Electronic force measuring systems are vital to nearly all industries, commerce, and trade. As load cells are the key component of a force measurement system, they must be accurate and functioning properly at all times. Whether performed as regular maintenance or as a reaction to a disruption in performance, knowing how to test a load cell can help to make informed decisions on repairing or replacing the component.
Why Do Load Cells Fail?
Load cells work by measuring the force exerted on them via voltage signals sent from a regulated power source. The control system device, such as an amplifier or tension control unit, then converts the signals to numeric values easily read on the display of the digital indicator. They are required to perform in near every environment which, at times, can present numerous challenges to their functionality.
Such challenges make load cells susceptible to faults and, on occasion, they may encounter problems that affect their performance. If faults do occur, then it’s best to first check the integrity of the system. For example, it’s not that uncommon to overload the scale’s capacity. Doing so can lead to load cell deformation, even result in a shock load. A power surge can disrupt a load cell, too, as can any moisture or chemical spill at a point of ingress on the scale.
Sure signs of a load cell malfunction include:
- Inability of the scale/device to reset or calibrate
- Inconsistent or unreliable readings
- Weights or tension that fails to register
- Random drifts at zero balance
- No reading at all
Load Cell Troubleshooting
If your system is performing erratically, check for any physical deformities. Eliminate other obvious causes of a malfunctioning system – a worn interconnecting cable, loose wire, fitting or connection to the tension indicator panel, and the like.
If a load cell fault is still occurring then a series of troubleshooting diagnostic measures should be performed. With a trusty, quality digital multimeter in hand with at least a 4.5-digit meter, you will be able to conduct tests for:
- Zero balance
- Insulation resistance
- Bridge integrity
Once a cause for the malfunction has been identified, your team can decide how to move forward.
Zero Balance
A zero balance test can help determine if the load cell has been subjected to any physical damage such as an overload, shock load, or metal wear or fatigue. Make sure the load cell is in a “no load” condition before starting. Once the zero balance reading is indicated, connect the load cell input terminals to an excitation or input voltage. Measure the voltage using the millivoltmeter. Divide the reading value by the input or excitation voltage to get a zero balance reading in mV/V. That reading should match the original load cell calibration certificate or the product’s datasheet. If not, you have a damaged load cell.
Insulation Resistance
Insulation resistance is measured between the cable shield and the load cell circuit. After disconnecting the load cell from the junction box connect all the leads together – input and output. Measure the insulation resistance with the megohmmeter and measure the insulation resistance between the connected leads and the load cell body, and then the cable shield, and finally between the load cell body and cable shield. The readings for the insulation resistance should be 5000 MΩ or more for bridge circuit to housing, bridge circuit to cable screen and housing to cable screen, respectively. Lower values indicate electrical leakage caused by moisture or chemical corrosion, an exceptionally low reading is a sure sign of a short circuit rather than moisture intrusion.
Bridge Integrity
The bridge integrity checks the input and output resistance and is measured with an ohmmeter across each pair of input and output leads. Using the original datasheet specs, compare the input and output resistance from “minus output” to “minus input”, and “minus output” to “plus input”. The difference between both values should be smaller than or equal to 5 Ω. If not, chances are there is a broken or shorted wire that could have been caused by a shock load, vibration, wear, or temperature extremes.
Shock Resistance
The load cell should be connected to a stable power supply. Then using the voltmeter, connect to the output leads or terminals. With care, push on the load cell or roller to introduce a mild shock load, be cautious to not apply excessive load. Observe the readings for stability and the return to the original zero balance readings. If the readings are erratic it’s a likely indication of a failed electrical connection or an electrical transient may have damaged the glue layer between the strain gauge and element.
