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How to Test a Load Cell

When the load cell faults occur. Firstly, it's necessary to carefully distinguish the system structure and whether the load cells have the following problems before removing the load cell from the weighing system:
  1. Check if there is a system transmission failure, the fault may be caused by dust, misalignment of mechanical parts or delayed component force transmission, rather than a load cell fault.
  2. Check if there are damages, rust or obvious wears at the force transmission part of the system. In winter days, it shall be noticed that whether the force transmission part is frozen, which affects the force transmission and relay of the system.
  3. Check whether the caging device of the system works and if the gap meets the requirements.
  4. Check whether the load cell cable is correctly connected to the junction box and display instruments, if there are broken wires or bad contacts between connecting wires. Focus on checking the connection reliability of nine-core plug of the bus and junction box.
  5. Check if there are faults in connection boxes and meters, particularly the potentiometers and terminals in junction box.
  6. Check whether the load cell is corroded or damped (particularly the patch hole area). Check the completeness of load cell cable. Check the environment at the entrance of load cell cable.
It is recommended that users should be equipped with the following instrumentation devices as necessary devices for detecting load cells:
  1. High performance calibrated digital multimeter (higher than four and a half digits), the detection precision shall be ±0.1Ω and ±0.01mv, detect the load cell's zero output and bridge integrity.
  2. Megger (insulation meter), to test the insulation impedance of load cells. 5000MΩ can be tested under a recommend measurement range of 50VDC.
Generally, load cell manufacturers would provide detailed input/output impedance, insulation impedance, zero output, output sensitivity and correct wiring code identification.
Now, test the load cell step by step as follows:
Load cell checking process
Step 1: Zero output check
Zero output refers to the output value of load cell without load. All kinds of load (including static loads, such as weighing body and force transfer parts) shall be removed while testing the load cell output. Load cell's zero point should be measured under the designed installation and application state of the load cell, so as to prevent the erroneous effects caused by load cell weight. Load cells should be connected to a regulated power supply, a excitation voltage recommended in product specifications or sample is prefer. Measure and record the output value of the load cell's millivolt voltage, then divide by the load cell's excitation voltage, and the zero output value (mV/V) is thereof obtained. Compare this output value with the one on the load cell certificate (if possible) or with the corresponding data in the load cell company's sample, then it can be judged whether the load cell's zero output is qualified.
Analysis: Permanent change of load cell zero position is usually caused by load cell overload or instantaneous impact. Periodic change of load cell zero position may be caused by dampness of the strain gauge or other reasons. Of course, such a condition can also be detected by testing the load cell's insulation impedance and the bridge resistance.
Step 2: Test the insulation impedance
In general, we should test the impedance between the load cell's lead-out and the load cell body (elastomer, shell). It shall be noticed that the load cell shall disconnect with other instruments apart from the junction box. Debug the insulation test box (instrument) and then connect one end of the instrument to the load cell cables (output wire, input wire, shielding wire), connect the other end to the load cell body (elastomer, shell). The impedance is generally required to be larger than 5000MΩ. Attention: The insulation box's pens are not allowed to test the input and output impedance of the load cell, because the output voltage of insulating box is higher than the withstand voltage of electrical components inside the load cell.
Analysis: We generally require the insulation impedance between the load cell lead-out wire and the load cell body shall be larger than 5000MΩ. Low insulation value may be caused by dampness of load cell bridge or partial bridge damage. Particularly low insulation impedance (≤1KΩ) may be caused by serious dampness of load cell which leads to short circuit between the bridge and load cell body or the breakdown damage of insulation layer. The direct manifestation of low load cell insulation is that the load cell output is unstable and would change with the environment change, or even affected by the printer.
Step 3: Testing bridge impedance
Testing the bridge impedance is to test the completeness of load cell bridge. When testing, the load cell should disconnect from the junction box and other testing equipment. Input and output impedance testing is to measure the impedance value at the load cell's input and output terminals respectively with a digital multimeter pen, then compare the testing value with the value on product qualification certificate. Confirmation of bridge symmetry is to measure the impedance between an input terminal and an output terminal with digital multimeter, a total of 4 impedance values could be obtained. In a fully symmetrically compensated load cell, the extreme difference of these four impedance values should not be greater than 1Ω (for the multimeter with lower accuracy, this value shall not be larger than 2Ω).
Analysis: Changes of bridge impedance is often caused by bridge damage or disconnection, failure of electronic components or internal short circuit. It may be caused by the breakdown of bridges by high current or voltage (lightning or welding current), or the shock or vibration fatigue, or ultra high temperature.
Step 4: Testing the load cell output
Connect the load cell to the regulated power supply independently with an excitation voltage of 10-15VDC. Connect the output end of the load cell to the millivolt meter (or turn the multimeter at the DC millivolt gear), load on the load cell according to the installation and use status of the load cell, then observe the output changes.
Analysis: Normally, the load cell should have a positive output. If the load cell output abnormally, it may be caused by wrong connection of the load cell, the falling off of the load cell strain gauge and elastomer matrix, or internal circuit breaking.

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