Why calibrate the load cell?
Load cells are special force sensors used to measure weight or force in various applications. They are an indispensable part of weighing systems in different industries such as aerospace, shipbuilding, and automobiles. They can obtain highly accurate weighing data. Calibration of load cells ensures accurate load cell readings in order to prevent undesirable consequences, which is why they must be checked and calibrated regularly.
The load cell showed signs of wear after several years of use. It has a certain relationship with the aging of the load cell caused by environmental factors such as frequency of use and temperature. Cable and machine failures, particle accumulation and material accumulation, mechanical defects, incorrect installation, and the impact of electricity can also cause inefficiency. This is why routine calibration should be performed to ensure the efficiency and accuracy of the load cell. In the case of infrequent calibration, the load cell may give incorrect readings and generate incorrect data.
Routine calibration of load cells helps to achieve accuracy of around 0.03 to 1%. As part of product responsibility, safety and compliance in the quality management system, load cells should be calibrated in accordance with national standards.
Calibrate the load cell
Before calibrating the load cell, first check whether the machine can provide correct measurement data.
We mentioned three key indicators to evaluate whether the load cell and sensor are working properly, including: when the system is empty, the weight indicator should return to zero. When the weight is doubled, the indicated weight must be doubled. No matter where the load is placed, the weight indicator should provide equal readings. As long as the above conditions are met, you can safely draw a conclusion that the load cell is working normally. Due to cable failure or improper installation, the load cell may not provide accurate readings. Therefore, before calibrating the load cell, please check the following: Check the cables and wires. Please remember that before the construction and welding work is completed, please place the virtual sensor on the position of the operational load cell. After performing the preliminary test, if you find that the load cell is causing a problem, perform the following test:
Check for physical damage in the load cell. In addition, check for dents and cracks on all four sides. Please note that if the load cell is not in its original shape and is changed-compressed, bent or stretched, the load cell needs to be replaced.
This test should be performed when there is no load and the system is disconnected from the weight controller. Measure the excitation lead of the input resistance and the signal lead of the output resistance. Compare the reading with the load cell specifications. Tolerance readings are usually caused by power fluctuations.
Zero point balance:
Residual stress in the sensing area usually results in a shift in zero balance. If the load cell is repeatedly overloaded in multiple operating cycles, residual stresses will accumulate. When the system is empty, check the output of the load cell with a voltmeter. It must be within 0.1% of the above zero output signal. If the zero balance tolerance band is exceeded, the cell may be damaged.
Connect the input, output, and ground leads. With the help of an ohmmeter, check the resistance between the load cell and the lead. If the reading does not reach 5000 megohms, disconnect the ground wire and repeat the test. If it fails again, the battery may be damaged. By performing these steps, you can not only ensure that the load cell is working properly, but you can also prevent and take preventive measures to prevent any potential damage.
How to calibrate the load cell?
Standard calibration will test the repeatability and linearity of the load cell, both of which are used to determine accuracy. The most commonly used is the ‘5-step’ method, where a known load is applied to the loading unit in an incremental manner, and output readings are performed at each step. For example, when the load is 20, 40, 60, 80, and 100 tons, a 100-ton load cell will take the reading. This process is repeated twice, and the difference in results is used to determine its repeatability/accuracy. Since most load cells are used for some form of readout/display to form a weighing system, the load cell and meter should always be calibrated together as much as possible.
(1) Place the bench frame on a stable and inelastic foundation, and the plane on which the bench frame is placed on the load cell should be basically level.
(2) Fix the load cell on the bench frame with the backing plate.
(3) Put on the hanger, and make the indenter of the hanger press positively on the indenter of the sensor.
(4) Hook the weight support to the hanging rack.
(5) Connect the bridge power supply of the load cell, and connect the output to a high-precision millivolt meter (the accuracy should be higher than 70% of the sensor’s nominal accuracy) (the current output value can also be measured when necessary).
(6) According to the measuring range of the sensor and the number of points to be measured, load (weight) and unload step by step to the weight support hook, and record the output data of the sensor. Thereby, performance indicators such as zero point output, linear accuracy, repeatability accuracy and hysteresis of the sensor can be detected, and it is natural to determine whether the sensor is normal and good or bad.