Physical ChecksPhysically checking the load cell is an important part of the testing process. Proper inspection involves checking the physical condition of the load cell to see if the cable has any signs of damage – such as fraying or breaks in the insulation. It is also important to check the load cell element for any deformations, scratches, or other signs of damage. Additionally, it is important to check the connection between the load cell and its electronics, as well as the state of the mounting bracket. Finally, it is important to ensure that there are no obstructions between the load cell and the unit to be measured.
Electrical ChecksTo electrically test a load cell before putting it to use, a multimeter is needed as well as a voltage source. Measure the resistance of the input and output leads of the load cell by setting up the multimeter in ohms. Compare the measurement values with the calibration certificate from the manufacturer to see if they closely match each other. Similarly, check the load cell for accuracy by measuring the millivolts signal from the input leads. With no force applied to the load cell, the value should be zero. Apply a calibrated dead weight as specified in the calibration certificate and compare the values again.
Set the multimeter in DC millivolts and connect the output wires of the load cell to the multimeter. Supply a voltage of 5V or 9V DC at the excitation leads and place a test weight on the load cell. The multimeter will register a change in voltage measured across the load cell’s output.
How to Measure Load Cell Output?Load cell output is measured using a digital meter. The digital meter connects to the output of the load cell. It converts the digital signals produced by the load cell into readable digital values. You can also measure the output of a load cell using a multimeter. However, a multimeter will delete the output voltage in millivolts, and will not convert it into force or weight.
How to Check Load Cell Resistance?A load cell test is performed using a digital multimeter. To test for load cell output resistance, the digital multimeter is connected between the positive signal wire and the negative signal wire of the load cell. The output between them should be equal to a value specified in the datasheet. To test for input resistance, check the signal between the positive excitation wire and the negative excitation wire – which should be equal.
A multimeter can also be used to check insulation. Ideally, the insulation resistance should be above 5 Giga Ω; if the resistance is below 2 Giga Ω, the insulation is likely bad.
Load Cell Value FluctuationLoad cell values can fluctuate for a variety of reasons. From physical damage such as shock loading and overloading for a longer duration to environmental conditions such as temperature, moisture, water ingress, or chemical corrosion, the load cell is likely to produce erroneous output. Measurement values will also fluctuate if the cables break or if there’s a short circuit.
To check what is causing fluctuations in the load cell value:
- Perform a visual check to identify the fault location
- Perform a zero-balance check to identify if the strain gauge has undergone permanent deformation
- Perform an insulation resistance check to further help identify if moisture is getting inside the load cell
- Perform a bridge resistance check to determine if there is a short circuit within the load cell
Load Cell Zero Drift or BalanceTo check for zero balance, the load cell is positioned with no load attached and the input is connected to a stable, low noise power supply. The mV/V value is determined by measuring the output voltage in mV and dividing it by the input voltage in V.
Zero drift is the condition where zero measurements of the load cell change randomly under no-load conditions. It can also happen when the apparatus is loaded, and this phenomenon is called ‘drift’. Several reasons such as mechanical errors, fluctuation in excitation voltage, and temperature variations could cause drift. To troubleshoot the load cell for zero drift, it is important to inspect the entire system.
Load Cell Negative ReadingA load cell negative reading occurs when the load cell is in an incorrect orientation. A load cell used to measure tension will not reflect a negative reading if it is installed upside down, for example, and will result in an erroneous reading. However, if the load cell is installed correctly and the readings are still negative, verify the wire connection according to the color code specified by the manufacturer.
There is usually an arrow on the load cell that shows the direction of loading.
Load Cell OverloadEvery load cell comes with a rated capacity, and loading a load cell beyond the rated value will overload it. The telltale signs of load cell overload are, among others:
Inconsistent display readings Readings not coming back to zero even after the load is removed Dramatic change of zero balance
Shock overload is one of the most damaging among overloads. Here the weight on the load changes to a significant degree in a very short period.
Most load cells endure some overload, and this value is called Safe Overload. Anything beyond that can lead to permanent damage.
- What is a Load Cell?
- Types of Load Cells
- Selecting a Load Cell
- Installing a Load Cell
- How to Wire a Load Cell?
- Amplifiers for Load Cells
- Data Loggers for Load Cells
- Load Cell Meter
- Load Cell Controller
- Signal Conditioners
- Weighing Trucks, Trains, and Aircraft
- Dynamic Load Measurement
- Testing Load Tolerance in Cables
- Critical Component in X-ray Laser