Load Cell Troubleshooting
Contents
1. bridge balance is obtained by comparing the resistance from output to input and output to input The difference between both values should be smaller than or equal to 10 Analysis 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 Document Number 11867 Revision 14 Dec 2011 ut load cell or cable Extremely low values lt 1k Q 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 ut For technical support contact in Americas Ic usa vishaypg com in Europe Ic eur vishaypg com in China Ic prc vishaypg com 3 result from over voltage lightning or welding physical damage from shock vibration or fatigue excessive temperature or from production inconsistencies www vpgtransducers com in Taiwan Ic roc vishaypg com VPG 08 VPG Transducers Test 4 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 l l l l l l i With a voltmeter connected to the output leads light2. VISHAY PRECISION GROUP VPG TRANSDUCERS Load Cells Application Note VPG 08 Load Cell Troubleshooting Scope Load cells are designed to sense force or weight under a wide range of adverse conditions they are not only the most essential part of an electronic weighing system but also the most vulnerable Load cells might be damaged because of shock overloading lightning strikes or heavy electrical surges in general chemical or moisture ingress mishandling dropping lifting on cable etc vibration or internal component malfunction As a direct result the scale or system might zero drift provide unstable unreliable readings or not register at all This application note is written to assist our customers with potential load cell problems It describes basic field tests which can be performed on site and provides the information necessary to interpret the results Proper field evaluation is absolutely critical to prevent similarly induced damage in the future Under no circumstances should fault location as described below be attempted on load cells installed in a hazardous area In General Carefully check the system integrity before evaluating the load cells e Check for force shunts might be caused by dirt mechanical misalignment or accompanying components such as stay or check rods e Check for damage corrosion or significant wear in the areas of load introduction e Check cable connectio
3. be removed from the load cell Low capacity load cells should be measured in the position in which the load cell is designed to measure force to prevent the weight of the element giving wrong results 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 any other load cell for multiple load cell systems 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 certificate if available or to the data sheet Analysis 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 www vpgtransducers com For technical support contact in Americas Ic usa vishaypg com in Europe Ic eur vishaypg com in China Ic prc vishaypg com 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 Document Number 11867 Revision 14 Dec 2011 in Taiwan Ic roc vishaypg com VISHAY PRECISION N croup VPG 08 VPG Transducers Load C
4. croup VPG Transducers Load Cell Troubleshooting Load Cell Evaluation Form Company Contact person Address City Country Tel Fax Repair order Date Load cell type Serial number Capacity Accuracy grade Short description of system failure and application Visual Inspection Label O OK O Unreadable O Missing Condition O Like new O Cable cut O J box damage O Broken welds O Visual mechanical overload O Dents cracks in parts Weld s O J box cable entry O Diaphragm Corroded parts O Housing element O Top bottom plate O Bellow tube cubs Affected by chemicals O No O Unknown O Yes Electrical Inspection Bridge Measurements Actual Specification Conclusion Zero balance mV V lt 1 of rated span O OK O Wrong Input resistance Q Q 1 o OK O Wrong Output resistance Q Q 1 a OK O Wrong Output to input Q o OK O Wrong Out put to input Q difference lt 1 O OK O Wrong Insulation Resistance Actual Specification Conclusion Bridge to housing MQ 25000 MQ O OK O Wrong Bridge to shield MQ 25000 MQ O OK O Wrong Shield to housing MQ 25000 MQ O OK O Wrong Expected Reason for Failure O Moisture ingress O Short circuit O Broken wire component O Excessive heat O Electrical transients O Mechanical overload O Excessive corrosion O Broken cable O Other Recommendation O Return load cell to supplier for further evaluation and repair if possible Return load cell to supplier for warranty O Load cell beyond econ
5. ell Troubleshooting Test 2 Insulation Resistance The insulation resistance is measured between the load cell circuit and element or cable shield Disconnect the load cell from the junction box or indicator and connect all input output and sense if applicable leads together Measure the insulation resistance with a megohmmeter between these four or six connected leads and the load cell body Repeat the measurement between the same 4 or 6 leads and the cable shield Finally measure the insulation resistance between the load cell body and cable shield Never use a megohmmeter to measure the input or output resistance as it normally operates at a voltage which exceeds the maximum excitation voltage by far Analysis The insulation resistance of all load cells should be 5000 MQ 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 Test 3 Bridge Integrity The bridge integrity is verified by measuring the input and output resistance as well as the bridge balance Disconnect the load cell from the junction box or measuring device The input and output resistance is measured with an ohmmeter across each pair of input and output leads Compare the input and output resistance to the original calibration certificate if available or to the data sheet specifications The
6. ly rap on the load cell with a small mallet to mildly shock it i Exercise extreme care not to overload low capacity load cells ut while testing their shock resistance i l l l l l l Watch the readings during the test The readings should not become erratic should remain reasonably stable and return to original zero readings Analysis Load Cell Troubleshooting VISHAY PRECISION N croup 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 Load Cell Evaluation Form The following load cell evaluation form should be used as a guide for the testing and evaluation of load cells We recommend that this form be included in the customer dossier and used as the basis to discuss the test results and diagnostics with third parties If a load cell is returned to VPG Transducers the Evaluation Form will assist our repair department in further diagnostics and repair of the cell www vpgtransducers com For technical support contact in Americas Ic usa vishaypg com in Europe Ic eur vishaypg com in China Ic prc vishaypg com in Taiwan Ic roc vishaypg com Document Number 11867 Revision 14 Dec 2011 VPG 08 VISHAY PRECISION M
7. ns to junction box and indicator e Check the measuring device or indicator with an accurate load cell simulator Visually inspect the load cells before performing the tests as described on the following pages Pay particular attention to signs of corrosion especially around the critical gauge area the integrity of the cable might be compromised due to cuts abrasions etc and the condition of the cable entry The following test equipment is required to properly evaluate a load cell e A high quality calibrated digital volt and ohmmeter with a measuring accuracy of 0 50 and 0 1 mV to measure the zero balance and integrity of the bridge circuit e Amegohm meter capable of reading 5000 MQ with an accuracy of 500 MQ at 50 volts to measure the insulation resistance Do not use megohm meters which supply more than 50 volts to the load cell in order to prevent permanent damage A means to lift the dead load weighbridge tank hopper conveyor etc off the load cell to be able to measure the zero balance or to remove the load cell s i e a crane hydraulic jack etc Load cells are produced according to specifications and tolerances which are described in the applicable data sheet More detailed information can be found on the calibration certificate which is packed with each load cell The calibration certificate mentions the exact values for the input and output resistance insulation resistance zero balance rated
8. omic repair g Document Number 11867 Revision 14 Dec 2011 For technical support contact in Americas Ic usa vishaypg com in Europe Ic eur vishaypg com in China Ic prc vishaypg com in Taiwan Ic roc vishaypg com www vpgtransducers com 5
9. output and the correct wiring code it provides an important reference for the values which can be measured and should be filed with the system documentation set Document Number 11867 Revision 14 Dec 2011 For technical support contact in Americas Ic usa vishaypg com in Europe Ic eur vishaypg com in China Ic prc vishaypg com in Taiwan Ic roc vishaypg com www vpgtransducers com 1 VPG 08 VPG Transducers VISHAY PRECISION QJ croup Load Cell Troubleshooting Test Procedures and Analysis The diagram below represents a proposed sequence for testing load cells after a particular system malfunction Isolate the fault location by moving a relatively small deadweight over each load cell or by disconnecting load cell by load cell Sudden change in Zero point Unstable readings random change in Zero point Test 2 Insulation Resistance Zero Balance Moisture or chemical ingress Mechanical overload Short circuit to housing screen Scale reads overload incorrect or not at all Erratic readings when load is applied or removed Test 3 Bridge Integrity Test 4 Shock Resistance Failed electrical connection Electrical overload or internal short circuit Broken wire or component Test 1 Zero Balance The Zero Balance is defined as the load cell output in a no load situation Therefore all weight including deadload has to
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