Electronic force measurement systems are vital to virtually all industries, commerce and trade. Since load cells are critical components of force measurement systems, they must be accurate and function properly at all times. Whether as part of scheduled maintenance or in response to a performance outage, knowing how to test a load cell can help make informed decisions about repairing or replacing components.
Why do load cells fail?
Load cells work by measuring the force exerted on them by a voltage signal sent from a regulated power source. A control system device, such as an amplifier or tension control unit, then converts the signal into an easy-to-read value on a digital indicator display. They need to perform in almost every environment, which can sometimes pose many challenges to their functionality.
These challenges make load cells prone to failure and, at times, they may experience issues that affect their performance. If a failure does occur, it’s a good idea to check the integrity of the system first. For example, it is not uncommon for scales to be overloaded with capacity. Doing so can deform the load cell and even cause shock loading. Power surges can also destroy load cells, as can any moisture or chemical spillage at the inlet on the scale.
Reliable signs of load cell failure include:
Scale/device will not reset or calibrate
Inconsistent or unreliable readings
Unrecordable weight or tension
Random drift at zero balance
did not read at all
Load Cell Troubleshooting:
If your system is running erratically, check for any physical deformities. Eliminate other obvious causes of system failure – frayed interconnect cables, loose wires, installation or connection to tension indicating panels, etc.
If the load cell failure is still occurring, a series of troubleshooting diagnostic measures should be performed.
With a reliable, high-quality DMM and at least a 4.5-digit gauge, you’ll be able to test for:
zero balance
Insulation resistance
bridge integrity
Once the cause of the failure 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 suffered any physical damage, such as overload, shock loading, or metal wear or fatigue. Make sure the load cell is “no load” before starting. Once a zero balance reading is indicated, connect the load cell input terminals to the excitation or input voltage. Measure the voltage with a millivoltmeter. Divide the reading by the input or excitation voltage to get a zero balance reading in mV/V. This reading should match the original load cell calibration certificate or product data sheet. If not, the load cell is bad.
Insulation resistance:
The 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 a megohmmeter, measure the insulation resistance between the connected lead wire and the load cell body, then the cable shield, and finally the insulation resistance between the load cell body and the cable shield. Insulation resistance readings should be 5000 MΩ or greater for bridge-to-case, bridge-to-cable shield, and case-to-cable shield, respectively. Lower values indicate leakage caused by moisture or chemical corrosion, and extremely low readings are a sure sign of a short, not moisture intrusion.
Bridge Integrity:
The bridge integrity checks the input and output resistance and measures with an ohmmeter on each pair of input and output leads. Using the original datasheet specifications, compare the input and output resistances from “negative output” to “negative input”, and “negative output” to “plus input”. The difference between the two values should be less than or equal to 5 Ω. If not, there may be a broken or shorted wire caused by shock loads, vibration, abrasion, or extreme temperatures.
Impact resistance:
Load cells should be connected to a stable power source. Then using a voltmeter, connect to the output leads or terminals. Be careful, push the load cells or rollers to introduce a slight shock load, being careful not to apply excessive loads. Observe the stability of the reading and return to the original zero balance reading. If the reading is erratic, it may indicate a failed electrical connection or an electrical transient may have damaged the bondline between the strain gauge and the component.
Post time: May-24-2023
