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Load Cell Sensitivity 101

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Load cell sensitivity refers to the relationship between the input force applied to a load cell and the output signal it generates. It is a measure of the load cell’s responsiveness to changes in the applied force and is expressed in units of mV/V (millivolts per volt) or micro-volts per volt.

When determining the accuracy of a load cell, load cell sensitivity is an important parameter. A higher sensitivity means that even small changes in the applied force will result in a larger change in the output signal, making the load cell more sensitive and accurate.

It is critical to understand that load cell sensitivity and accuracy are closely related. A high sensitivity load cell will generate a larger output signal for the same applied force, which can increase the accuracy of the measurement. In general, the accuracy of a load cell is a combination of its sensitivity and the quality of its design and construction. Interface specializes in precision accuracy, which is important when considering the use case for your load cell.  As defined by the specifications, a high-quality load cell with appropriate sensitivity will provide accurate and consistent measurements, while a load cell with low sensitivity or poor quality may provide less accurate measurements.

Most load cells are designed to measure force in one certain direction, which is determined by the way the load cell is mounted. Inappropriate loading will cause side and eccentric load, which risks reducing the life of load cells and distorting measurement results.

Eccentric load sensitivity is measured by eccentric load, which is any load applied parallel to but not concentric with the primary axis. Side load is any load at the point of axial load application at 90 degrees to the primary axis.

To achieve a desired level of accuracy, it is important to choose a load cell with the appropriate sensitivity for the application. Load cell sensitivity can be affected by factors such as temperature, temperature gradients, and environmental conditions, so it is important to take these factors into account when selecting a load cell.

What conditions impact load cell sensitivity? Load cell sensitivity can be impacted by several factors, including:

  • Temperature: Changes in temperature can cause thermal expansion or contraction of the load cell material, affecting the output signal and reducing accuracy.
  • Temperature gradients: The presence of temperature gradients within the load cell can cause differential expansion or contraction of different parts of the load cell, further affecting the output signal and reducing accuracy.
  • Environmental conditions: Exposure to harsh environments, such as moisture, vibration, and shock, can cause damage or degradation to the load cell, reducing its sensitivity and accuracy.
  • Load cell orientation: The orientation of the load cell can impact the output signal, especially in applications where the load is applied at an angle.
  • Mechanical stresses: The presence of mechanical stresses, such as bending or twisting, can affect the output signal and reduce accuracy.
  • Aging: Over time, the load cell may experience degradation or wear and tear, reducing its sensitivity and accuracy. This is where regular calibration plays a role in the lifetime of your load cell.

It is important to consider these factors when selecting a load cell and to properly maintain and calibrate the load cell to ensure optimal sensitivity and accuracy over time.

Calibration is a process that involves adjusting the output signal of a load cell to ensure that it accurately reflects the applied force. Calibration improves load cell sensitivity by correcting for any errors or inaccuracies in the output signal, ensuring that the load cell provides accurate and consistent readings over time.

During calibration, a series of known loads are applied to the load cell, and the corresponding output signals are measured. These measurements are used to create a calibration curve that represents the relationship between the applied force and the output signal.

Calibration helps to correct for various factors that can affect load cell sensitivity, such as temperature, environmental conditions, and mechanical stresses. By adjusting the output signal to accurately reflect the applied force, calibration helps to ensure that the load cell provides accurate and consistent readings, even in challenging conditions. Interface recommends calibration of every load cell at least once a year for regular sensitivity maintenance.

Interface engineers design high accuracy, quality load cells with appropriate sensitivity that provides accurate and consistent measurements. It is important to consider both sensitivity and accuracy when selecting a load cell for an application and to regularly calibrate the load cell to ensure that it continues to provide accurate and reliable measurements over time.

Load Cell Basics Sensor Specifications

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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.

 

The Five Critical Factors of Load Cell Quality

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

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

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

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

Five Most Critical Factors of Load Cell Quality

#1 Repeatability

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

#2 Longevity

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

#3 Accuracy

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

#4 Sensitivity to Off-Axis Loads

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

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

#5 Access to Prominent Force Measurement Experts

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

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

Contributor:  Ken Vining, Chief Engineer and Head of Quality