The force measurement system is crucial to almost all industrial, commercial, and trade sectors. As the key component of the force measurement system, weighing load cells must be accurate and always functioning properly. Knowing how to test weighing load cells can help make informed decisions when it comes to maintenance or component replacement, whether it is done as a routine maintenance or in response to performance interruption.
The working principle of weighing load cells is to measure the voltage signals sent from a regulated power source that are applied to them. Then the control system equipment converts the signal into digital values that are easy to read on a display screen. Weighing Load cells need to perform in almost every environment, sometimes posing many challenges to their functionality. These challenges make weighing load cells vulnerable to failure, and sometimes they may encounter issues that affect their performance. If a failure does occur, it is best to check the integrity of the system first. For instance, overloading of scale capacity is not uncommon. This can cause the weighing load cell to deform, and even result in an impact load. Surges can also damage weighing load cells, and any water or chemicals spilled at the entry point of the scale can interrupt the weighing load cell.
Zero balance calibration
Zero balance calibration can help determine if weighing load cells have sustained any physical damage, such as overstress, impact loads, or metal wear or fatigue. Before starting, make sure that weighing load cells are in a "no-load" state. Once the zero balance reading is indicated, connect the weighing load cell input terminals to the excitation or input voltage. Use a millivolt meter to measure the voltage. Divide the reading by the input or excitation voltage to get the zero balance reading. The reading should match the original weighing load cell calibration certificate or product data sheet. If not, the weighing load cell is damaged.
Measure the insulation resistance between the cable shield and the weighing load cell circuit. Disconnect the weighing load cell from the junction box and connect all leads together - input and output. Measure the insulation resistance with a megohmmeter, measuring the insulation resistance between the connected leads and the weighing load cell body, then the cable shield layer, and finally the insulation resistance between the weighing load cell body and the cable shield layer. The insulation resistance readings for bridge circuits to the case, bridge circuits to the cable shield, and the case to the cable shield should be 5000MΩ or higher. Lower values indicate leakage caused by moisture or chemical corrosion, while very low readings are a clear indication of a short circuit rather than moisture intrusion.
Check the input and output resistance of the bridge, and measure it with an ohmmeter on each pair of input and output leads. Compare the input and output resistance from "negative output" to "negative input," as well as "negative output" to "positive input," using the raw data table specifications. The difference between the two values should be less than or equal to 5Ω. If not, there may be wire breakage or short circuit caused by impact loads, vibration, wear, or extreme temperatures.
Weighing Load cells should be connected to a stable power source. Then, using a voltmeter, connect to the output leads or terminals. Be careful, push the weighing load cell or roller to introduce a slight impact load, being careful not to apply too much load. Observe the stability of the reading and return it to the original zero balance reading. If the reading is unstable, it may indicate an electrical connection failure or an electrical transient may have damaged the adhesive layer between the strain gage and components.
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