Advanced Load Cell Troubleshooting

With placements in numerous industries and every type of environment, load cells undergo challenges that may impair their functionality. Some common causes of load cell malfunction include:

• Overloading scale capacity, which results in shock and load cell deformation

• Lightning strikes or electrical surges

• Chemical or moisture ingress

• Improper handling

Symptoms of load cell malfunction might be zero drift, the inability to calibrate and reset the scale, unreliable readings or weights that don’t register. After checking for obvious causes of load cell failure, such as worn cables, loose wire fittings or visible damage to the load cell itself, you can perform a series of signal measurements. Resistance measuring is most commonly applied to troubleshoot failing load cells. However, it will not expose all the information needed to cost-effectively repair a bad load cell. Below is a list of tests that can be used with one another to fully understand the problem.

Millivolt Return

Checking the millivolt return involves using your voltmeter to measure the voltage across the plus and minus signal lines.

Resistance Readings

Checking resistance involves taking a series of resistance readings with your voltmeter for each load cell. This can help determine if there is a fault in the internal load cell circuitry.

Leakage Test

This test involves using a megohmmeter and twisting all the wires together. One lead will attach to the bundle of wires, and the other will attach to the load cell body. The reading is expected to go off the meter. If the lights on the meter stay in range, it indicates leakage or perhaps water ingress.

Delta Weight/Signal Test

Delta (∆) weight can also be used in diagnostics. This weight and signal test looks at the difference in millivolt readings that correspond to a change in weight on the scale. For example, you might take a reading without weight on the scale, followed by a reading with 100 pounds, the value change can be compared with what it should be relative to a new system.

Tap Test

The tap test can be done simply by monitoring the indicator while lightly tapping the load cell with the handle of a screwdriver or a rubber mallet, but never a hammer. Drastic jumps in the display would indicate the presence of a loose component on the inside of the load cell. Ensure all taps are light enough to not damage the load cell.

Unknown Color Codes or Capacities

You may need to verify color code and capacity when troubleshooting load cells. In many instances, this information may be known; however, information identifying the model and manufacturer may be rubbed off or otherwise difficult to decipher. In these cases, follow the steps below to determine color code and capacity:

How to Determine Color Code for an Unknown Load Cell Measure all six resistance readings using all possible combinations of leads and write the results down. Look for the highest resistance reading, usually the excitation pair. The pair of leads with the next highest readings will be the signal leads. There should be four sets of resistance readings that are all about the same. These will be the individual bridge resistance readings and they will typically be about 70 to 80% of the bridge resistance readings. Using the information obtained from your tests, determine polarity. If one of the excitation leads is black, it will usually be the minus excitation. Connect the excitation voltage to the load cell and read the signal output. Apply force to the load cell in the normal direction and watch to see if the signal increases.

How to Determine the Capacity of an Unknown Load Cell Connect the load cell excitation leads to an indicator or other power supply. Measure millivolt signal return without weight on the scale or load cell (zero). Add a known amount of weight and read the millivolt signal return. Subtract the initial millivolt signal reading from the reading with weight. Divide the signal by the weight. This will result in a millivolt per unit of weight value (pounds, kilograms or grams). Use this value to calculate the full-scale capacity based on standard mV/V values. For example, if the excitation voltage was 10 VDC and the load cell might be 3 mV/V, then 30 mV would be the expected output at full scale. Set up an equation to solve for the weight of the unknown load cell.

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