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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.

ADDITIONAL TECHNICAL RESOURCES

Interface Technical Support Information and Troubleshooting

Interface Product Selection Guides

Interface Installation Guides and Operation Manuals

Interface Software and Drivers

Interface Product Catalogs

Interface 101 Blog Series and InterfaceIQ Posts

Interface Industry Solutions and Applications

Interface Recorded Webinars

What is Static Error Band Output?

Static error band (SEB) measures the accuracy of a measuring device. Under static loading conditions, it is defined as the maximum deviation of the device’s output from a best-fit line through zero output. SEB includes the effects of non-linearity, hysteresis, and non-return to minimum load.

Static Error Band (SEB) Definition: A band encompassing all points on the ascending and descending curves centered on the best-fit straight line. It is expressed in units of %FS.

SEB is typically expressed as a percentage of full scale (FS), the maximum load the instrument can measure. For example, a load cell with a SEB of 0.1% FS would have a maximum error of 0.1% of its full-scale capacity.

SEB is an essential specification for measuring instruments used to make precise measurements, such as load cells, pressure transducers, and temperature sensors. A high SEB indicates that the device is inaccurate, and its measurements may be unreliable.

How to Calculate SEB

  • Collect a series of calibration data points for the instrument under static loading conditions.
  • Plot the calibration data on a graph, with the instrument’s output on the y-axis and the applied load on the x-axis.
  • Fit a best-fit line through the calibration data points.
  • Calculate the maximum deviation of the calibration data points from the best-fit line.
  • Express the maximum deviation as a percentage of the full scale.

SEB is a helpful metric for comparing the accuracy of different measuring instruments. It is also important to note that SEB is only one measure of an instrument’s accuracy. Other factors, such as repeatability and reproducibility, should also be considered when selecting a device for a particular application.

What is SEB Output?

SEB output is the computed value for output at capacity derived from a line best fit to the actual ascending and descending calibration points and through zero output. It measures the accuracy of a measuring instrument under static loading conditions.

SEB Output Definition: The output at capacity is based on the best fit straight line.

The SEB output is the maximum deviation of the calibration points from this best-fit line. SEB output is typically expressed as a percentage of full scale (FS). SEB output is an essential specification for load cells and other measuring instruments used to make precise measurements.

Why Interface Uses SEB Output Instead of Terminal Output

In the absence of alternate specific instructions, Interface uses the SEB output instead of the terminal output in straight-line scaling of a transducer to a digital indicator or analog signal conditioner. On average, the SEB output line yields the least error over the transducer range relative to the calibrated points.

SEB stands for Static Error Band and is a band on either side of a straight line through zero that is positioned to have equal maximum error above and below the line. The line extends from zero to the SEB output. The line considers both ascending and descending calibration points.

The plot below allows error visualization relative to the SEB and terminal output lines for a typical load cell calibration curve with ascending and descending points.

In this example, the SEB equals 0.03%FS, and the SEB line is no more than 0.03%FS away from any calibration point. The terminal line, in contrast, has a maximum deviation from calibration points of 0.05%FS. The plot shows that the ascending calibrated curve and the SEB line cross near 80%FS, often a more common measurement area in an application than 100%FS.

Source: Levar Clegg

Benefits of Using SEB Output

  • SEB output is a more accurate measure of the load cell’s accuracy than terminal output.
  • SEB output is less sensitive to environmental factors and noise than terminal output.
  • SEB output is easier to understand.
  • SEB output confirms that the measurements are accurate and the results are reliable.

How does a test engineer use SEB Output when selecting a load cell and instrumentation system?

Test engineers use SEB Output when selecting a load cell and instrumentation system to ensure the system is accurate enough for the intended application. The selection of a load cell is often based on an SEB Output that is less than the required accuracy of their application. For example, if an engineer needs to achieve measurements with an accuracy of 0.1%, they will select a load cell with a SEB Output of less than 0.1% FS.

It is crucial to consider the instrumentation system’s accuracy to measure the load cell’s output. The instrumentation system should have an accuracy equal to or greater than the accuracy of the load cell.

For additional information about specification values, be sure to watch this short clip from our Demystifying Specifications Webinar Recap

Test and measurement professionals can select an accurate, reliable, valuable load cell and instrumentation system following these tips.

Demystifying Specifications Webinar Recap

Interface recently hosted an online technical seminar that detailed product specification basics, key values, terms to know, how to read a datasheet, what specs matter most in force measurement applications.

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

Throughout the webinar Demystifying Specifications, Brian Peters and Jeff White offered important tips on what to consider for high-speed, durability, precision, and specialty product requirements. They highlighted what to look for on the product datasheet when choosing a load cell or instrumentation device. This includes variables in specifications related to expected performance of transducers and instrumentation based on frequency, environment, and other critical testing application considerations. They also answered the most frequently asked questions of our applications engineers related to specifications and datasheets.

Demystifying Specifications Webinar Topics

  • Specification Basics
  • Specifications and Values in Force Measurement
  • Decoding Datasheets
  • Detailing Product Specs for Load Cells
  • Detailing Product Specs for Instrumentation
  • Detailing Product Specs for Specialty Sensor Products
  • Applying Specifications to Applications
  • Specification Tips
  • FAQs and Resources

The entire webinar, Demystifying Specifications, is now available to watch online.

Four Types of Specifications

Interface provides four types of specifications for every product we make and sell: functional, technical, performance and design.

  1. Functional specifications describe the intended functionality or behavior of a product, whether a sensor, instrument or accessory.  They outline what the product or system should do and how it should perform its tasks. Functional specifications typically include applications, product requirements, and expected use case results.
  2. Technical specifications provide detailed information about mechanical aspects of a product or system. They may include information about the materials, dimensions, technical standards, performance criteria, capacities, and other technical details necessary for the design, development, and implementation of the product or system
  3. Performance specifications define the performance requirements and criteria that a product or system must meet. This is critical in force and measurement. They specify the desired performance levels, such as speed, accuracy, capacity, efficiency, reliability, or other measurable attributes. Performance can be defined by a specific range, with maximum standards for peak performance. Performance specifications help ensure that the product or system meets the desired test and measurement goals.
  4. Design specifications outline the specific design criteria and constraints for a product or system. These specs provide guidelines and requirements related to the visual appearance and can also reference the model details found in a product’s engineering CAD STEP file. 

Specifications Commonly Found on Interface Product Datasheets

  • Models based on Form Factor
  • Measuring Range (Capacity)
  • Measurement Units: US (lbf) Metric (N, kN)
  • Accuracy (Max Error)
  • Temperature: Operating Range, Compensated Range, Effect on Zero and Effect on Output (Span)
  • Electrical: Rated Output, Excitation Voltage, Bridge Resistance, Zero Balance and Insulation Resistance
  • Mechanical: Safe Overload, Deflection, Optional Base, Natural Frequency, Weight, Calibration and Material
  • Dimensions
  • Options
  • Connector Options
  • Accessories

Key Force Measurement Specification Terms to Know

Nonlinearity: The algebraic difference between OUTPUT at a specific load and the corresponding point on the straight line drawn between minimum load and maximum load.  Normally expressed in units of %FS.

Hysteresis: The algebraic difference between output at a given load descending from maximum load and output at the same load ascending from minimum load. Normally expressed in units of %FS.

Static Error Band (SEB): The band of maximum deviations of the ascending and descending calibration points from a best fit line through zero output. It includes the effects of nonlinearity, hysteresis, and non-return to minimum load. Expressed in units of %FS.  SEB Output is a best fit straight line output at capacity.

Nonrepeatability: The maximum difference between output readings for repeated loadings under identical loading and environmental conditions.  Expressed in units of %RO. In practice there are many factors that affect repeatability that ARE NOT included in the nonrepeatability specification.

Creep:  The change in load cell signal occurring with time, while under load and with all environmental conditions and other variables remaining constant. Expressed as % applied load over specific time interval. Logarithmic effect that is also symmetric on load removal. Stated specifications may differ and are not for the same time interval.

Eccentric and Side Load Sensitivity: Eccentric Load – Any load applied parallel to but not concentric with the primary axis. Results in moment load. Side Load – Any load at the point of axial load application at 90° to the primary axis. Error influences are reported in terms % and %/in.

Watch the event to understand why these specification details matter and some of the important variables to consider when comparing, using or troubleshooting different measurement products.  During the event, we provided a list of resources that are helpful when looking for specification information or definitions. The complete list is below.

ADDITIONAL RESOURCES

Interface Product Selection Guides

Interface Technical Support Information and Troubleshooting

Interface Load Cell Field Guide (Free Copy)

Interface Installation Guides and Operation Manuals

Interface Software and Drivers

Interface Product Catalogs

Interface 101 Blog Series and InterfaceIQ Posts

Interface Industry Solutions and Applications

Interface Recorded Webinars

Are Load Cells Used in Vacuum Environments?

Vacuum testing labs are essential for ensuring that products and materials are safe and dependable in vacuum environments. A vacuum environment is an area where there is little or no matter. This means that there are very few gas molecules present, and the pressure is incredibly low. Vacuum environments are often created using vacuum pumps, which remove gas molecules from an enclosed space.

Vacuum environments are used to simulate the conditions that products and materials will experience in space or other high-altitude environments. These types of testing labs typically have a vacuum chamber that can be evacuated to an incredibly low pressure. The vacuum chamber is then used to evaluate products and materials for a variety of properties. Engineers use vacuum environments in testing for reduced contamination, improving heat transfer, and to reduce the weight of products.

Tests performed in vacuum labs are used to determine the rate at which gases are released from a product or material and the ability of a product or material to withstand a vacuum without leaking. Thermal cycling tests are done to assess the ability of a product or material to withstand changes in temperature in a vacuum environment. Other tests are done to understand how the test article withstands exposure to radiation.

Vacuum testing labs are used by a variety of industries, including aerospace, medical, and defense. These labs are common for material process testing and used in R&D. Vacuum testing helps to identify potential problems with products and materials before they are used in a real vacuum environment. Engineers use this type of testing to improve the performance of products and materials and ensure they meet the required standards. Contact Interfaced to explore your options.

Can load cells be used in a vacuum environment?

Load cells can be used in a vacuum environment. However, not all load cells are created equal or suited for this specialized use case. Some load cells are designed that make them appropriate for vacuum environments, while others are not. Load cells that are not engineered to perform in vacuum environments may not be able to withstand the low pressures and outgassing that can occur in a vacuum. Using quality load cells that are manufactured by force measurement experts in sensor technologies is important in any consideration. It is critical to review the specifications and requirements with a qualified applications engineer.

Key considerations when choosing a load cell for a vacuum environment:

  • Outgassing: Load cells that are used in vacuum environments will have low outgassing rates. This means that they will not release gases into the vacuum chamber, which can contaminate the environment and interfere with measurements.
  • Mechanical strength: Load cells must be able to withstand the low pressures that can occur in a vacuum. They will also be able to withstand the conditions that can be generated by vacuum processes, such as outgassing and condensation. Form factor and model material of the load cell are important in choosing a load cell for this use case.
  • Temperature range: Load cells will need to operate in a wide range of temperatures. This is important because vacuum chambers can be very cold, especially when they are first evacuated, or when they are used to simulate high altitudes or space.

If you are looking for a load cell that can be used in a vacuum environment, please review with Interface application engineers to determine if the model fits your test requirements. We also can offer custom solutions to ensure that the load cell maintains the accuracy and performance specifications based on your exact test plan.

Can a load cell be vented for use in a vacuum testing lab?

Technically yes, you can vent a load cell to be used in vacuum. This allows the internal cavity of the load cell to equalize with external vacuum. However, this does not prevent outgassing and can cause the gages and wiring to be subject to humidity and condensation.

Cabling is extremely important when using any sensor in this environment. There are options to make the load cells wireless using Bluetooth technology.

Caution: Interface recommends that all our products used in this type of environment are designed, built, and calibrated for use in this environment. Venting an existing load cell can alter the performance and damage the cell.  By designing the load cell with venting for use, we can ensure that it will meet the vacuum test range.

Interface also can install thermocouples to work with the sensor to detect temperature in this type of testing environment. In fact, our engineers have designed load cells to package the thermocouples inside the form factor for convenience and performance benefits.

Interface engineers have worked with testing labs for decades. We are available to assist with any use case requirements to determine the best measurement solution.

Demystifying Specifications Webinar

Interface’s technical force measurement webinar Demystifying Specifications details descriptions, terms, values and parameters found in product datasheets for load cells, torque transducers, instrumentation and specialty products. Learn from our experts what specifications need critical review, recommendations based on product categories, and the insider point of view on what is most important in terms of specifications for different use cases and tests.

Interface Force Measurement 101 Series Introduction

In our ongoing commitment to provide valuable resources through self-help guides and online reference materials, we are introducing our 101 Series.

This new online resource is an easy-to-use guide for load cell basics and force measurement topics. The series is a collection of content in various formats that detail subjects related to test and measurement.

Interface prioritizes helping our customers understand the inner workings of our expanding line of sensors, accessories, and instrumentation by creating guides, technical manuals, and solution applications for force measurement.

The Interface 101 Series will introduce you to relevant subjects about our products and how we can help you get the most accurate and reliable force data in the industry by using our solutions.

Our new 101 Series guide is an effortless way to navigate through high-level test and measurement topics. Each section of the new 101 Series includes a featured 101 IQ blog on a single subject, as well as quick links to videos, case studies, white papers, application notes, product information, technical specifications and more related to that subject.

The goal in creating the 101 Series is to provide a basic understanding on how our products are used for various test and measurement applications across all industries. The references are an effective way to learn about the broad depth of Interface products like our precision load cells, torque transducers, multi-axis sensors, calibration systems and instrumentation. We also provide relevant test and measurement content related to types of force measurement testing, components, systems, and materials used in engineering highly accurate measurement technologies.

There are thousands of references found throughout our site, like our design files for product engineers and digital instrumentation set-up videos for lab techs. It is our pledge to develop material that support our 35,000 products, as well as provide educational content like the 101 Series and our ForceLeaders Webinars you can watch on-demand.

Included below are the current 101 IQ Blogs you will find featured on the 101 Series online guide. We will add additional references to this 101 Series, as we post new subjects. Go to Force Measurement 101 Series to bookmark this reference.

101 Series IQ Blogs

You can find additional reference materials related to our products and services including manuals, product catalogs, technical references, and events.  Go to our online support to find helpful educational and advanced resources like our technical glossary, engineering tips and installation guides.

If you are mostly interested in why you should choose Interface, here is a good reference to start.

If you are not able to find the information you need or you have a specific question about our products or services, be sure to contact us to help.

Shunt Calibration 101

Calibration is a critical stage to ensure proper accuracy and reliability of any force measurement device. There are many ways to calibrate and different types of calibration. In the standards of maintaining our quality and precision requirements, Interface calibrates every test measurement device we manufacture including our load cells and torque transducers. Every device is shipped with the most detailed calibration certifications in the industry.

With our experience and expertise, we understand that sharing what we know is beneficial to our customers and partners in the test and measurement industry. One of the means by which we do this is through a series of technical white papers.  A popular white paper that was written years ago still stands the test of time, as it provides a deep dive on the topic of shunt calibration. Click on the title “Shunt Calibration for Dummies,” to access the full white paper.

What is Shunt Calibration?

Shunt calibration is a technique for simulating strain in a piezo-resistive strain gage Wheatstone bridge circuit by shunting one leg of the bridge. The bridge may be internal to a discreet transducer or composed of separately applied strain gages. The resulting bridge output is useful for calibrating or scaling instrumentation. Such instrumentation includes digital indicators, amplifiers, signal conditioners, A/D converters, PLC’s, and data acquisition equipment. Care must be taken to understand the circuits and connections, including extension cables, in order to avoid measurement errors.

Benefits of Shunt Calibration

The biggest reason to use shunt calibration is the flexibility and low-cost it offers the user. In this method of calibration, the bridge circuit is already there, and you don’t need to make and break cable connections to run it. This means that a shunt calibration can be applied conveniently and at any time during the test program. It is often used in situations where the user is calibrating control system equipment that will be communicating with a transducer or to confirm that the transducer is functioning properly.

Expected Shunt Calibration Repeatability in Modern Transducers

An important question that comes up regarding calibration is what type of repeatability can I expect from shunt calibration? Included below are the specifications outlining expected repeatability:

Procedure for a repeatability test performed:

  • 100 Klbf Load Cell specimen loaded in compression.
  • 12 test cycles of 4 mV/V hydraulically applied physical load and 1 mV/V Shunt Cal on two bridge legs.
  • Rb = 350 ohm, Rs = 88750 ohm, 20 ppm/°C, internal to load cell.
  • Measurements over 3 days.
  • Interface Gold Standard HRBSC instrumentation.

Results of test

  • Std Dev of physical load measurement: 0.004%.
  • Std Dev of Shunt Cal: 0.001% pos, 0.001% neg.

The topics that are illustrated in examples and discussion points for this white paper include:

  • Basic Bridge Circuit and Formulas
  • Resistor Examples
  • Tolerance
  • Cables
  • Errors
  • Permanent Zero Balance Shifts
  • Transducer Toggles
  • Instrumentation
  • Procedures for Repeatability in Tests

If this is a topic that of interest, download this technical reference guide for further exploration and calculation examples in shunt calibration.

As a leader in calibration services, Interface has an A2LA ISO 17025 accredited calibration lab located at the company’s headquarters in Arizona. Many depend on Interface for expert recalibration, which we recommend to do annually for optima maintenance. Our own calibration lab has the broadest capability and highest quality of calibration and repair services available. We understand the criticality of proper calibration and traceability and have the experience and expertise necessary to meet your exacting needs.

Additional Calibration Resources

Extending Transducer Calibration Range by Extrapolation

Additional Interface Calibration Grade Solutions

Gold Standard® Calibration System

This refreshed white paper is a tribute to the contributions of LaVar Clegg.

Interface White Paper Highlights Contributing Factors to Load Cell Accuracy

Core to everything we do at Interface is within our foundational pillars of quality, service, accuracy, and innovation.

When it comes to precision load cells, our team of experienced engineers is fully committed to designing and building the best force measurement products in the industry. In fact, it is commonly known that with our published specifications, we often exceed them. It is why Interface products retain the industry-leading reputation for precision performance.

With innovation and imagination in our values, Interface team members look for ways to continuously test boundaries and explore possibilities, never losing sight of accuracy. We are students of the industry and understand the precise details and mechanics that go into force measurement product development to make our solutions are the most accurate on the market.

Critical to serving our customers is understanding the contributing factors of load cell accuracy and matching the products that best fit their requirements. There are many factors that can disrupt accuracy or skew data, such as the environment in which the load cell is being used, the type of load cell application use, even the mounting process.

All of these specifications must be correctly identified when choosing the right product for the project or product to ensure accurate results. To help our fellow community of engineers, manufacturers, and product designers to navigate the force measurement world, we have developed a new Contributing Factors to Load Cell Accuracy White Paper. Click here to get your copy today!

This Interface technical white paper includes a breakdown of the most critical factors of load cell accuracy, which includes:

  • Creep
  • Side and eccentric load
  • Temperature
  • Humidity
  • Mounting process

Chief Engineer Ken Vining provides valuable insights into some of the steps to take to avoid accuracy failure, such as:

  • Utilizing a golden part
  • Preventing load cell misuse
  • Setting a preventative maintenance schedule
  • Recalibration

This valuable resource provides a quick reference to understand load cells and the intricate details that come with their proper and accurate use. Included below is a link to access the downloadable PDF of the white paper. This technical white paper is an addition to our long-standing commitment to providing expert resources for those using or researching use case applications for load cells.

CLICK HERE TO ACCESS TO THIS NEW WHITEPAPER TODAY

The Interface Load Cell Field Guide is also available on Amazon.