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Understanding Load Cell Temperature Compensation

The performance and accuracy of a load cell is affected by many different factors. When considering what load cell will work best for your force measurement requirements, it is important to understand how the impact of the environment, in particular the temperature impact on output.

An important consideration when selecting a load cell is to understand the potential temperature effect on output. This is defined as the change in output due to a change in ambient temperature. Output is defined as the algebraic difference between the load cell signal at applied load and the load cell signal at no load. You can find more detailed information in our Technical Library.

Temperature affects both zero balance and signal output. Errors can be either positive or negative. To compensate for this, we use certain materials that are better suited for hot or cold environments. For instance, aluminum is a very popular load cell material for higher temperatures because it has the highest thermal conductivity.

In addition to selecting the right material, Interface also develops its own proprietary strain gages, which allows us to cancel out signal output errors created by high or low temperatures.

In strain gage-based load cells, the effect is primarily due to the temperature coefficient of modules of elasticity of the force bearing metal. It is common in the industry to compensate for this effect by adding temperature sensitive resistors external to the strain gage bridge which drop the excitation voltage reaching the bridge. This has the disadvantages of adding thermal time constants to the transducer characteristic and of decreasing the output by 10%.

Our load cells are temperature compensated for zero balance. By compensating for zero balance, we can flatten the curve in the relationship between temperature and zero balance. An uncompensated load cell has a much more severe curve, which ultimately affects accuracy and performance.

Interface offers thousands of load cell designs, standard use and for hazardous environments. For instance, rocket engine tests subject our load cells to extremely high temperatures. For use in various maritime industry projects, they can be used in very cold coastlines and even submerged in cold water. No matter where you are, environment influences the load cells performance.

If you are concerned about temperature, Interface provides specifications for every load cell we manufacture. The Interface specification datasheet, as see referenced here, is available for download by product. It always includes all the necessary data required to understand the load cell’s ability to perform at the highest-level including compensation range, operating range, effect on zero balance and effect on span.

One thing that is also unique about our products is that while most competitors only compensate for hot temperatures (60 to 160 degrees Fahrenheit), Interface covers both hot and cold thermal compensation from 15 to 115 degrees Fahrenheit, including adjust and verify cycles.

Be sure to tune into Load Cell Basics, where Keith Skidmore discusses temperature compensation.  He notes during this informative presentation that if the temperature is changing during a test that can affect the zero and the output of the load cell. How much effect depends how much temperature is changing and how well the load cell is compensated against the errors, which can be either positive or negative. Good news is they are repeatable from test to test so if you have large temperature swings you can characterize the system and then subtract out the shift if you know the temperature effect on zero.

Interface Application Engineers are available to answer questions regarding the effect of temperature on force measurement data, or the different ways we can help design a solution to compensate for your environment.

Load Cell Basics Sensor Specifications

When selecting a load cell, it’s critical to understand the major factors impacting sensor accuracy. These factors are determined by the materials and components used in the construction of the load cell, the calibration, instrumentation, the accessories such as cables, and mounting installation. Each must be considered in the specific use cases for the load cells.

During our virtual event, Load Cell Basics, applications expert Keith Skidmore detailed everything you need to know about load cells and how to choose the right load cell.  First, Keith highlighted four vital application considerations:

Mechanical – Dimensions and Mounting

Electrical – Output and Excitation

Environmental – Temperature and Moisture

Performance – Accuracy and Thermals

Taking a deeper dive into performance, an important consideration when selecting your load cell are the sensor specifications and how they impact accuracy. The sensor specifications relate to the max error of various parameters. The specifications are always included in product spec sheets and are expressed using the following values – %FS, %RO, %, %/°F, time-related. Specifications listed relate to the max error for accuracy and temperature.

Interface provides all sensor specification data for our load cells in the product datasheets found on each product page for easy download.

The information we provide for every load cell typically includes:

Accuracy:

  • Static Error Band
  • Non-linearity
  • Hysteresis
  • Non-repeatability
  • Creep
  • Side Load Sensitivity
  • Eccentric Load Sensitivity

Temperature:

  • Compensated Range
  • Operating Range
  • Effect on Zero
  • Effect on Span

Further definitions can be found in our online technical glossary.  Here are a few that we highlighted in the Load Cell Basics webinar.

Static Error Band: 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 Output: The output at capacity based on the best fit straight line.

Non-repeatability: The maximum difference between output readings for repeated loadings under IDENTICAL LOADING AND ENVIRONMENTAL CONDITIONS. In practice, there are many factors that affect repeatability that ARE NOT included in the non-repeatability specification. It is normally expressed in units of %RO.

Non-linearity: The difference in the output from a straight line. It is normally expressed in units of %FS.

Hysteresis: The difference in the ascending versus descending curves. This is normally expressed in units of %F.

Understanding these factors and the maximum error for your specific project is critical to selecting a load cell and getting the best possible data out of it.

To learn more about sensor specifications for load cells, review the product specs on each datasheet or in our product catalogs. For additional help, call to speak with our application engineers at 888-557-2533.

To view additional online events, please go to our events page.

 

How to Choose the Right Load Cell

Load cells are used to test and confirm the design of hardware, components, and fixtures used across industries and by consumers. From the structural integrity of an airplane to the sensitivity of a smartphone touchscreen, there’s a load cell available to measure force. In fact, here at Interface we have over tens of thousands of products used in force measurement, for all types of different applications.

How do engineers and product designers go about choosing the right load cell for a specific application or testing project?

Have no fear, Interface has put together a short guide on choosing the load cell that is right for you. This blog will cover the basic questions to answer when selecting a product, as well the most important factors affecting load cell choice.  Be sure to watch the online video, Load Cell Basics, that highlights key factors of consideration when choosing the right load cell for additional insights.

The basic questions you need to consider when selecting a load cell include:

  • What are the expected loads? What is the minimum and maximum load you’ll be measuring?
  • Is there any potential for higher peak loads than what you intend to measure? What are these expected peak forces?
  • Is it tension, compression, or both?
  • Will there be any off-axis loads? If so, what is their geometry? Do you want to measure them too?
  • Will it be a static, dynamic or fatigue measurement?
  • What is the environment in which you’ll be conducting your test? Will the load cell need to be sealed?
  • How accurate do your measurements need to be? Do they need to be at the highest accuracy of ±0.02-0.05% or within ±0.5-1%?
  • What additional features, accessories and instrumentation does your application require to complete a test?
  • Do you need standard electrical connectors or customized options? What about additional bridges or amplifiers?
  • How are you planning to collect and analyze the data output from the load cell?

Next, these are the most important factors affecting accuracy, which will have a heavy influence over the load cell you choose. It’s important to understand how your application and the load cell will be affected by each of the factors, which include:

  • Mechanical – Dimensions and Mounting
  • Electrical – Output and Excitation
  • Environmental – Temperature and Moisture

One of the most important factors in choosing the right load cell is understanding how it will be mounted for testing or as a component within a design. There are a wide variety of mounting types including threaded connections, inline, through hole or even adhesive. Understanding the mounting type that suits your application is critical to getting the correct data because a poorly mounted load cell will distort the results and can damage the load cell.

The mounting process also requires you to understand which direction the load is coming from, in addition to any extraneous loads that may be present. The load cell mating surface is also an important factor. For example, when using our LowProfile® load cells without a pre-installed base, the best practice is to ensure that the mating surface is clean and flat to within a 0.0002-inch total indicator reading and is of suitable material, thickness, and hardness (Rc 30 or higher). Also make sure that bolts are torqued to the recommended level.

If you’re conducting a fatigue measurement, it’s also important to address the frequency and magnitude of load cycles with your load cell provider. Factors to address include single mode versus reverse cycles, deflection versus output resolution, and material types. Interface offers a wide variety of fatigue-rated load cells that are perfect for these types of applications.

Another consideration in choosing the right load cell is the electrical signal. Load cells work by converting force into an electrical signal. Therefore, it’s important to understand the electrical output type necessary for your application, which could include millivolt, voltage, current or digital output. You can find the excitation voltage data on our website for each of our load cells. Additional considerations include noise immunity, cable length and proper grounding.

The environment is also a critical factor in ensuring accurate performance of your load cell. Interface provides load cells in a variety of material types including aluminum, steel, and stainless steel. Each material has a variety of properties that make them more suitable for different environments. For a more in-depth perspective on the different strengths and weaknesses of materials, please read our blog titled, Considerations for Steel, Stainless Steel and Aluminum Load Cells. For applications where load cells need to be submerged in liquid or enter an explosive environment, we also have a variety of harsh environment and IP rated load cells, in addition to load cells suitable for high humidity or splash resistance. Learn more about our intrinsically safe load cells here.

Learn more about choosing the right load cell in these online resources:

WATCH: Load Cell Basics with Keith Skidmore

WATCH: How to Choose a Load Cell with Design Engineer Carlos Salamanca

READ: Load Cell Field Guide

VISIT: Interface Technical Library

To learn more about choosing the right load cell for any application, connect with our applications engineers about the force measurement needs for your next project at 480-948-5555.

Looking Forward to the New Year

2020 has been a challenging year for most. Despite the disruption every person and industry has faced this past year, Interface continues to keep our eyes on the future. What can we do to meet our customer’s needs today and tomorrow?

One of the ways that Interface has accomplished this intense focal point is by setting our sights on the increasing number of requests for specific drawings and engineered to order force, torque, and specialized product lines. These requests provide us unique visibility in evolving product development concepts and the overall T&M landscape for the customers we serve.

Through Interface’s continued investments in R&D, analyzing the marketplace, and anticipating customer needs, we’ve identified key trends that we believe will have a significant positive impact in meeting our customer’s current requirements and preparing for future demands.

The first trend we see today is our customer’s increased focus on miniaturization. Innovation using sensor technologies continues to shrink in dimensions yet require the same or greater level of precision performance. We see this in all types of devices and hand-held technologies, industrial automation, robotics, as well as in automotive and consumer product inventions.

The trend towards miniaturization in the medical device industry has been happening for quite some time; however, the capabilities and precision requirements continue to become more stringent and demanding of higher levels of performance. For example, with trends in at-home care remote monitoring has caused medical device manufacturers to develop mobility for devices. They’re also taking diagnostic machines and turning them into a handheld version so doctors can administer medical tests quickly and efficiently at the point of care, rather than sending patients out of home for administering or tracking.

To address product development challenges related to devices that becoming more compact, Interface is engineering and manufacturing precision components that excel in confined space. We’ve already begun to address this in 2020 with the release of our ultra-precision ConvexBT™ Load Button Load Cells.

ConvexBT is a first of its kind load button load cell, providing better temperature resistance and more enhanced eccentric load rejection than other competitive products. For many years, 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. Read more about the product release here.

The next key trend we are prepared to address in 2021 and beyond is test and measurement systems that address complex and multi-dimensional testing requirements. Historically, load cells and torque transducers have addressed force, torque, and weight in a variety of single product groups. And these devices would be added to a test series with a host of other sensors or get multiple outputs through a variety of tests. More and more customers want measurement devices that include additional outputs from the sensors to account for things like acceleration, temperature, humidity, and more.

We see this in growing requests for adding accelerometers in our load cells and torque transducers. In industries like automotive, we often use a torque transducer to measure rotation with components like car engines and tires. By adding an accelerometer, the device can be used to measure both torque and acceleration. This addresses our customer’s desire to simplify their test process and it provides more data in a single test to expand use cases and applications.  As this desire for more data and fewer testing devices grows, Interface is continuing the expansion of engineered to order and customized load cells to allow for dual measurements in a single component.

This trend is closely related to the need for a complete system. Systems are often completely customized to a single project or program that provides the measurement device, instrumentation, cabling, and housing. These systems can also be mobilized with wireless components for field applications.  The group of custom solutions engineers have created multiple versions of these systems used in energy, aerospace, and T&M labs.

Despite the challenges of the last year, Interface is hyper-focused on innovation and the ability to address the evolving needs of our customers. If you want to measure it, then Interface is here to help you create the right solution to get exactly what is required for your invention, product improvement or test. How can we help you win in 2021?

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.

Addressing Off-Axis Loads and Temperature Sensitive Applications

As technology progresses, one of the main differences we see over years and years of iteration on a wide range of consumer and commercial technology is miniaturization. There are hundreds of thousands of examples of advancing technology that went from the room-size of a mainframe decades ago, to a handheld device today. As technology grows more compact and convenient, the equipment used to design, test and manufacture these devices has to follow suit.

This is one of the driving factors for Interface to increase the product types and capabilities in our Load Button Load Cells and Interface Mini Load Cell products. Interface’s Load Button Load Cells are designed for customers who require the measurement of forces in a very confined space. They provide the most accuracy in as little space as possible. Diameters range from 1 inch to 3 inches, with heights from 0.39 inch to 1.5 inches.

For many years, load button load cells have been sensitive to off-axis, eccentric or misaligned loads. This means that if the load is not exactly perpendicular to the surface it’s resting on, the data could become skewed or inaccurate. All on-axis load generates some level, no matter how small, of off-axis extraneous components. This can cause a variety of challenges including slight inaccuracies and reduction of the load cells overload capacity.

With Interface’s family of load button load cells, we bring premium accuracy and repeatability, even under eccentric angular loading. The shaped load button has a spherical radius to help confine misaligned loads to the primary axis of the cell. Our design engineers and force measurement experts have purposefully tested applications under a wide variety of load conditions to ensure that the our load button series can continue to deliver premium performance. We have been extremely pleased with the results and continue to advance our offerings, including the soon to be released precision focused ConvexBT load button load cells.

The load button load cells’ size has in the past precluded the use of internal temperature compensation. We have redesigned our ultra precision product line of load buttons to ensure that this is no longer something the user has to account for when testing a product in certain environmental conditions.

Interface engineers have eliminated this issue by taking the technology out of the load button load cells cable and designing it back into the product. This ensures that temperature-sensitive applications do not suffer from errors caused by the load cell being exposed to different environmental conditions than the cable.

These new features open new possibilities to test compression force on a broader range of products and environments. To learn more about our ultra precision Load Cell Load Buttons and how it can make a difference in your design and testing process when dealing with tight and confined spaces, contact our Application Engineer experts here.

To see the complete line of Load Button Load Cells, visit here.  Watch for the release announcement of our new ultra precision ConvexBT product coming out this month.

Contributors:  James Richardson, Interface Mechanical Engineering Manager and Ted Larson, VP Product Management and Marketing

Source:  Interface Load Cell Field Guide