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Load Cell Basics Technical Q&A Part Two

Interface hosted a series of ForceLeaders webinars this past year covering topics that included multi-axis sensors, instrumentation, torque transducers, custom solutions and more.  We plan to continue this popular series into the new year, based on the feedback we have received about topics of interest and support requirements.

One of the features that we like to include in our events is answering a list of top questions, as well as questions we get from the participants.  In our event, Load Cell Basics, we covered a lot of interesting inquiries as well as the top questions we get about our precision load cell products.

We highlighted the first group of questions in our recent post, Load Cell Basics Technical Q&A Part One. And there is more, below highlights additional questions we addressed during the live event.   A recap of the webinar is also online, reviewing key topics we discussed during the one-hour learning event with force measurement experts Keith Skidmore and Brian Peters.

Is There a Better Way to Tare a Load Cell?

Tare is defined as a deduction from the gross weight of a substance and its container made in allowance for the weight of the container, which is sometimes also called the vessel.  For purposes of this reference, we are noting the container is the actual body of the load cell.

Whether you tare a load cell by electronics or mechanical, preload taring of the system is typically done through instrumentation.  You want to ensure that whatever mechanical tare load you have on the sensor isn’t going to impede on your usable capacity range. As far as the load cell is concerned, there really is no need to accommodate any sort of mechanical tare. These are linear devices if you stay within the range.  On the mechanical preload side, it is important how you preload some of attachments especially for the LowProfile load cells. We recommend to preload if you can. It is what we do for best results so that the load path doesn’t change through the threads, which can affect very small amounts of linearity hysteresis

Does Frequency Measurement Limit Impact How Fast the Load Cell Responds?

This is a function of the mechanical flexure design for that sensor. It is important to note that each sensor is a mechanical spring. Depending on the stiffness or the deflection characteristics of a load cell, you will have different natural frequency characteristics. This information is typically documented on our product data sheets by model and configuration.  Generally, the stiffer the sensor the better it’s going to perform in cyclic applications.

Adding to this, Keith notes that load cells are great at hundreds of hertz. If you are asking about tens of thousands of hertz, they are not. There is a spot where very high frequencies of these standard load cells aren’t as effective. Most of our typical testing customers who are applying cyclical loads to a test specimen will confirm that low profile or strain gage-based load cells are great for their frequency requirements.  Work with our application engineers if you have a frequency requirement that is extremely high.

Tare and Working Up an Error Budget

This question comes in from an engineer that is working up a calculated error budget that has a mechanical tare load on that system already. Before you start your test, if you tare it how does that factor into your initial preload in the system? Will it impact your error budget? The answer is that yes, it can have some impact. You can zero out your temperature effect on zero; however, if you have a lot of preloads, you can’t. You must go back to zero to zero it out. This can impact your error budget. Basically, if you zero out, it eliminates the temperature effect. You cannot make the same assumption in your budget if you have considerable preloads.

Does Interface Recommend Particular Amplifier Instrumentation Products?

We do have a wide variety of amplifier solutions. They range from small embeddable amplifiers up to inline options that are DIN rail mounted.  We also have various digital output instrumentation productions available for ethernet, ether cat, Modbus, and USB.

It is important in your consideration to understand that there are errors that can impact instrumentation performance. Selecting the right instrumentation should be a function of the application and what are the changes in testing environment. Resolution is critical in your decision if you are doing a digital type conversion. There’s a lot of parameters to consider. Our recommendation would be to look at what is available from Interface and contact our application engineers to review your specific requirements.

What is the Most Frequent Problem When Installing a Load Cell?

One of the issues are asked about is usually related to the mechanical installation. It’s not as simple as just a nut, bolt, and threaded connection. There is a lot more to it and it’s often and discounted how important it is to ensure things are aligned properly so that you don’t have any parallel load paths. All the load must go through the load cell to make a proper measurement. For example, if you have a mounting plate and the plates are riding on the heads of the screws of the load cell, it is going to shunt the load. Mechanical installation is often missed in the test set-up design. It is often not given enough weight in importance of your test.

We also often see that users have either inadequate torque in an assembly or perhaps too much torque for the lower capacity load cells.  It’s advised to be careful of the installation torque. We recommend that if you aren’t familiar with mechanical installation, ask us for our detailed installation instructions. We provide guidance in terms of recommended torque values and proper methods of where to attach the sensor. It’s easy to over torque a load cell, as they are designed to see axial loads. We have seen users people crank on things and transmitting too much torsional load through creating performance issues that might not show up right away.

Do Cables and Amplifiers Affect Results of Load Cell Calibration?

The answer is absolutely it can impact your results. It doesn’t mean it will, but it can affect performance and measurement accuracy. For example, if you change the cable length or have an instrument that is a different temperature with cable running out of an oven into cooler temperature room, these things can influence the measurement. Even if the load cell is at a constant temperature but the cable is changing temperature, it will cause a signal to change. That’s not something you want in your measurement.

You may mitigate temperature by shielding the cable or you could have a fatter cable which could have less of an effect. Some of these things can be calibrated out. If you add a long cable and then apply loads to your load cell, it considers that there is a longer cable, and you can completely calibrate out the effect of the cable length on the signal dropping due to the resistance. Some instruments use sense to measure the voltage at the load cell. In that case, it is insensitive to any cable effect. You can change cable lengths and it doesn’t make any difference.

Is a 10kn (2,250) lb. Force Application Too Much for a 2,000 lb. Load Cell?

Yes, it’s above the calibrated range and thus you can’t guarantee performance. Load cells will typically stay linear over capacity but there’s no guarantee on the accuracy. This is especially true if you overload with frequency. You’re starting to exercise the load cell in areas where we’d recommend you stay away from to keep the results accurate. Oftentimes, you’re better off going with a larger sensor. Pay attention to the actual output. That’s one advantage of most of our products, especially in the LowProfile range.

Is There Maximum Sampling Frequency for Strain Gage Load Cells?

It is going to depend somewhat on the model of the load cell. Hundreds of hundreds of hertz are certainly adequate. If you start getting close to the natural frequency posted on our product data sheet, we recommend you review the application with our application engineers.

We know engineers who use load cells for safety impact testing. For example, you might need to sample 10 kilohertz. The load cell doesn’t make a difference for how fast you sample the signal, but those cells can only respond in typically a millisecond response time. There are no moving parts or active circuitry; however, typically testing is going to test something until it fails. It is suggested to sample it at a couple thousand hertz to make sure you capture that exact point where the system fails or spikes. If you do have some concern over creep error that’s introduced, record what that non-zero value is as soon as you release that load. That is an accurate measurement of how much creep error was introduced being symmetric.

We exercise our load cells before calibration. It’s relatively quick and typically done in hydraulic frames which are automated to test where they cycle the load in short cycle intervals, like 30 seconds. It’s really the magnitude of the exercising, not how long it lasts. If load is being tested at 120% for an hour, that’s no better than 10 seconds, other than it introduced a lot of creep for an hour. Exercising means you must wait longer for the creep to return, so if the load is a fast cycle, you can check the zero quickly.

If you have additional questions for our experts, we recommend that you visit our technical support resources. We have commonly asked questions, installation guides and manuals and more.  We are also here to support you. Contact our technical support team if you need assistance.

Additional Resources

 

Load Cell Field Guide

Load Cell Basics Sensor Specifications

Load Cell Basics Technical Q&A Part One

Interface 1200 Precision LowProfile Load Cell Series Product Highlight

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