25 September 2017

Load Cell Troubleshooting

The following tests are required to make to find a faulty load cell,if connected 2 or more load cells please disconnected all the load cell
 Zero balance
 Insulation resistance
 Input and output resistance
4  Bridge Integrity
 Shock resistance


1  Zero balance measurament
1.1  Shouldn’t any weight on the load cell
1.2  The load cell should be connected to a stable power supply, Preferably a load cell indicator with an excitation voltage of at least 10 volts.
1.3   Measure the voltage across the load cell’s output leads with a millivoltmeter and divide this value by the input or excitation voltage to obtain the Zero Balance in mV/V.Compare the Zero balance to the original load cell calibration certifcate ( if available ) or to the data sheet.
Changes in Zero Balance usually occur if the load cell has been permanently deformed by overloading and/or excessive shocks.Load cells that  experience progressive zero output changes per time period are most likely undergoing a change in the strain gauge resistance because of chemical or moisture intrusion. However, in this case the insulation resistance and/or the bridge integrity will also be compromised.


2  Insulation Resistance
2.1  Connect all cables together.
2.3   Measure the insulation resistance between the cables and the load cell housing with megaohmmeter
2.4  The insulation resistance of all load cells should be 5000 MΩ or more for bridge circuit to  housing,
Bridge  circuit to cable screen and housing to cable screen.A lower value indicates electrical leakage, which is usually caused by moisture or chemical contaminations within the load cell or cable.Extremely low values (≤1kΩ) indicate a short circuit rather than moisture ingress.Electrical leakage results usually in unstable load cell or scale reading output. The stability might vary with temperature.


3  Input and output resistance
3.1  Disconnect the load cell
3.2  Measure the resistance between SIG- and EXC-.
3.3 Measure the resistance between SIG+ and EXC+.
commonly used standard values:Input resistance: 350Ω and 700Ω , Output resistance: 350Ω and 700Ω


4  Bridge Integrity
4.1  Measure the resistance between SIG- and EXC-.
4.2  Measure the resistance between SIG+ and EXC+
4.3  The difference between the two values must be no greater than 1 Ω.
Changes in bridge resistance or bridge balance are most often caused by a broken or burned wire, an electrical component failure or internal short circuit. This might result from over-voltage (lightning or welding), physical damage from shock,  vibration or fatigue, excessive temperature, or from production inconsistencies.


5  Shock Resistance
The load cell should be connected to a stable power supply, preferably a load cell indicator with an excitation voltage of at least 10 volts. Disconnect all other load cells for multiple load cell systems. With a voltmeter connected to the output leads, lightly rap on the load cell with a small mallet to mildly shock it. Exercise extreme care not to overload low capacity load cells while testing their shock resistance. Watch the readings during the test. The readings should not become erratic, should remain reasonably stable and return to original zero readings. Analysis Erratic readings may indicate a failed electrical connection or a damaged glue layer between strain gauge and element as a result of an electrical transient.

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