A lifting load cell is essentially a device inserted into the load path, which is normally installed in the vicinity of the junction between the lifting hook and the load. It is a device that changes mechanical force into electricity, which is subsequently translated into weight. They are completely invisible when they work well. But when they don’t work well, things can get pretty serious.
How Strain Gauges Really Work in Measuring Weight in Lifting Load Cells
Most lifting load cells utilise a technology called the strain gauge. It is essentially based on the principle that the resistance level of any electrical conductor increases when it is subjected to any type of mechanical pressure. Inside the load cell casing, strain gauges are installed and wired to particular areas on the metallic load cell component, which is designed to flex in a particular way when subjected to increases in weight.
These changes in resistance level are minuscule, measured in terms of micro-ohms. To measure these changes in resistance level effectively, the strain gauges are wired in what is called the Wheatstone bridge configuration.
The quality of measurement is obviously dependent on the quality of the load cell itself. Low-grade load cells may have an aluminum alloy case, which is lightweight but may suffer from temperature drift and fatigue. Better-grade load cells may have alloy steel or stainless steel cases that are less susceptible to temperature and environmental influences. The bonding of the individual strain gauges is also important. The quality of bonding will have a huge influence on the reliability and drift of the load cell, which may not become apparent until later during calibration.
Calibration Drift and When It Matters
Load cells do drift over time. Mechanical fatigue, temperature effects, and simple aging of the components cause a load cell to become less accurate in its ability to relate the actual weight to the output. This may not matter for some purposes. The actual weight is 8 tonnes instead of 8.2 tonnes. You may be able to live with that. However, for critical purposes, it may be important to recalibrate regularly to maintain accurate records.
The problem is how often do you actually need to recalibrate a load cell? The answer is that it is a good idea to recalibrate once a year. However, some may need to be recalibrated more often and some may not need recalibration at all. The answer is that a baseline needs to be established and regular monitoring carried out to determine how quickly a load cell is actually drifting in a particular application.
This requires proper equipment in the form of dead weights or a master load cell and a test rig that is able to apply loads to the device. This is not something that can be done in the field with rough estimates and a pencil and paper.