoltage), excess temperature or physical damage from improper handling.
Note: 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 load cell’s normal input and output voltage and can therefore damage the load cell.
Shock Resistance Tests
A shock resistance test is carried out if the load cell gives erratic output display when load is applied or after load is removed. The steps are:
- Connect the load to a stable voltage source.
- Connect a voltmeter to the output terminal.
- Gently tap on the load cell 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 could be a failed electrical connection or possibly a glue layer between the strain gauge sensor and element caused by an electrical transient.
If the load cell is still defective after all the above tests for faults are performed, it can be returned to the OEM for repairs, or replacement if the problem cannot be fixed. Tacuna Systems supports all of its products and is available to help troubleshoot and resolve any product issues.