Calibration is a crucial component of force measurement, but its necessity is often underrated or misunderstood, writes David Ayling, former owner of load cell manufacturer Straightpoint, and president of FAD Equipment Store.
Load monitoring products are increasingly popular and necessary tools at indoor and outdoor facilities of all types, in a multitude of industries. Planned or unplanned load movement is made inherently safer by rigging a dynamometer or monitoring device to the application or lift.
Load cells—the wireless Radiolink plus is Straightpoint’s most popular product—are calibrated upon manufacture or distribution, and recalibration is the periodic repetition of that process to ensure safety and reliability of measurement.
Calibration is the correlation of the readings of an instrument against a known standard (Usually in the form of another loadcell) to check accuracy. For load cells and weighing instruments this usually means applying a known load up to the rated capacity of the unit under test, either using masses or a hydraulic force generating machine with a reference load cell traceable to national standards. Importantly, the requirements for traceability will change from one country to the next.
Regardless, if an instrument is not calibrated, it cannot provide a true and accurate measure of the load - this can be potentially unsafe and negatively effect productivity.
Here in the U.S., standards are set by the American Society of Mechanical Engineers, or ASME. ASME’s “mission is to serve diverse global communities by advancing, disseminating and applying engineering knowledge for improving the quality of life; and communicating the excitement of engineering.”
In other words, “ASME aims to be the essential resource for mechanical engineers and other technical professionals throughout the world for solutions that benefit humankind.”
ASME is widely referenced in industry because it ensures that certain steps are taken into consideration in the design and manufacturing of lifting equipment.
Straightpoint, an FAD Equipment Store vendor, has universal testing machines made by U.S. company MTS, which combine with customized tooling, creating a top-of-the-range rig tailored to the specific requirements of force measurement, load monitoring and suspended weighing load cell equipment.
Calibration of all test rigs is carried out annually by a third-party calibration provider to necessary standards. Full traceability is assured. When Straightpoint creates any certificate of calibration, they make reference to the testing machine used and the traceability back to the third-party calibration body.
A test is completed by applying a load in a testing machine that has got the third-party standard within it. The process involves calculating what the load is; rigging the load cell; applying the load in a smooth and progressive way; holding it; and coming back to zero. At Straightpoint, that process is completed three times for accuracy in each calibration.
Calibration frequency
All Straightpoint products are provided with proof test and calibration certifications, and alert users when recalibration is required once a year. This can be completed quickly and cheaply by a network of recalibration laboratories across North America.
However, calibration frequency is an important facet of calibration because guidance varies. Straightpoint advises that the frequency of calibrations should be determined by the user of the load cell based on a number of considerations:
- Frequency of use
- Severity of service conditions
- Nature of loading
- Experience gained using other similar products in the same application
Once these factors are established the user may then establish the calibration frequency:
- Normal use: calibration every 12 months
- Severe use: calibration monthly to quarterly
- Special service: as recommended after consultation with Straightpoint’s engineering team and user.
A written record of the most recent periodic inspection shall include the condition of the load cell.
These guidelines fall in-line with ASME recommendations.
It’s worth expanding on one or two of the above bullet points. When you’re using a load cell, various stresses are put through the dynamometer’s body. It might be a tensile force in the case of the aforementioned Radiolink plus, a shear force (load shackle) or compression force (compression load cell). It is possible that usage will stretch or distort the load cell body, while the return to zero ratings might lose accuracy.
Severity of service conditions or environment in which the load cell is used can also impact the frequency at which it should be calibrated. A load cell used once daily offshore will be exposed to more demanding conditions than one used once daily in a sterilised factory environment, for example. However, manufacturers, like Straightpoint, make their products to extremely high standards with Ingress Protection ratings. Further, all Straightpoint products have built-in temperature compensation circuits that respond to operating temperatures.
End users can also apply experience gained using other similar products in the same application. For example, if they had a fleet of load cells that perform similar tasks, in like environments, they can make a calibration frequency judgement based on historic usage. It might be that this experience tells them all load cells of a certain capacity used for specific tasks, should really be recalibrated on a quarterly basis.
As an aside, all Straightpoint products are lightweight—the 55,000-pound capacity Radiolink plus weighs just 11 pounds—and shipped in a carry-case or wooden crate, depending on capacity. Following use, the equipment can be returned to a box and placed back in a rigging store before the next use.
Does that answer your calibration questions?
