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Why Is Load Cell Zero Balance Important to Accuracy?

Several factors go into the accuracy and consistent performance of a load cell. These factors include non-linearity, hysteresis, repeatability, creep, temperature, environmental effects, and zero balance.

Every Interface load cell’s design and specifications account for all these factors. Understanding each of these factors is important, especially considering the use case.

Specifications are detailed descriptions that outline the characteristics, features, and qualities of our products, systems, or services. Product specifications detailing performance, capabilities, capacities, and dimensions are included on all datasheets. Products have internal specifications tested during manufacture, typically with full traceability.

Zero balance is considered an electrical load cell specification value. It is essential to consider when selecting the type of load cell for any application.

Load cell zero balance is the signal of the load cell in the no-load condition. It is defined as the output signal of the load cell with rated excitation and no load applied. It refers to the amount of deviation in output between true zero and an actual load cell with zero load. It is usually expressed in the percentage of rated output (%RO). Zero balance is a test that can be done to understand calibration on a load cell.

Load cells constantly reset to zero after every measurement to maintain accuracy. If it does not, then the results will prove to be inaccurate. The zero balance must be within the error margin indicated on the calibration certificate. Interface sensors are typically +/-1.0%.

This is important to test because zero balance will tell you if a load cell is in working order or has been damaged or overloaded. A computed zero balance of 10-20% indicates probable overload. If the load cell has been overloaded, mechanical damage has been done that is not repairable because overloading results in permanent deformation within the flexural element and gages, destroying the carefully balanced processing that results in performance to Interface specifications.

While it is possible to electrically re-zero a load cell following overload, it is not recommended because this does nothing to restore the affected performance parameters or the degradation of structural integrity. If the degree of overload is not severe, the cell may sometimes be used at the user’s discretion. However, some performance parameters may violate specifications, and the cyclic life of the load cell may be reduced.

To perform a zero balance test, The load cell should be connected to a stable power supply, preferably a load cell indicator with an excitation voltage of at least 10 volts. Disconnect any other load cell for multiple load cell systems. Measure the voltage across the load cell’s output leads with a millivoltmeter and divide this value by the input or excitation voltage to obtain the zero balance in mV/V. Compare the zero balance to the original load cell calibration certificate or the datasheet. Every Interface product has a detailed datasheet available on the product page of the sensor.

ADDITIONAL TECHNICAL DEFINITIONS

Zero float is the shift in zero balance resulting from a complete cycle of equal tension and compression loads. It is normally expressed in the units of %FS and characterized at FS = Capacity.

Zero stability is the degree to which zero balance is maintained over a specified period with all environmental conditions, loading history, and other variables remaining constant.

Learn more about the specification values that define load cell accuracy in this short clip from our  Demystifying Specifications Webinar.

Get your free copy of the Interface Load Cell Field Guide to learn more about factors affecting load cell accuracy. If you are concerned about the zero balance of your Interface load cell due to inaccurate results or recent damage, please get in touch with us at 480-948-5555.

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The Five Critical Factors of Load Cell Quality

Accurate data and high-quality test and measurement programs have many contributing factors. None are more important than the devices and equipment used on the test line. At Interface, we understand this better than anyone else.

Quality is why our force measurement products are used and known across multiple, highly regulated, and complex industries for providing the most reliable and accurate data anywhere. It is also why Interface is recognized as the preeminent leader in load cell quality.

How do we reach this high standard that we continue to hit with every product that leaves our facility? It is our overriding commitment to quality and consistency. The most important aspect of this is the fact that Interface controls the entire manufacturing process of our load cells. Many providers outsource certain components like that strain gages. We build the strain gages, the load cells, integrate the strain gages into the load cells and we do our own test, calibration, and quality inspection on each device.

Through our development process, which has been created and perfected over 52 years, we have learned what makes a great load cell. To start, Interface Chief Engineer Ken Vining outlines the top factors in load cell quality.

Five Most Critical Factors of Load Cell Quality

#1 Repeatability

Repeatability is first on the list and it is what our customers consider the most important aspect of buying an Interface load cell. Anyone can develop a load cell that is accurate for the first 10 to 15 measurements, but as environmental factors and stress are inflicted upon the load cell it needs to last. Due to our experience in this industry, we understand how certain temperatures, loads and other factors can diminish the accuracy of a load cell. This is one of the reasons we work so closely with our customers. Every application is different, and if we understand the application, we can deliver a custom load cell that withstands the various stressors over time without providing diminishing returns. This ensures that our customers receive the same, high-quality data after 10 years of use that they received on day one.

#2 Longevity

Like repeatable data accuracy over time, the load cell also needs to feature a high-quality and ruggedized build to last physically. Constant application of weight, pressure or torque can diminish the build quality and strength of a load cell if it does not meet the material requirements of the application. This can also reduce accuracy and lead to higher costs if customers must replace their load cells regularly. Interface has worked across a wide variety of industries and we understand the materials necessary for nearly any environment. With proper use, build quality and routine maintenance, load cells should last a very long time. In fact, Interface still has load cells in use in the field from when we started building quality product more than five decades ago.

#3 Accuracy

Data accuracy is affected by a litany of factors in load cells. In fact, we wrote an entire white paper on this very topic called, “Contributing Factors to Load Cell Accuracy.” Once again, the application of the load cell is what determines the conditions that affect accuracy. These conditions include creep, side and eccentric load, temperature, humidity, the mounting process and more. Interface can customize each of our load cells to ensure these conditions are accounted for to maintain premium accuracy.

#4 Sensitivity to Off-Axis Loads

A typical load cell is designed to measure load in one direction. However, nearly any project using force measurement test processes is going to introduce an off-axis load. If the load cell is not designed to adjust for this and compensate for what is called moment, the data output will be skewed. This is another reason that customers need to be extremely specific when discussing the application of the load cell. There are several ways to compensate for moment; however, most of these adjustments are physical and occur in the design and manufacturing process. With a correctly calibrated and designed load cell, off-axis loads will be eliminated and will not affect the accuracy of the data.

Special Note: Our recent release of the new ConvexBT product, the first to market miniature load button load cell that is designed to solve for off-axis (eccentric) loading. Read more here.

#5 Access to Prominent Force Measurement Experts

Every factor of quality listed above is realized and accomplished through a close and transparent relationship between customer and force measurement provider. Every application dictates a different force measurement solution. When we understand the application, we can select the right type of load cell or customize an off the shelf load cell to meet the quality and accuracy needs necessary for any project. This is why a customer’s access to a force measurement expert is an integral part of load cell quality.

Every force test and measurement project can create a different challenge and developing an accurate and reliable load cell to meet those challenges can be tough. Therefore, Interface considers these five factors, and hundreds more, for every product we engineer and build. This is our unwavering commitment to quality and customer satisfaction.

Contributor:  Ken Vining, Chief Engineer and Head of Quality

Advancing Load Button Load Cell Capabilities with ConvexBT

Demands for high precision testing utilized for compact designs and in confined spaces is growing. The requirements for quality, accuracy and most importantly reliability are what has driven the experienced engineers at Interface to create the newly released ConvexBT™ Load Button Load Cell product line.

The revolutionary design of the ConvexBT is a first of its kind load button load cell, providing better temperature resistance and more enhanced eccentric load rejection. Miniature load cells categorized as load buttons have been sensitive to off-axis, eccentric or misaligned loads. This means if the load is not exactly perpendicular to the surface it is resting on, the data could become skewed or inaccurate.

Interface designed the ConvexBT™ Load Button Load Cell to confine misaligned loads to the primary axis of the cell providing superior performance in comparison to similar products on the market in repeatability, better data and reproducible results.

As technology advances, there is a growing demand to make devices and products more compact and convenient. This trend is happening across industries and is especially prevalent in medical, industrial automation and products reliant on advanced communications technology. To design and validate these products, our customers need force-sensing solutions that can fit in confined spaces and provide extremely accurate data. This is the driving force behind the development of ConvexBT, the next generation in force measurement device.” – Ted Larson, VP product management and marketing, Interface.

CONVEXBT FEATURES AND SPECIFICATIONS

The newly released ConvexBT product comes in two different sizes: 3/8-inch, and 1/2-inch, which are all manufactured using 17-4 PH heat treated stainless steel. These options provide a wide measurement range from 10 to 250 lbf, a compensated temperature range of 60° to 160°F, and an operating temperature range of -40° to 175°F.

Additional specifications for ConvexBT include:

  • 2.00 ± 20% mV/V rated output
  • ± 0.25 non-linearity as a percentage of full scale
  • ± 0.25 hysteresis as a percentage of full scale
  • ± 0.50 static error band as a percentage of full scale

Other load cell load buttons designs have also been extremely sensitive to temperature conditions. Interface has redesigned its ConvexBT ultra-precision product line of load buttons to ensure that this is no longer something the user has to account for by taking the sensing technology disrupted by temperature out of the cable, and designing it directly into the load button.

The new available ConvexBT models include the following capacities:

  1. ConvexBT Model LBSU-10 lbs 3/8″
  2. ConvexBT Model LBSU-25 lbs 3/8″
  3. ConvexBT Model LBSU-50 lbs 3/8″
  4. ConvexBT Model LBSU-100 lbs 1/2″
  5. ConvexBT Model LBSU-250 lbs 1/2″

Additional model capacities will be available this year.  You can view the complete product specifications as well as technical guide by visiting the product page here.

ConvexBT was developed through a combination of intense research into growing technology trends in force measurement and actively collaborating with our customers to understand their unique challenges, By introducing the industry’s most advanced and versatile ultra-precision load button load cells, we are solving the test and measurement challenges associated with miniaturization of existing and new technologies.” – Greg Adams, CEO at Interface

The revolutionary ultra-precision line of ConvexBT™ Load Button Load Cells uniquely uses multi-point calibration for testing force on miniaturized products and within confined spaces where accuracy is paramount to success and safety. The requirements are critical to common buyers of miniature load cells, especially for use in medical devices, robotics and in industrial automation applications

In addition to its ability to solve test and measurement challenges with compact devices, another key benefit of ConvexBT is its versatility in that it can be used as a traditional test and measurement solution. It can also be installed into OEM components and devices as an advanced miniature sensing solution to collect accurate real-time force data on the product as it is in use.

ConvexBT is available now under the product family of Interface Mini® Load Cells. The product is part of a growing line of Interface Load Button Load Cells. The new ConvexBT model LBSU specifications are available here: /product-category/load-button-load-cells/.

Read more about Off-Axis Loads and Temperature Sensitive Applications here.