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Interface Calibration 101

Calibration of force and torque sensors is critical to receiving accurate data from measurement testing.  Calibration is the comparison of the instruments output against the known standard of measurement. For a load cell, it is the comparison of the load cell output against the standard test loads.

The calibration ensures the sensor is performing accurately and set for ideal output based on the capacity and configuration of the design. A standard calibration tests repeatability and linearity, which are both used to determine the accuracy. Calibration tests are run to identify any potential measurement errors caused by zero offset, non-linearity, hysteresis, non-repeatability, and shifts from zero.

The team of calibration experts at Interface are specifically trained to provide this specialized service as part of our quality and lab standard requirements of operations. Interface is ISO/IEC 17025:2017 and A2LA Accredited for Torque and Force Calibration in accordance with the recognized International Standard ISO/IEC 17025:2017.

Interface certifies all our load cells with accredited calibration before releasing the product for shipment. Certificate information includes tabulated measurement variables data, zero balance, computed nonlinearity and hysteresis, and traceability statements. We are often recognized as the most complete calibration certification in the industry

In a recent customer survey, we found that one of the most important roles that Interface force measurement solutions play is for our calibration services. This is due to the accuracy, reliability, and dependability over long periods of time of the load cells we manufacture and service.

Why Calibrate?

Calibration is important based on many different factors, including continued performance, safety, and compliance with ISO or industry specific standards. Interface’s standard recommended calibration interval is to recalibrate every 12 months. The frequency of calibrations should be determined by the following use case factors that may affect measurement accuracy:

  • Measurement quality and allowable tolerance range
  • Level of stress to which the equipment is subjected
  • Stability of past calibrations
  • Required measuring accuracy
  • Quality assurance requirements

Interface Calibration Services

Interface provides calibration and repair services on load cells and torque transducers, including devices made by other sensor manufacturers. In fact, Interface performs more than 100,000 calibrations every year. This includes new products as well as devices sent to Interface for recalibration and repair. Repairs include a complete evaluation of the device prior to repair and calibration upon completion.

In addition, here are some other benefits for choosing Interface as your calibration services partner:

  • ISO 17025 Accredited
  • Scheduled Repairs for Ongoing Inventory Management
  • Custom Calibration Services
  • RMA Tracking and Permanent Archive of Test Data
  • NMI Certified Gold and Platinum Standard Reference Load Cells
  • Interface Gold Standard Calibration Software Used for Data Collection and Analysis
  • Full-Service Machine Shop for any Mechanical Requirements

Calibration Services Process

The Interface calibration team consists of a team of professionals dedicated to an optimized calibration and repair process for timely management of our customer’s requests. We begin the calibration service process with our technical services group who manage the request and RMA process, in addition to evaluation and troubleshooting.

We evaluate all products at our headquarters with our team of experts. The sensor goes through a thorough inspection process to identify any necessary repairs and to ensure device is in working condition in preparation of a calibration. This includes an electrical test. Based on the extensive evaluation, if the device is found to be unrepairable, there is no charge. We also will work to find a replacement unit. All work beyond the evaluation is sent to the customer for approval.

The final step in our calibration services process is the actual calibration. Our calibration team is considered the most experienced in the industry due to the sheer volume of work product they calibrate every day.  Interface also is heavily invested in conducting all our calibrations with the most advanced equipment, including our proprietary Gold Standard® and Platinum Standard® systems. These machines ensure that the transducers are calibrated to the most accurate ability possible before returning to the customer.

Even the most high-end manufactured load cells and finely tuned components endure accuracy degradation over continued use. To ensure your sensors are always ready for peak performance, check out our calibration services request form. We also provide a full range of calibration grade products for metrology and testing labs. You can also give us a call at 480-948-5555 to discuss your specific calibration needs.

 

Interface 1200 Precision LowProfile Load Cell Series Product Highlight

As a premier force measurement solutions manufacturer of more than 53 years, Interface has developed a giant catalog of more than 30,000 products that are standard, engineered-to-order and completely custom. We’ve seen nearly every challenge and complexity presented by our customer’s application requirements.

Load cells are what we know, inside and out. By design, testing, manufacturing, and calibrating, Interface is the trusted leader in quality load cells. This allows us to keep our finger on the pulse of the marketplace and develop products that meet the unique needs of a wide variety of industries.

From our earliest days, one product that has become truly an industry standard in load cells by popularity and by far the top pick for use in test labs and by product testing engineers is the 1200 Precision LowProfile® Load Cell.

When we originally released our first of its kind LowProfile Load Cell, Interface became an instant market leader in precision load cells.  In fact, our 1200 Standard Precision LowProfile Load Cell, designed for eccentric load compensated tension and compression force measurement, remains our most frequently purchased product every year. This pancake-style load cell is world-renowned for durability, accuracy, and performance.

The 1200 Model Series features and benefits include:

  • Proprietary Interface temperature compensated strain gages
  • Performance to .04%
  • High output – to 4 mV/V
  • 0008%/°F (.0015%/°C) temperature effect on output
  • Low deflection
  • Shunt calibration
  • Barometric compensation

In addition, it’s popularity amongst customers is due to the product’s versatility and customizability. There are multiple models available including the 1240, 1244, 1252, 1260, 1280, and 1290. In addition, the series is compatible with a host of base options, connectors, bridge options and overload protection. And like with all Interface products, our custom solutions team can work directly with you to meet your specific needs.

The 1200 series is used across a variety of industries for test and measurement applications. Included below are a few examples of the product in action expressed as application notes. Industries covered in these examples include aerospace, agriculture, and waste management.

AIRCRAFT LIFTING EQUIPMENT

An aerospace company wanted to check if the valves on their aircraft lifting equipment were working safely and properly. Interface’s recommended installing a 1200 Standard High Capacity Load Cell in between the aircraft testing rig and the lifting jack. The load cell can measure the load’s force safety valve when the lifting equipment opens. Results are then sent to the 9890 Strain Gage, Load Cell, & mV/V Indicator, where the customer can see it displayed in real-time. Using this solution, the customer was able to determine that the aircraft lifting equipment was working properly. Since they are ensured of its safe functionality, it can now be used on real aircrafts that need to be lifted. Read more here.

SILO GRAIN DISPENSING

A customer wants to measure and record the grain being put in and out of their grain dispensing container, as it dispenses content into a carrier truck for transportation. The customer also preferred a wireless solution. Interface suggested installing a WTS 1200 Standard Precision LowProfile™ Wireless Load Cells at the legs of the grain dispensing container. The 1200 can measure the distribution correlation of the grain as it inputted and outputted from the container. Results are transmitted and displayed using the WTS-BS-1-HA Handheld Display for multiple transmitters and logged and graphed using the WTS-BS-4 USB Industrial Base Station. Using this solution, the customer was able to log and graph the measurement results of the grain content that the silo dispenses into the grain dispensing container, and also when the grain is dispensed into the carrier truck.  Get more details here.

WASTE MANAGEMENT CONTAINER WEIGHING

A waste management company wanted to measure the capacity of their waste containers to know when it is time to dispose the waste. Interface provided Model WTS 1200 Standard Precision LowProfile™ Wireless Load Cells to be installed at the bottom of each waste container leg to measure the sum weight of the container. The real time weight data is then transmitted to the WTS-BS-4 USB Industrial Base Station with the supplied Log100 software. Using this solution, the customer was able to determine when their waste container was at full capacity to dispose of the waste, or to transfer it in a timely manner.

To learn more about load cell basics and watch our recorded webinar, go here.

Load Cell Basics Technical Q&A Part One

Interface recently hosted a webinar, Load Cell Basics, where our experts answered a series of questions from event attendees.  In the interest of sharing what we know and addressing some frequently asked inquiries, we are offering a new series in our Interface IQ blog entitled Interface Technical Q&A.  At the conclusion of our presentation, we had several questions related to the basics of load cells.  We will be answering them in two posts.  Here is the first list.

Why Does Interface Use Proprietary Strain Gages?

Strain gages are a key component to any load cell. Making our own strain gages gives Interface full control over our design and production, ensuring we can meet our specifications with certainty in quality, accuracy, and dependability. Interface strain gages are precision matched to the load cell material to counteract the temperature effect on output. We have different load cell materials and that means we use different strain gages for each type.

Our strain gages are simple in design, and we don’t use compensation resistors making them a more reliable circuit.  They don’t have to go through resistors; thus, the accuracy and precision measurement is more reliable and capable of faster thermal and higher outputs.

What Type of Temperature Testing Do You Do on Interface Load Cells?

Interface performs both hot and cold thermal compensation from 15˚ – 115˚F, including adjust and verify cycle. Most other manufacturers of load cells provide only the hot side (60˚ – 160˚F) testing. The slope of the curve is much flatter near room temperature and identical at both ends of the slope.  The result is minimal variation across the entire thermal range.  We also offer custom calibrated ranges by request. Read more about temperature compensation here.

Is Temperature Compensation Achieved Using Dummy Gages?

The simple answer is no. Interface does temperature compensation of output by matching the strain gage to the material temperature compensation of zero. This is achieved by using a wire in the circuit that opposite resistance changes to the circuit.

Do You Have Software to Read TEDS Available to Use with Interface Load Cells?

Interface does offer instruments that can use the TEDS data and instruments that can write the TEDS data to the chip. Interface has software that hooks up or attaches to the TEDS chip reader writer so that when we program a chip during manufacturing of load cells in our factory, we have software we use to complete this function.  We also do offer this software for commercial use.

Is it Acceptable to Regularly Use a Load Cell Above Its Specified Capacity?

The short answer is no.  You want to reserve the “above capacity” for accidents. We are aware that people will do this in practice and the load cell will typically work reasonably well above capacity.  The problem is you want to handle that the answer is no you’re consuming your safety factor and you don’t have any extra head room for something that goes wrong.  Also, using the load cell above capacity doesn’t exactly follow the same calibration curve.  As an example, if you are using a 10k load cell with a 10k calibration and you are running tests at 12k, you may have higher errors.  You can ask us to calibrate the load cell to the 12,000 lbs.; however, you must note this can reduce the safety factor and that is why we do not recommend it.

Does Variable Altitudes Impact Performance?

Interface does have the ability to handle different pressure scenarios. Interface sends our products out ventured, if necessary, to be able to tolerate pressure change without causing any zero shift. If you do have applications where you will have either high pressure requirements or pressure change throughout the test, we certainly have solutions that can accommodate that requirement.  Work with your application engineer to ensure you have the exact testing requirements detailed when you are ready to buy your next load cell.

Can Cables Influence Temperature Errors?

If you’re adding cable to the load cell and for example, you have a 10-foot cable you want to add 100 feet more of cable, that adds resistance. The lower gauged cable, the better. A 22-gauged cable is better than a 28-gauged cable, so use a thicker cable and it will have less resistance impact on the measurement.

Is Creep Due to Material or Strain Gages?

Essentially all the elements in the sensor and everything mechanical will respond to creep.  The goal is to match the creep behavior of the bridge to the creep behavior of the load cell material or the flexure. Yes, creep affects both, but you want the result to trend in the same direction so that you get minimal error or change in the signal because of creep.

Are All Load Cells Intrinsically Safe?

They are intrinsically safe devices; however, anytime there is a hazardous environment or intrinsic safety requirements, please contact an application engineer to review the sensor use case and to factor in all aspects of your testing and use environment.  The load cells are typically working off 5-volt 10-volt and you need to look at all aspects of the location. For more information about our specialized line visit load cells for harsh environments.

This is the first in a new series of Interface Technical Q&A.  Do you have a question for our technical experts?  Send an email to digmktg@interfaceforce.com and we will add it to a future post.  If you have an immediate question, be sure to contact us today and let us know how we can support you.  You can also reference our technical support resources online for help.

Contributors:  Keith Skidmore and Brian Peters

Additional Resources

Load Cell Basics Webinar Recap

Interface Load Cell Field Guide

Interface Solutions Designed for Infrastructure Challenges

All infrastructure, big and small, needs to be designed with safety and durability in mind. Take for instance the massive amount of design, engineering and quality control that goes into a suspension bridge requires testing before and after it’s built. Not only does it need to be constructed with supreme accuracy, but it also needs to be tested and monitored constantly to ensure it’s safe for use, especially as often infrastructure projects are exposed to extreme elements.

Among the various tools and technologies used to build and test infrastructure designs, sensors play a substantial role. Interface has served infrastructure industry suppliers and customers since our founding more than 50 years ago. Force and torque measurement products including Interface load cells, torque transducers, load pins, load shackles, tension links and instrumentation are involved in a wide variety of infrastructure applications including construction and maintenance of bridges, roads, transportation systems, communication structures, water and electrical facilities, and numerous inventions that are used to build, test, support, maintain and monitor performance of these critical projects around the world.

Accuracy and reliability of Interface solutions are a key factor in measuring structures and components that are exposed to hazardous or weather-related conditions, heavy loads and constant use. Our precision load cells, rugged load pins, wireless and digital instrumentation, along with multi-axis sensors and robust torque transducers are a top choice for those engaged in infrastructure engineering projects and testing.

A few examples of how Interface products have been used on infrastructure applications over the years are noted below. The following examples are also found with many others here /solutions/infrastructure/. 

Concrete Dam Flood Monitoring

A customer was looking for a solution to monitor a concrete dam and be notified when it reached high flooding levels. Interface provided WMC Miniature Sealed Stainless Steel Load Cells with multiple WTS-AM-1E Wireless Acquisition Modules connected to the load cells. This solution proved to be small enough and perfect for measuring compression and tension on the dam. The WMC Modules are installed on the arch of the dame and transmit data and notify the customer through Interface’s Wireless Telemetry System when flooding occurs. Check out the application here.

Hydropower Turbine Generator Monitoring

When a customer wants to monitor and detect turbine generator faults in their hydroelectric power plant located on a river, Interface can provide a T2 Ultra Precision Shaft Style Rotary Torque Transducer and attach it to the turbine generator with Interface’s Shaft Style Torque Transducer Couplings. When water from the river pushes through the penstock to the outflow, it moves the turbine blades, creating electricity through the generator shaft. Torsion measurements can be graphed and logged with the 9850 Torque Transducer and Load Cell Indicator catching any unusual fluctuations and vibrations. Using this solution, the customer can monitor, graph, and log the torque measurement results of the turbine generator. Learn more here.

Bridge Seismic Force Monitoring Solution

A customer wanted to monitor seismic activity that occurs to a bridge using force sensors to continuously monitor activity before, during and after earthquakes. They also wanted a wireless solution to avoid running long cables on the bridge. Interface provided an LP Load Pin custom made to fit the need. The load pin was used in conjunction with our WTS Wireless Telemetry System to monitor the force on the load without cables. Using this solution, the customer was able to monitor continuously, log results to the cloud and review the data. Read more here.

This is just a brief example of the applications throughout the infrastructure industry that Interface supports. A long list of additional applications that use Interface products includes:

Highways and Bridge Construction and Monitoring

Concrete Dam Measurement and Flood Monitoring

Transportation Heavy Equipment Testing

People Movers for Airports

Train Brakes Testing

Power Generation Equipment

Geotechnical Monitoring

Road Load Tests

Weight Bridges and Transportation Scales

Truck and Aircraft Weighing

Housing Mainframe and Skyscraper Construction Monitoring

Building Foundation Capacity Measurement

Bridge Seismic Force Monitoring

In-Motion Rail Weighing

Cranes and Heavy Object Lifting

Structural Testing and Telecommunication Structures

Conveyor Belts

To learn more about Interface and our solutions for Infrastructure and other key industries, please visit our solutions page at www.interfaceforce.com/solutions/.

Interface Force Measurement Solutions Featured in Quality Magazine

Choosing a force measurement device and getting the most out of it is a tricky process, even for the most seasoned engineers. So, when Quality Magazine asked our Chief Engineer and VP of Quality, Ken Vining, to share his knowledge of force measurement, he decided to put together a guide on what to look for in force measurement equipment and how to use and maintain your equipment properly.

In his Quality Magazine article titled, “Selecting and Using a Force Measurement Device: Everything you need to know,” Vining explains the contributing factors to force measurement device quality and accuracy, as well as a few tips and tricks to make sure you’re getting the best possible accuracy and longevity out of your device.

Included below is a brief introduction from article:

Force measurement devices like load cells, torque transducers and data acquisition devices are used across industries to design and test hardware. They’re a key factor in the product development process because the force, torque and weight data they collect helps to ensure products are accurately constructed, work as intended, are safe for use, and can withstand the test of time. In highly regulated and complex industries like medical and defense, this data becomes even more important because any miscalculation in the design of a product can put lives at risk.

The first thing to understand is every project requiring a load cell or torque transducer has different variables affecting accuracy and quality. And for every situation in product development and testing, there is a load cell to fit your precise need. Therefore, the most important step in ensuring accurate and high-quality data is speaking to a force measurement expert about the details of a project.

There are five key factors you need to know related to data accuracy, and three factors related to force measurement device quality. I’ll explain why each factor can contribute to inaccuracies and what to look for when selecting a device based on material selection, build quality, and environmental factors… READ MORE

Additional Ken Vining feature:

/the-five-critical-factors-of-load-cell-quality/

For additional information on selecting and using your force measurement device, please contact our solutions experts.

Load Cell Basics Webinar Recap

Interface applications and load cell expert Keith Skidmore was the featured presenter at the latest ForceLeaders Forum hosted event, Load Cell Basics. In his comprehensive presentation, he highlights key subjects including fundamentals of load cell design, sensor specifications, use cases, troubleshooting and valuable performance related topics.

The entire event is now available on the Interface YouTube channel.

In this 60-minute virtual event, Keith highlights commonly asked questions from both new load cell users as well as for advanced engineers and force measurement pros.

What will you learn watching the online Load Cells Basics event?

  • Load cell designs and how they work
  • Capacities, models and how to choose the right load cell
  • Factors that can impact sensor accuracy
  • Performance, moment compensation, creep, and eccentric load sensitivity
  • Calibration and troubleshooting
  • Use Cases and FAQs

The team concluded the event by answering a series of questions from the participants. They addressed advanced technical and set-up questions, as well as frequently asked inquiries about common troubleshooting issues.

Here is a sample of questions that you can find answered in the Load Cell Basics recorded event:

  • Is the temperature compensation achieved using dummy gauges?
  • Does Interface offer or have their own software to read the TEDs?
  • Can we assume that all load cells are intrinsically safe for hazardous locations?
  • What is the IP protection rating for the electrical connection?
  • What is better way to tare load cells, by electronics or mechanical preload?
  • Which is the frequency measurement limit and how fast does the load cell respond?
  • What are recommended amplifier instrumentation brands?
  • What is the most frequent problem when installing a load cell?
  • Does the cables and amplifiers affect the results of the load cell calibration?
  • For an application to 10 kN (2250 lbf), is it too much to use a 2000 lb load cell or should we use the next higher capacity?
  • What is the maximum sampling frequency for strain gage load cells?

WATCH THE EVENT AND Q&A HERE: https://youtu.be/_oHvfAzHMig

If you have additional technical questions or would like to talk about your specific application requirements, contact our Interface Application Engineers here. 

Additional resources for troubleshooting can be found here.

Our Interface Load Cell Field Guide is also helpful for troubleshooting and advanced technical support references.  You can order here.

Interface Load Cell Indicators

At Interface, our claim to fame is that we offer the most accurate and reliable force measurement devices on the market, from load cells to torque transducers and everything in-between. However, no test is complete without the system used to gather the data to evaluate performance results. That’s why we provide a wide variety of instrumentation solutions that include signal conditionersoutput moduleshigh speed data loggersportable load cell indicators, and weight indicators to complete any testing system.

Among the Interface instrumentation products, the most frequently purchased with a force measurement devices are our load cell indicators in various models including handheld, digital, wireless, multichannel, programmable, output modules, analog and bidirectional.

What is a load cell Indicator?

A load cell indicator is a high-accuracy device connected to the output of a load cell to amplify and display the value of the measured load force and weight. Load cell indicators are often needed where the force, load, or weight measurement needs to be visually displayed for the user and displaying the results on a PC is not feasible or desired in the testing environment. For example, testing in the field or confined spaces can make it impossible to connect directly to a PC. In these situations, indicators are used to quickly review and capture force data in real-time.

A few key benefits of load cell indicators include that they provide stable excitation voltage and converts force measurement sensor signals to a digital display. Commonly available features include analog or digital output, selectable digital filtering, peak and valley monitoring and set-point outputs. Additionally, each load cell indicator can be used to connect to four (or more) digital load cells and can display individual readings or the sum of all connected load cells.

Need a load cell indicator?

Interface offers a wide variety of load cell indicators in multiple configurations. Interface indicators come in single to multi-channel weight transmitters and can be found in handheld and portable designs. Things to consider when selecting an indicator are internal sample rate and update rate of the output. A few of our most popular indicators include:

9890 Strain Gage, Load Cell, & mV/V Indicator

Model 9890 is a powerful multipurpose digital load cell meter ideal for weight and force measurement applications. With a max current of 350 mA at 10 V, it can support up to 12 load cells making it perfect for multipoint weight measurement purposes. The dual-scale capability allows for displays in two different units of measure. See a demonstration video here.

9320 Battery Powered Portable Load Cell Indicator

Model 9320 is a bipolar 7-digit handheld meter featuring two independently scalable ranges, peak and valley monitoring, display hold, mV/V calibration, and a power save feature. Typical battery life exceeds 45 hours of continuous use and 450 in low power mode. IEEE1451.4 TEDS Plug and Play compliant.

482 Battery Powered Bidirectional Weight Indicator

Model 482 is battery powered, bidirectional, and comes in a NEMA 4X stainless enclosure. Standard options include 523,000 internal counts, 0.8-inch LCD 6-digit display and a measurement rate that goes up to 40 Hz. Available options include analog and relay outputs.

1280 Programmable Weight Indicator and Controller

The Interface 1280 Series programmable digital weight indicator with color touchscreen, web server view and multiple protocol types delivers uncompromising speed for today’s operations and expansive options for tomorrow’s requirements.

INF4-Ethernet IP Weight Transmitter and Indicator

The Interface INF4-Ethernet IP weight transmitter and indicator has a six-digit red LED display (8 mm height), space-saving compact design, four buttons for the system calibration, and a six-indicator LED.

See all the indicator options here.

Load Cell Indicator Application Note

The application note below provides an example of the benefit of a load cell indicator in real-world use within the medical industry.

A pharmaceutical tablet producer wanted to monitor the forces applied by the tablet forming machine to understand the relationship between raw material, die set, forming force, and motor-cycle speed. The goal was to improve productivity and efficiency of the tablet forming process while reducing losses, such as cracked tablets or voids, by adding a dimension of feedback that could be used to assign specific press adjustment criteria for given inputs.

An Interface Model WMC Sealed Stainless Steel Mini Load Cell (10K lbf Capacity) was mounted in the section of the downward press bar. The machine was modified to accomplish this. The load cell was then connected to a Model 9320 Portable Load Cell Indicator to collect the needed data.

The indicator was selected as the data collection device because a laptop could have interfered with the test cycle due to space restrictions. The output of the load cell was connected to the 9320 Portable Load Cell Indicator and set aside so that the cable did not interfere with the cycle and no snagging would occur. A cable tie was used to stow aside the cable and to ensure there was enough clearance for the entire cycle.

After analyzing the data, the tablet producer was able to quantify adjustment levels by monitoring which forces produced the most optimal results for a given cycle speed, die set, and raw material. Productivity and efficiency were greatly improved by the enhancement of the data feedback.

To learn more about Interface load cell indicators and for a complete list of products, you can download our instrumentation brochure here. You can also read more about instrumentation options in test and measurement in this post.

Load Cell Test Protocols and Calibrations

In the Interface Load Cell Field Guide, our engineers and product design experts detail important troubleshooting tips and best practices to help test and measurement professionals understand the intricacies of load cells and applications for force measurement devices. In this post, our team has outlined some helpful advice for testing protocols, error sourcing and calibrations.

The first step in creating test protocols and calibration use cases is to define the mode you are testing. Load cells are routinely conditioned in either tension or compression mode and then calibrated. If a calibration in the opposite mode is also required, the cell is first conditioned in that mode prior to the second calibration. The calibration data reflects the operation of the cell only when it is conditioned in the mode in question.

For this reason, it is important that the test protocol, which is the sequence of the load applications, must be planned before any determination of possible error sources can begin. In most instances, a specification of acceptance must be devised to ensure that the requirements of the load cell user are met.

Typical error sources in force test and measurement are usually identified as being related to:

  • Lack of protocol
  • Replication of actual use case
  • Conditioning
  • Alignment
  • Adapters
  • Cables
  • Instrumentation
  • Threads and loading
  • Temperature
  • Excitation voltage
  • Bolting
  • Materials

In very stringent applications, users generally can correct test data for nonlinearity of the load cell, removing a substantial amount of the total error.  If this can’t be done, nonlinearity will be part of the error budget.

An error budget is the maximum amount of time that a technical system can fail without service level consequences. In force test and measurement, it is sometimes referred to as uncertainty budget.

Nonlinearity is the algebraic difference between output at a specific load and the corresponding point on the straight line drawn between minimum load and maximum load.

Nonrepeatability is essentially a function of the resolution and stability of the signal conditioning electronics.  Load cells typically have nonrepeatability that is better than the load frames, fixtures and electronics used to measure it.

Nonrepeatabillty is the maximum difference between output readings for repeating loading under identical loading and environmental conditions.

The remaining source of error, hysteresis, is highly dependent on the load sequence test protocol.  It is possible to optimize the test protocol in most cases, to minimize the introduction of unwanted hysteresis into the measurements.

Hysteresis is the algebraic differences between output at a given load descending from maximum load and output at the same load ascending from minimum load.

There are cases when users are constrained, either by requirement or product specification, to operate a load cell in an undefined way that will result in unknown hysteresis effects. In such instances, the user will have to accept the worst-case hysteresis as an operating specification.

Some load cells must be operated in both tension and compression mode during their normal use cycle, without the ability to recondition the cell before changing modes. This results in a condition called toggle, a non-return to zero after looping through both modes. The magnitude of toggle is a broad range. There are several solutions to the toggle problem, including using a higher capacity load cell so that it can operate over a smaller range of its capacity, use a cell made from a lower toggle material or require a tighter specification.

ONLINE RESOURCE: INTERFACE TECHNICAL INFORMATION

For questions about testing protocols, conditioning, or calibration, contact our technical experts. If you need calibration services, we are here and ready to help.  Click here to request a calibration or repair service today.

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