Quality Control: Load Cell Handling, Storage and Preservation Do’s and Don’ts

Force measurement devices, such as load cells and strain gauges, are becoming increasingly versatile as they meet growing manufacturing requirements. However, these devices are delicate, and their accuracy is derived from the smallest deformations in their structures. Their proper handling, storage, and preservation are imperative to maintaining a load cell’s long-term reliability and accuracy. Therefore, any quality control processes for using these devices must include procedures for handling, preserving and storing load cells. This article explains these quality control best practices.

Definitions

1. Measurement Device – A device having known dimensions and used to evaluate the dimensions of a work piece. For the purposes of this article, measurement devices are the various types of load cell and strain gauge instruments.
2. Handling – General movement or deployment of a measurement device with the goal of preventing damage or deterioration.
3. Storage – (Pertaining to) the specific location where a device is stored.
4. Preservation – The act of applying appropriate methods to preserve the functionality of the measurement device.

The Dos and Don’ts of a Measurement Device’s Service Life

Many circumstances can prevent a measurement device from providing a reliable result. Interference from other processes (for example, EMI) is one such circumstance. Damage to the device is another culprit. This damage may be from improper use or excessive wear, or caused by extreme environmental conditions. Moreover, these devices generally suffer from careless handling before/after use due to the sensitivity and fragility of their electronic components. This includes improperly storing load cells between deployments.

Each measurement device is designed to endure some level of misuse. After all, accidents happen. However best practices should exist at all levels among all personnel from technicians to project managers to avoid problems. These quality control procedures for a load cell’s service life and for storage between applications should be well documented. Moreover, this documentation should be the basis of training for all personnel handling these devices. Before any measuring device goes into service, these personnel should thoroughly understand and commit to adherence to these procedures. After all, money has been spent on this valuable equipment to accomplish an important task. Improper handling wastes not just this expense but time in unnecessary troubleshooting, replacement and process downtime.

The sections below offer guidance for the “dos” and “don’ts” when handling, preserving and storing load cells.

Handling Load Cells

The following should be part of documented quality control processes for properly handling a load cell.

While Installing In a New Application

1. Device Rated Capacity. Choose a device with rated capacity such that the application’s performance range mostly falls near the center of the device’s measurement range.
   
2. Intrusion Protection (IP) Rating. Ensure the load cell’s documented IP rating is compatible with its operating ambient conditions.
   
3. Rough Treatment Avoidance. Avoid dropping the measurement device or accidentally bumping it into other objects as this may permanently alter its accuracy.
   
4. Carrying. Use proper care when moving the measurement device; do not carry it by its cables.
   
5. Load Alignment. Accurate load cell output signals require that the load force be completely axial to its intended load path. Therefore, mount the load cell to ensure proper load alignment. Prevent load shifting with mounting hardware.
   
6. Cable Length. Each measurement device has an established cable length by design. Avoid altering this length, as this alters the output signal.
   
7. Large Structure Installation. When installing a measurement device in an application where it is integral to a larger structure (such as a hopper or tank frame), substitute a dummy device until the structure is complete.
   
8. Fittings. Use only high-quality fittings rated for the highest loads encountered in the application or process. Before final assembly, verify that all threaded combinations that connect the measurement device are compatible.
   
9. Bolt Torque. Install all bolts and fasteners per the torque requirements in the device’s literature. This prevents excessive compression on the load cell body, or conversely, its shifting.
   
10. Installation Force. Do not excessively twist or physically force the load cell body during installation.
    
11. Welding. When the system requires electric welding for any reason, ground the electrode near the weld to prevent a current path through the measurement device.

While In Operation

1. Safe Capacity Limits. Measurement devices have a specified measurement range by design, and perform best in the middle of that range. Measurements approaching the maximum device range increase the potential for both harm to the device and unreliable results. Avoid overloading the measurement system; consider how transitory environmental forces such as wind or vibrations may cause a temporary overload.
   
2. Impact Loads. Avoid sudden impact loads (e.g., dropping rather than placing a load on a weigh platform). This could momentarily exceed the rated capacity or shock the device, causing permanent damage and/or incorrect results.
   
3. Prevention of Damage from Debris. Ensure that the measurement device and/or mounting brackets are free of debris and oil.
   
4. Maintenance Calibration. As with all measurement devices, calibrate periodically during the service interval, with traceability to a known standard.
   
5. Visual Checks. Make scheduled visual inspections to the measurement system. Look for damage such as corrosion or cracks, and possible damage to the wiring assembly including moisture intrusion. Replace load cells that exhibit damage.

When Removal Is Necessary

1. Troubleshooting. When a load cell performs unpredictably or outside its usual tolerances, follow a formal, step-by-step troubleshooting procedure, such as the one described in our article, How to Test for Faults in Load Cells.
   
2. Defective Device Documentation. Document specifics relating to the removed device’s failure and other usage information, using a tag attached to the device. Also include applicable traceability information.
   
3. Cutting Connecting Wires. When removing a defective device from a system, leave sufficient wiring to allow the manufacturer to perform corrective action testing.
   
4. Replacement Device Handling. When removing a measurement device from the system, calibrate the replacement device before installation. Alternatively calibrate the entire system as a whole, before the measurement system returns to service. The article, Calibrating the Force Measuring System describes these methods.

Storing Load Cells

The intent of storing a measuring device is to preserve it for future use. To ensure the measurement device is in a ready-to-go condition when needed, include these steps in quality control procedural manuals. These actions prevent environmental damage between uses.

1. Device Cleaning. Before storing, ensure the device is free of debris, oil or other deposits.
   
2. Moisture Control. Provide a storage location with humidity control to minimize moisture accumulation around the load cell or its wiring.
   
3. Temperature Verification. Before storing the device, verify that the location’s ambient temperature meets the device’s storage temperature requirements in its product literature.
   
4. Storage Security. Sometimes rodents can invade storage locations and cause damage. The wiring assembly on a measurement device is a prime target. Keep the wiring assembly in rodent-proof sheathing or the entire device in rodent-proof containers.
   
5. Vibrations. Vibrations affect not just the sensitive internal electronics but the delicate physical bonds between them and the load cell body. Select a location that is safe from excessive vibrations or seismic activity, and store in packaging that cushions the device.
   
6. Storage Location Monitoring. Monitor storage conditions to ensure their maintenance within the acceptable limits in the device documentation.
   
7. Visual Examination. Avoid leaving devices in storage for long periods without scheduled visual checks for damage. Likewise conduct visual checks after unexpected events where environmental conditions could have affected the measurement device. Such events include failures in climate control systems or seismic activity.

Preserving Load Cells

Preservation methods apply to devices both stored or in service, and should include those in this section. These ensure the future usability of the measurement device.

1. Clear Documentation. Establish well-documented corporate procedures for the use, maintenance and care of the measurement device.
   
2. Personnel Training. Train the appropriate personnel to consistently execute established, well-documented corporate procedures for use of these devices.
   
3. Environmental Requirements. Because load cells are used in harsh environments (temperature extremes, depths of water, wind components, etc.), verify the load cell’s environmental ratings are compatible with the application requirements of each new deployment.
   
4. Moisture Avoidance. Keep all wiring dry, as moisture can provide performance problems for a measurement device, particularly through wiring assembly.
   
5. Remedial Actions. Routinely examine measurement devices for corrosion or surface wear. If required to ensure its durability, preserve the measurement device by applying a protective coating per the manufacturer’s recommendations.

Conclusion

Measurement devices are highly accurate and precise sources of valuable process data. Properly handling, preserving and storing load cells will ensure they provide reliable results throughout their lifetime.

To summarize, all the recommendations above fall into the following five categories:

1. Documentation. Maintain all documentation about the measurement device for future reference. These include the Certificate of Calibration, product datasheet and manual, and most importantly corporate procedures for the measuring application. Quality control without proper documentation is impossible.
   
2. Personnel Training. Appropriately train all personnel using above-mentioned documentation to ensure consistent, proper care of measurement devices.
   
3. Environmental Requirements. Ensure the load cell operates and exists in an environment within its ratings given in its data sheet.
   
4. Installation. Install the measuring system with all recommended hardware and according to all instructions.
   
5. Visual Checks. Establish a practice of routine visual checks on the measurement device system, its assembly and wiring.

References

1. TO 32-1-201, Technical Manual: Care, Use and Maintenance; Measuring Tools, Class 5200, March. 1997
   
2. MIL-STD-1312-21, Military Standard, Fastener Test Methods, Method 21, Shear Joint Fatigue, July 31, 1985