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I’ve Got a Load Cell – Now What? Episodes 5 and 6

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Reviewing the test and measurement video I’ve Got a Load Cell – Now What?, today we are highlighting Episodes 5 and 6 in the favored series.

These two installments highlight instrumentation and installation tips with best practice recommendations for the most popular load cell, the 1200 LowProfile, in addition to the 1100 LowProfile, SM S-Type and WMC Stainless Steel Miniature Load Cell products.

Once you have selected measurement device, it is equally important to review your instrumentation options. You will need a way of interpreting the output or the voltage from the load cell. This requires connecting to some type of instrumentation. If you already own an instrumentation device, you need to ensure that it is useable with any new force measurement device.

Interface offers a large line of instrumentation, from simple indicators to more complex multi-channel data acquisition systems used with multi-axis sensors, load cells and torque transducers. In addition to watching the video, be sure to check out our Instrumentation Selection Guide.

If you are selecting new instrumentation, there are a few questions to help qualify what will work best. For example, do you need a simple device to read analog outputs or advanced wireless instrumentation used with calibration-grade equipment? What type of software will you need to analyze the output data? Will you be connecting the instrumentation to a computer? As discussed in, I’ve Got a Load Cell Episode 5, here are some basic considerations for instrumentation.

Instrumentation Selection

Step 1: Electrical Wiring – Review the electrical wiring diagrams (also available on each product page). If you need to review the wiring based on engineered-to-order or custom options, contact our application engineers.

Step 2:  Signal Leads – Are you using a 4 or 6-wire configuration? A 6-wire lead provides sense. Some instrumentation options will support 6-wire signal leads. Most applications allow you to just pair with excitation.

Step 3: Cables – Identify the type of cable required with the right number of leads. For example, our standard 6-wire cable will support the sense function. We recommend shielded conductors when you order your cables.

Step 4: Grounding – Avoid ground loops in any wiring.

Step 5: Excitation Voltage – Review the sensor’s voltage of excitation on the datasheet to identify the load cell calibration. We recommend to properly match with the instrumentation’s capabilities in voltage with the load cell.

Step 6: Set-Up – Once you connect your instrumentation, you need to scale it. Review your options in the installation instructions. Some instrumentation options will take an mV entry and others will require a known load or shunt calibration. Reference your device’s calibration certificate.

In the I’ve Got a Load Cell, Now What? series Episode 6, we highlight some important diagnostics, installation tips and best practices. This video highlights examples of installation for some of our most popular load cells.

During the selection of your load cell, which usually begins with capacity and capabilities requirements, it is important to consider performance influencing factors and installation.

Use Case Considerations

  • Environmental Considerations and Exposure
  • Fatigue or Non-Fatigue
  • Overload Protection Needs
  • Off-Axis Loads
  • Dimensions
  • Mounting and Base Requirements
  • Output
  • Installation Set-Up

If you already have an older load cell, we recommend doing some back load cell health checks before putting it back into use. How can you evaluate the health of your load cell? You can do some simple diagnostics with load cells. Here are three recommended diagnostics:

  1. Zero Balance Check – If the load cell has been in use for some time, it is good to determine if there has been any potential overload or damage to the load cell. If the zero balance has shifted up or down from the nominal, that is an indication the load cell should be evaluated or repaired prior to using it in test applications.
  2. RCal or Shunt Calibration Check – Any shift from the specifications or certificate requires further evaluation.
  3. Bridge Evaluation – Some instrumentation will allow you to do a bridge evaluation. If you have any concerns on the performance or accuracy of the output, contact Interface to discuss tools that you can use to evaluate the bridge of the load cell.

The next section of Episode 6 highlights the 1200 LowProfile features, installation tips, and mounting options for assembly.

Mechanical Installation Tips

Once you have your load cell hooked up and your instrumentation scale, it is time to put it to work. You want to review the attachment, including thread engagement and mounting. If you are threading into the live end, the center hub on the low profile load cell, make sure you have enough threads engaging into the load cell itself. We recommend that you thread it into where the studs bottom out, then back up a half a turn. Next step is to preload tension load by about 130% of capacity and jam the jam nut. We want to ensure that we do not have any repeatability issues due to thread engagement of the flexure. If you cannot provide a preload, reference the torque values in the installation guide.

Mounting to Base, Structure or Plate Tips

  • Use Grade 8 or Better Hardware
  • Mount to Total Flatness of .002″
  • R30-33 Hardness Scale
  • Follow the star patter to proper torque values
  • Preinstalled Bases

Interface bases help in making integration to any assembly much easier. Interface manufactures bases with the same high-performance materials and specifications for hardness and flatness as our load cells. The bases offer threaded holes, which make it easier for mechanics like hydraulic actuators.

Brian Peters and Elliot Speidell continue to detail the assembly and best practices for three additional products. You can watch more installation tips and recommendations related to the 1100 Model Ultra Precision Compression Only LowProfile Load Cells, SM S-Type Tension and Compression Load Cells and our popular WMC Sealed Stainless Steel Miniature Load Cell.

Interface publishes all our installation guides, diagrams and resources for support online. Go to our support resources for additional help.

Be sure to subscribe to our YouTube channel to get the latest videos on products, applications and user tips.

Load Cell Basics Technical Q&A Part One

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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