Load cells are designed to detect force or weight under unfavorable situations; they are not only an essential component of an electronic weighing system but also the most susceptible.
Ingress of chemicals or moisture, improper handling, vibration, internal component failure, overloading (shock), lightning strikes, or heavy electrical surges generally can all cause damage to load cells.
The scale or system may drift, produce unstable or incorrect readings, or fail to record.
The performance of a load cell force (or weight) measuring system is based on the physical installation's integrity, accurate connectivity of the components, the proper performance of the main components that comprise the system, and system calibration.
If the installation was initially operational and properly calibrated, troubleshooting could start by checking each part separately to see if it has been damaged or has malfunctioned.
The fundamental parts are:
- Load cells
- Mechanical supports and load connections
- Interconnecting cables
- Junction boxes
- Signal conditioning electronics
Load Cells not installed according to the manufacturer's guidelines may not perform as expected. It is always beneficial to check:
- Installing surfaces for cleanliness, flatness, and alignment
- All mounting hardware torque
- Load cell orientation: Live end attached to the load to be measured, dead-end connected to mechanical reference or load forcing source. (A dead end is an end that is mechanically closest to a connector or cable exit.)
- Connecting the load to the load cell requires Interface-recommended hardware (thread diameters, jam nuts, swivels, etc.).
- One of the most important rules is that there can only be one load path. This load path must pass via the load cell's load axis. Although it may seem simple, this issue is frequently disregarded.
The proper load cell operation is also affected by the electrical system. The following are examples of common problem areas:
- Loose or soiled electrical connections or faulty color-coded wire connections.
- Failure to employ remote excitation voltage sensing over long connections.
- Incorrect excitation voltage setting. (The optimal setting is 10 VDC, as this is the voltage used to calibrate the load cell at the factory. The maximum permissible voltage is 15 or 20 volts depending on the model. Some battery-powered signal conditioners use as low as 1.25 volts to preserve energy.)
- Bridge circuit Loading. (Extremely accurate load cell systems necessitate highly accurate read-out instruments. These types of devices often have very high input impedances to prevent circuit loading errors.).
- Input and output resistances. If these are more significant than 3 k, the load cell is often faulty due to electrical surges or lightning.
- Separate resistance of strain gauges.
- Make sure the input and output resistance are within the product's permitted tolerance by measuring them with a multimeter (resistance accuracy of 0.025 or better).
- Check the certificate of calibration. A difference of more than 0.1 indicates a faulty load cell.
- Check the insulation of the wire leads connecting to the load cell's metal body using a multimeter. The insulation is poor if the resistance is less than 2 Giga. The ideal insulating resistance is better than 5 Giga.
- Connect the load cell to a steady power source and measure the mV/V output as in the previous step to check for zero return. Ensure that it is within the acceptable tolerance. Load the load cell for 5 seconds at 50% to 100% capacity. Check if the mV/V output returns to the acceptable tolerance after removing the load.
- Place the load cell without a load applied to check for zero balance. Connect the input to a low-noise, steady power source. Measure the output voltage in mV using a multimeter, then divide it by the input voltage in V to obtain mV/V. Refer to the load cell's calibration certificate to determine if this mV/V value falls within the load cell's tolerance range.
TEST PROCEDURES AND ANALYSIS
A quick diagnostic check of a load cell is relatively simple. The process is straightforward, and only a few pieces of equipment are necessary. If the load cell is found at fault, the device should be returned to the manufacturer for more evaluation and essential repairs. An ohmmeter can be used for many of the checks. Isolate the location of the problem by passing a moderately light deadweight over each load cell or by disconnecting each load cell.
TEST 1: ZERO BALANCE
The load cell's output under "no-load" conditions is referred to as the zero balance. Therefore, removing all weight (even dead load) from the load cell is necessary. Low-capacity load cells should be tested in the place where the load cell is designed to measure force to avoid the weight of the element generating incorrect results.
Load cells with a low capacity should be measured where they are meant to measure force so that the element's weight doesn't provide inaccurate results. A steady power source should be used to power the load cell; ideally, this is a load cell indicator with an excitation voltage of at least 10 volts. If you have more than one load cell, disconnect the other ones. Calculate the Zero Balance in mV/V using a millivoltmeter to measure the voltage across the load cell's output leads, then divide the result by the input or excitation voltage. Check the Zero balance against the data sheet or original load cell calibration certificate.
If the load cell has been irreversibly damaged by overloading or excessive shocks, changes in Zero Balance typically happen. The strain gauge resistance is most likely changing in load cells that endure progressive zero output changes over time due to chemical or moisture intrusion. But this will also lead to a decrease in insulation resistance and a breakdown in the bridge's structural integrity.
TEST 2: INSULATION RESISTANCE
The load cell circuit and element or cable shield are used to test the insulation resistance. After removing the load cell from the junction box or indicator, you should connect the input, output, and sense (if applicable) leads together. Use a megohmmeter to measure the insulating resistance between these four to six connected leads and the load cell body. Repeat the test between the same four or six leads and the cable shield. Finally, check the insulation resistance between the cable shield and the body of the load cell.
A megohmmeter should never be used to measure input or output resistance because it typically runs at voltages far higher than the maximum excitation voltage!
Please disregard the housing and screen insulation tests if the shield is connected to the load cell body.
All load cells should have insulation resistance of 5000 megohms or more between the load cell body and cable screen, the bridge circuit to the housing, and the bridge circuit to the cable screen.
A lower value means electrical leakage, typically due to moisture or chemical contamination within the load cell or cable. Extremely low values (1k) mean that there is a short circuit instead of moisture ingress. Electrical leakage typically produces unsteady output from load cells or scales. The temperature may affect the stability.
TEST 3: BRIDGE INTEGRITY
The input and output resistances, and the bridge balance, are measured to ensure the bridge's integrity. Remove the load cell's connection to the junction box or measuring instruments. An ohmmeter measures the input and output resistance across each pair of input and output leads. Check the input and output resistance in relation to the specifications on the datasheet or the original calibration certificate if available. Comparing the resistance from -output to -input and -output to +input yields the bridge balance. The difference between the two values ought to be ≤ 1Ω.
The most frequent reasons for changes in bridge resistance or balance include burned or damaged wires, failed electrical components, and internal short circuits. This could happen due to excessive voltage (from lightning or welding), physical damage from shock, vibration, fatigue, or high temperatures.
TEST 4: SHOCK RESISTANCE
The load cell needs to be connected to a steady power source; ideally, this is a load cell indicator with an excitation voltage of at least 10 volts. For systems with multiple load cells, disconnect all other load cells. Using a small mallet, lightly hit the load cell to shock it while a voltmeter is connected to the output leads. Carefully avoid overloading low-capacity load cells when testing their shock resistance. During the test, pay attention to the readings. The readings should not deviate from their initial zero readings, become erratic, and should remain generally stable.
Unstable readings caused by an electrical transient may suggest a broken electrical connection or a damaged glue layer between the strain gauge and the element.