What Is a Load Cell? How Does a Load Cell Work?

Physical Inspection

How does your load cell look? If it is covered with rust, corroded or badly oxidized on its exterior, corrosion has likely worked its way into the strain gauge area. If the general and physical condition has a good appearance, then you likely need to look at specifics: sealing areas, the element itself and the cable.

In most load cells, areas of the load cell are sealed to protect the internal components from contamination by water and chemicals. To see if any seals have been degraded, look closely at the strain gauge seals.

Is rust concentrated on part of the welded cover? If there is no cover, do you see any tiny holes in the potting? These are indications there has been contamination to the gauge area. Check the load cell cable entrance for signs of contamination as well.

Other items to look for include: element damage, bends or cracks, cracks in the weld, or abrasions in the metal. It may be necessary to remove the load cell and check it for physical distortion against a straight edge.

No inspection would be complete without thoroughly reviewing the cable. The cable should be free of cuts, crimps and abrasions. If your cable is cut and in a wet environment, water or chemicals can "wick" up the cable into the strain gauge area, causing load cell failure. If your physical inspection fails to uncover any identifiable damage, a more detailed evaluation is required.

Zero Balance

This test is effective in determining if the load cell has been subjected to a physical distortion, which can be caused by overload, shock load or metal fatigue. Before beginning the test, the load cell must be in "no load" condition. In other words, either the cell should be removed from the scale or the dead load should be counterbalanced. This test is effective in determining if the load cell has been subjected to a physical distortion, possibly caused by overload, shock load or metal fatigue. 

Once the cell is no longer under any load, disconnect the signal leads and measure the voltage across the negative signal and positive signal. The color code for determining negative- and positive-signal leads is provided on the calibration certification with each load cell. The output should be within the manufacturer’s specifications for zero balance, usually ± 1% of full scale output. During the test, the excitation leads should remain connected, with the excitation voltage supplied by the digital weight indicator. Be certain to use the exact same indicator that is used for the cell’s daily operation to get a reading specific to the application. 

The usual value for a 1% shift in zero balance is 0.3 millivolt, assuming 10 volts excitation on a three millivolt per volt output load cell. To determine your application’s zero shift, multiply the excitation volts supplied by your indicator by the millivolt per volt rating of your load cell. When performing your field test, remember that load cells can shift up to 10% of full scale output and still function correctly. If your test cell displays a shift under 10%, you may have another problem with your suspect cell, meaning further testing is required. If the test cell displays a shift greater than 10%, it is likely it has been physically distorted and should be replaced.