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Understanding Uncertainty in Load Cell Calibration

In force measurement testing, accuracy is the most critical factor in ensuring the data you collect can help to identify challenges, failures and opportunities in the product design and development cycle. Here at Interface, we have mastered the art of load cell accuracy by employing a vertically integrated manufacturing process that allows us to control the development of our products most critical components.

Even the most high-end manufactured load cells and finely tuned components endure accuracy degradation over continued use. Therefore, we have also invested in equipment and talent with deep expertise in load cell recalibration, as well as offering gold and platinum standard calibration systems to customers. Recalibration is recommended on an annual basis, or of course, whenever our customers feel they need to confirm they are getting the right data out of their load cells.

One of the key factors of calibration and recalibration is understanding how to estimate practical uncertainty in load cell calibration. Measurement uncertainty is defined as an estimate of the range of measured values within which the true value lies or, alternatively, the degree of doubt about a measured value. In every application, there will be an uncertainty requirement on the force measurement. The equipment used to make the measurement must be traceable to a realization of the SI unit of force (the newton) within this required uncertainty.

Each application is different in terms of its uncertainty requirement. For instance, an application testing force in the aerospace and defense or medical sector will include a much more stringent uncertainty requirement than something like a commercial scale used to measure someone’s weight or food. It is critical to understand the uncertainty requirement on the application to ensure the force measurement device used is calibrated to handle the project.

How does one go about estimating uncertainty in load cell calibration? The first thing to understand is the GUM, a guide to the expression of uncertainty in measurement. This guide establishes general rules for evaluating and expressing uncertainty in measurement that are intended to be applicable to a broad spectrum of measurements.

Next, we have included a list of different considerations, as we measure uncertainty here at Interface. These factors will help guide you as you determine uncertainty for yourself. This list includes:

  1. Determine what parameter is to be measured and the units of measure.
  2. Identify the components of the calibration process and the accompanying sources of error.
  3. Write an expression for the uncertainty of each source of error.
  4. Determine the probability distribution for each source of error.
  5. Calculate a standard uncertainty for each source of error for the range or value of interest.
  6. Construct an uncertainty budget that lists all the components and their standard uncertainty calculations
  7. Combine the standard uncertainty calculations and apply a coverage factor to obtain the final expanded uncertainty.

It is also important to consider the different methods of load cell calibration. There are three different methods, and each has an approximate feasible expanded uncertainty. The different calibration methods include:

  • Direct dead weight – this method is the best for accuracy at 0.005% uncertainty, but it is slow, and the equipment is space inefficient.
  • Leveraged dead weight – middle of the road for accuracy at 0.01% uncertainty, and slow and space inefficient.
  • Hydraulic force generation comparison – this method has reasonable accuracy at 0.04% uncertainty and is also the fastest and most space-efficient option.

The final point is the sources of error in calibration. Error is defined as the difference between the measured value and the true value. There is a long list of different factors that can cause error and increase uncertainty. These factors may include drift, creep, misalignment, or environmental factors such as temperature. To compensate for this, it is important to understand the various formulas that can be used to find the true value based on the given measurement and the various factors for error.

To learn more about uncertainty and the different ways users can address uncertainty and overcome it, please give us a call at 480-948-5555, or visit our website to contact our Application Engineers.

Contributor:  LaVar Clegg, Interface

Source: NCLSI Measurement Training Summit 2014

Faces of Interface Featuring Dan McAneny

Our Faces of Interface series provides readers with an inside look at all the brilliant and talented people that work for our 52 year-old company.  There is also a team of experts and experienced individuals that are part of our sales engineering network. These representatives and distributors are integral to the work we do, and they are considered part of the Interface family.

In this post, we are featuring Dan McAneny, co-founder and sales engineer at Tritek Solutions, one of Interface’s sales representatives covering the Pacific Northwest. Dan has been working with Interface for many years, so we have gotten to know him pretty well and are grateful to have him part of our team.

Dan began his career as a design engineer in New York after graduating with a bachelor of science in Electrical Engineering from Lehigh University. He spent a few years in this design engineer role before quickly realizing that he had a passion for being out in the field and working directly with customers, rather than behind a desk.

His search for sales engineering positions took him out west to work for a rep company in Los Angeles, California, that sold electronic test equipment. This position helped Dan develop his proficiency in sales, as well as better understanding of the test solutions critical to the evolution of technology. When the LA company went out of business in 1988, Dan and his good friend and co-worker decided to start Tritek Solutions.

Tritek Solutions is a manufacturers’ representative with sales and administrative staff capable of providing a complete sales and support solution. The company focuses on test instruments, systems, specialized components and board-level products.

The company quickly ascended, growing into Northern California, the Rocky Mountain region, and the Pacific Northwest. And after 28 years in Southern California, Dan and his wife decided to take their talents to Seattle, Washington, where he could focus his work with Tritek in the Pacific Northwest.

Dan learned about the diverse and innovative technology ecosystem in this region and has developed a wide range of experience and expertise on how to serve customers in various industries such as aerospace and defense, medical, industrial, automotive and construction. In fact, this is the aspect of his job that he enjoys the most, working with customers in all of these different technology sectors in Seattle to learn about their challenges in bringing new innovation to market, and offering synergistic solutions available from the many manufacturers he represents.

“In a single day, I could be standing next to a 777X airplane for an hour, and then the next hour, I could be discussing a solution for developing delivery drones or ventilator production. The possibility of learning about and working with customers on hundreds of new technology is something I enjoy every day.”

When Dan began working with us many years ago, he noticed that our presence in the Pacific Northwest was mainly focused on large aerospace OEM’s and knew he could help us break into a larger variety of markets. Dan has been successful in proactively searching for and securing new opportunities for Interface force measurement products across new markets, innovators and advancing technology sectors.

“Working with Interface has been a pleasure. Their more than half-century in business has provided them with a deep understanding for efficiently working with reps like myself, and the quality of their products and engineering talent makes it easier because I know my OEM customers know and trust the Interface brand.”

When he is not making a killing helping original equipment manufacturers (OEMs) find test, engineering and manufacturing solutions for their product development process, Dan enjoys taking advantage of the beautiful outdoors in the Pacific Northwest. Dan’s wife of 30 years and two sons age 23 and 25 enjoy road and mountain biking, hiking and swimming together, as well as exploring the hidden gems the region has to offer.

Stay tuned to our blog for new Faces of Interface ForceLeaders profiles to learn more about our valued network and team members. You can subscribe to receive the blog weekly at www.interfaceforce.com/blog/.