Load Cell Failures and Definitive Troubleshooting Methods

Loadcell Failures in Dosing Systems and Definitive Solution Methods

 

In industrial facilities, the most vital 'sensory organ' enabling thousands of dollars worth of gravimetric (weight-based) dosing and loss-in-weight systems to communicate with their environment is the load cell sensor, which converts mechanical weight into electrical signals. No matter how good your PLC automation or perfectly designed helical motor assembly is, if these sensors do not operate accurately, without interference, and stably, it means the system will completely fail and become obsolete (a piece of junk that makes inaccurate measurements).

 

In production facilities that are pushed to their limits and operate 24/7, load cell failures are generally categorized into two types based on their source: electrical interference or mechanical physical shocks. This article will examine, in detail, the most common load cell failure types encountered by expert industrial maintenance, repair, and automation circuit technicians in the field, practical troubleshooting methods, and precise solution techniques from an electronic engineering perspective.

 

Mechanical Damage: Excessive Dynamic Loading, Shock Impacts, and Metal Fatigue

 

 

In addition to being electronic components, load cells are essentially mechanical devices that operate on the principle of millimeter-sized strain gauges formed at specific points on blocks of aviation aluminum or high-strength stainless steel, cast with special formulas. When a heavy load or engine block far exceeding the capacity (typically 150% of technical capacity) is placed on a weighing bunker by a crane, the sensor housing loses its strain gauge.

 

Permanent Deformation (Zero Drift) and Stopper Protection

 

If a sudden impact (shock) occurs in the bunker due to the force of material falling from above or a hard steel object falling, the metal block undergoes permanent molecular deformation and cannot be restored. This event, known in technical terminology as 'Zero Drift', causes the system to display thousands of kilograms of weight or fail to tare even when the scale is empty. To prevent this devastating and costly equipment damage, professionally designed dosing chassis must be fitted with 'mechanical overload protection lugs' (overload stops/jacking bolts) at the correct angles to protect the loadcell with millimeter precision.

 

Fault Indication Read on HMI or Display	Possible Mechanical Reason in the Infrastructure	Definitive Solutions and Interventions for Field Technicians
The tare (zero) value is constantly shifting up/down in the same direction.	Plastic (permanent) deformation/cracks in the sensor metal due to overload.	The deformed load cell should be replaced immediately with a new one, and a robust, adjustable mechanical stopper should be installed underneath it.
The screen constantly displays a nonsensical negative (-) value while under load.	The load cell equipment is mechanically mounted upside down (inverted) or the signal (+) (-) cables on the panel are connected in reverse.	The mechanical mounting direction or wiring should be checked (The pressure direction arrow on the sensor housing should always point downwards or towards the force).
While the weight physically remains unchanged, the digital value on the screen flickers once per second.	The chassis is being subjected to periodic, rhythmic machine vibrations or wind impacts from the outside.	The platform should be de-aligned, and the lower anti-vibration (rubber) damping pads should be thoroughly checked to ensure they haven't burst.

 

Electrical Interference, Noise (EMI), and the Vital Impact of MCC Panel Grounding

 

 

The analog voltage signal passing through the load cell cables and transmitted to the industrial amplifier (reader) is surprisingly small (typically 2-3 millivolts per kg weighed, or mV/V). The massive kW frequency inverters (motor drives/VFDs), heavy switching contactors, and power transformers operating in industrial MCC (Motor Control) electrical panels in heavy industrial facilities emit an incredibly large, invisible electromagnetic noise field and power pollution (EMI - harmonics).

 

Signal Isolation and Protection of IT/OT Layers

 

These wild electrical noise waves, interfering with an unshielded load cell analog signal cable like an antenna, cause the weight displayed on the screen to become distorted, and the PLC system to malfunction by accelerating the motor. The outer metal braid shield of the cables must be grounded to a very good copper busbar in the panel, and before entering the PLC's analog input (4-20mA / 0-10V) card, the signal/communication tray must be completely physically separate (at least 30 cm away) and routed through a separate cable tray, not through the thick, high-power cable ducts in the factory.

 

Practical Loadcell Integrity and Megger Testing with a Multimeter for Field Technicians

 

When the system suddenly stops during the night shift and the PLC gives a weight error, you can easily and electronically test whether the sensor is completely faulty using a digital multimeter (avometer). The ohm (electrical resistance / Ohm) values between the power input (Excitation +/-) and measurement output (Signal +/-) cable groups of the device should be measured and checked to see if they exactly match the values on the device's factory datasheet (catalog) label.

 

Furthermore, a high-voltage short circuit or Megger (insulation) test between the sensor's metal housing and the internal copper wire ends can clearly determine whether it has absorbed micro-level water or ambient moisture (causing Ohm leakage). Preventing liquid/moisture leakage is crucial for the device's lifespan, especially in wet areas where CIP (Clean-in-Place) cleaning takes place in the food industry.

 

Electronic Test Step Applied	Generally Expected (Example) Value	Fault Diagnosis and Technical Commentary
Input (Excitation) Impedance Resistance Measurement	Example: Around ~400 Ohms according to the label value.	If the measured value does not match the catalog value or is infinite (open circuit), the sensor's internal solder joints are broken or the cable is cut.
Output (Signal) Impedance Resistance Measurement	Example: Around ~350 Ohms according to the label value.	If the output value is significantly different, the sensitive internal strain gauge reading circuit has suffered severe physical damage due to high voltage or impact.
Metal Housing (Chassis) - Cable In-Cable Insulation Test	< 10 MegaOhms (Insulation is too low and the resistance has been broken)	Definitely, water/moisture has seeped into the system or housing, or the thin cable insulation has melted due to heat, making contact with the housing and causing a leakage current.

 

Frequently Asked Questions

 

If the long loadcell cable running from the sensor to the control panel breaks or shortens, can it be spliced using a terminal block soldered in the middle?

Unless absolutely necessary for an emergency night shift, normal splicing should never be done. Heat, oxidation, and internal resistance differences at the splicing point (soldering or copper twisting) will severely distort that weak millivolt signal, instantly shifting the machine's calibration. IP68 fully sealed special splice boxes and solderless terminals, or a new sensor with a single, original length cable, must be used.

 

What precautions should be taken when performing TIG/Gas Metal Arc Welding (GMAW) around the machine chassis or in an area very close to the sensor?

The enormous current flowing from the welding pliers, which carries a very high electrical current, to the ground will instantly burn out the microscopic electronic circuit inside the sensor if it tries to pass through the stainless steel load cell as the shortest path. Before welding, a thick copper shunt (jumper) cable must be run over the load cell to bypass the current, or the sensor must be physically removed from its mounting location and taken to a safe place.

 

Why does the loadcell display persistently show a negative value of -450 kg on the dosing machine's scale, even when it's completely empty?

There could be only three reasons for this: the read cables (Signal + and Signal -) entering the panel were connected to the terminals in reverse during installation, the metal sensor was physically mounted upside down (top to bottom), or the dosing scale rubbed against and jammed against a plumbing pipe underneath, causing the sensor to receive a reverse upward pressure force instead of a downward one.

 

Do constantly wet and humid environments, such as fruit juice or pickling factories, permanently shorten the load cell lifespan?

Standard IP65 or IP67 (aluminum and silicone-coated only) protected sensors will, over time, infiltrate microscopic moisture through their silicone seals in humid environments with continuous steam or intense chemical cleaning. In such environments (Food and Chemical), fully stainless IP68/IP69K armored models filled with argon gas, hermetically sealed (laser-welded, seamless) covers that are impossible to ingress of water, must be used.

 

How much does oxidation or moisture affect the measurement in a junction box where multiple load cells are connected and grouped in parallel?

It causes direct and catastrophic damage. In these boxes where 3 or 4 load cells are connected simultaneously and their signals are collected, the green/white copper oxidation that occurs over time in the tiny terminals, or even a single drop of moisture seeping in, causes the total signal voltage to be trapped and lost (creating resistance) inside the box, resulting in consistently inaccurate and incomplete weighings on the display.

 

What should be done immediately after an old or faulty load cell is replaced with a new one by the facility maintenance team?

Calibration coefficients in the PLC based on the characteristics of the old sensor become completely invalid. After installing the new hardware, the mechanical balances of the system must be adjusted, and the zeroing (dead tare) and full scale (span/gain) calibration parameters of the scale must be re-trained from the PLC menu using a new, state-approved and sealed set of reference weights (e.g., by hanging 25 kg M1 class standard cast iron masses one by one).

 

After troubleshooting, you can read our article "PLC Automation and SCADA in Dosing" to see how all this equipment is managed through automation and software .

 

For customized powder dosing systems for your factory and all the necessary technical information, please review our Dosing Systems product.