Load cells are not only the most important but also the most vulnerable component of an electronic weighing system. Load cells face obstacles that may hinder their operation due to their use in a variety of industries and environments. The following are some of the most typical causes of load cell failure:
- Overloading the scale, resulting in shock and deformation of the load cell
- Lightning strikes or electrical surges
- Ingress of chemicals or moisture
- Improper treatment
Zero drift, difficulty to calibrate and reset the scale, inconsistent readings, or weights that don’t register are all signs of a load cell malfunction. A series of signal measurements should be performed after checking for obvious sources of malfunction, such as worn cables, loose wire connections, or distortions in the load cell itself. Resistance measuring, a different sort of diagnostic test, is most typically used to troubleshoot failed load cells. Although this is practical and will reveal some information, it will not reveal all of the information required to fix a defective cell in a cost-effective manner. The tests listed below can be used in conjunction with one another to produce an issue summary.
Resistance Readings
To check resistance, use your voltmeter to take a series of resistance readings for each load cell in question. This can assist in determining if the internal load cell circuitry has malfunctioned.
Millivolt Return
Using your voltmeter to check across the plus and minus signal lines is all it takes to check the millivolt return.
Leakage Test
This test entails twisting all of the wires together with a megohmmeter. One lead will connect to the wire bundle, while the other will connect to the cell body. The reading is expected to deviate from the metre. If the meter’s lights stay inside the range, it means there’s a leak or maybe water infiltration.
Tap Test
Simply monitor the indicator while softly tapping the load cell with the handle of a screwdriver or a rubber mallet to perform the tap test. The existence of a loose component on the inside of the load cell would be indicated by large jumps in the display. Ensure that all taps are light enough to avoid damaging the load cell. Never use a hammer in this situation.
Delta Weight/Signal Test
In diagnostics, delta (∆) weight can also be employed. The difference in millivolt readings that correlate to a change in weight on the scale is examined in this weight and signal test. For instance, you may take a reading without putting any weight on the scale and then another reading with 100 pounds on the scale. The differences in values can be compared to what they should be in a new system.
Unknown Color Codes or Capacities
It’s possible that you’ll need to double-check the colour code and capacity when troubleshooting load cells. This information may be readily available in many cases; nevertheless, information identifying the model and maker may be rubbed off or otherwise impossible to see. Follow the methods below to establish the colour code and capacity in these situations:
How to Determine Color Code for an Unknown Load Cell
- Write down the findings of all six resistance readings using all conceivable lead combinations.
- Look for the resistance reading pair with the highest resistance—this is usually the excitation pair. The signal leads will be the pair of leads with the next highest readings.
- There should be four sets of resistance readings, all of which should be roughly the same. Individual bridge resistance values will typically account for 70 to 80 per cent of the total bridge resistance readings.
- Determine polarity using the information from your testing.
- It’s normally the minus excitation if one of the excitation leads is black.
- Connect the load cell to the excitation voltage and read the signal output.
- Watch to see if the signal increases as you apply force to the cell in the normal direction.
- How to Determine the Capacity of an Unknown Load Cell
- Connect the load cell excitation leads to a power supply or an indicator.
- With no weight on the scale or cell, measure millivolt signal return (zero).
- Read the millivolt signal return after adding a known quantity of weight.
- Subtract the weighted reading from the initial millivolt signal reading.
- Subtract the weight from the signal. As a result, a millivolt per unit of weight value will be generated (lb, kg, g).
- Based on normal mV/V values, use this number to compute what the full-scale capacity might be. For example, if the excitation voltage was 10 VDC and the cell was 3 mV/V, the predicted output at full size would be 30 mV. Create an equation to solve for the unknown cell’s weight.
