Handling, Storage and Preservation Do’s and Don’ts in Force Measurement Devices
Force measurement devices, such as load cells and strain gauges, are becoming increasingly complex as they meet growing manufacturing requirements. Their various designs meet the needs of a wide range of industrial applications. However, these devices are delicate, and their accuracy is derived from the smallest deformations in their structures. Proper handling, storage, and preservation is imperative to maintaining a load cell’s long-term reliability and accuracy, and preventing corrosion. Therefore, any quality control for processes using these devices should include procedures for handling, preserving and storing load cells.
- 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.
- Handling – General movement or deployment of a measurement device with the goal of preventing damage or deterioration.
- Storage – (Pertaining to) the specific location where a device is stored.
- 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. These include damage from improper use or excessive wear, interference from other processes, or degradation caused by environmental conditions. Moreover, electronic measurement devices generally suffer from careless handling due to the sensitivity and fragility of the electronic components. And damage can result over time from improperly storing load cells between uses.
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. Quality control procedures for a load cell’s service life and for storage between applications should be well documented. Moreover all personnel should be formally trained to thoroughly understand and adhere to these procedures before any measuring device goes into service. 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.
To help avoid these unnecessary costs, the sections below offer guidance for the “dos” and “don’ts” when handling, preserving and storing load cells.
Handling Load Cells
To avoid physical harm to a load cell, follow these guidelines, which should be part of well-documented procedures for handling these devices:
- System Rated Capacity. Select a measurement device with rated capacity such that the application’s performance range mostly falls near the center of the device’s measurement range.
- Rough Treatment Avoidance. Avoid dropping the measurement device or accidentally bumping it into other objects. Rough handling may permanently alter the accuracy the measurement device.
- Carrying. Use proper care when moving the measurement device. Do not carry the measurement device by its cables.
- Load Alignment. Optimum output accuracy requires the force of the load to be completely axial to the load cell’s intended load path. Therefore, mount the load cell to ensure the force aligns with the intended load path. Include mounting hardware to prevent shifting of the load.
- Cable Length. Each measurement device has an established cable length by design. Avoid altering the measurement device’s cable length as this may cause a loss of sensitivity in the output data and provide incorrect results.
- Installation. While installing a measurement device system in an application, particularly one where the measurement device is part of a larger structure such as a hopper or tank application, use a dummy device until the final installation is complete.
- 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 to the measurement device are correct.
- Torque. Install all bolts and accessories according to the torque requirements for the measurement device. This prevents excessive compression on or movement by the installed load cell body.
- Installation Force. Likewise, take steps to prevent installation damage due to excessive twisting, over-straining, or physical forcing of the measurement device.
- 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.
- Safe Capacity. Measurement devices have a specified measurement range by design, and perform best in the middle of that range. Measurements approaching the maximum device range invite the potential for harm to the device and/or providing unreliable results. Avoid overloading the measurement system, and consider how transitory environmental forces such as wind, vibrations and the like may add a temporary overload.
- Impact Loads. Avoid sudden impact loads (e.g., dropping rather than placing a load on the platform). This could momentarily exceed the rated capacity or shock the device, causing permanent damage and/or incorrect results.
- Calibration. As with all measurement devices, calibrate periodically during the service interval, with traceability to a known standard to ensure valid results.
- Visual Checks. It is good practice to routinely make visual checks to the measurement system for damage such as corrosion or cracks. This includes possible damage to the wiring assembly such as moisture intrusion, cuts, etc.
- Removal. When removing a measurement device, document specifics relating to its failure or other usage information, using a tag attached to the device. Also include applicable traceability information. When removing a defective device from a system, leave sufficient length of wiring to allow the manufacturer to perform corrective action testing.
- Replacement for Mechanical Failure. 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. Calibrating the Measuring System describes these methods.
Storing Load Cells
The intent of storing a measuring device is to preserve it for future use. Therefore, it is important to take steps to ensure the measurement device is in a ready-to-go condition when needed. Such actions prevent rusting, corrosion or other environmental damage. They may include partial or complete disassembly, covering, and protection from vibrations. Storing load cells properly includes the following steps:
- Moisture Control. Provide a storage location with humidity control to minimize accumulation of moisture.
- Temperature Verification. Before storing the device, verify that the location’s ambient temperature meets the device’s storage temperature range rating in its product literature.
- Storage Security. Sometimes rodents can invade storage locations causing damage. The wiring assembly on a measurement device is a prime target. Secure the wiring assembly to prevent damage.
- Vibrations. Vibrations can affect not just the sensitive electronics but the delicate physical bonds that may exist between the electronics and load cell bodies. Select a location that is safe from excessive vibrations or seismic activity, and store in packaging that cushions the device.
- Device Cleaning. Before storing, ensure the device is free of debris, oil or other deposits.
- Storage Location Monitoring. Verify that the storage conditions are being maintained within the acceptable limits of the device.
- Visual Examination. Avoid leaving devices in storage for long periods without visual checks for damage. This is particularly true after unexpected events where environmental conditions could have affected the measurement device. Such events include failures in climate control systems or seismic activity.
Again, procedural manuals should formally incorporate these guidelines.
Preserving Load Cells
Preservation methods apply to devices both stored or in service, and should ensure the future usability of the measurement device. Procedures for preserving load cells must include the following:
- Clear Documentation. Establish well-documented corporate procedures for the use, maintenance and care of the measurement device.
- Personnel Training. Train the appropriate personnel to consistently execute the established and well-documented corporate procedures.
- Environmental Requirements. Because load cells are used in harsh environments (temperature extremes, depths of water, wind components, etc.), verify the environmental rating is compatible with application requirements.
- Moisture Avoidance. Keep all wiring dry, as moisture can provide performance problems for a measurement device, particularly through wiring assembly.
- Prevention of Damage from Debris. Ensure that the measurement device and/or mounting brackets are free of debris and oil.
- 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.
Measurement devices are highly accurate and precise equipment. They provide valuable process data while in use. Properly handling, preserving and storing load cells will ensure they will continue to provide reliable results.
To summarize, all the recommendations above fall into the following five categories:
- 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.
- Personnel Training. Appropriately train all personnel using above-mentioned documentation to ensure consistent, proper care of measurement devices.
- Environmental Requirements. Ensure the load cell operates and exists in an environment within its ratings given in its data sheet.
- Installation. Install the measuring system with recommended hardware and instructions.
- Visual Checks. Employ a practice to routinely conduct visual checks on the measurement device system, its assembly and wiring.
- TO 32-1-201, Technical Manual: Care, Use and Maintenance; Measuring Tools, Class 5200, March. 1997
- MIL-STD-1312-21, Military Standard, Fastener Test Methods, Method 21, Shear Joint Fatigue, July 31, 1985