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Henry Zumbrun

Metrology

How Load Cell Stability Can Kill Your Uncertainty Budget

If something can be stressed, its reference standard stability must be considered

Published: Tuesday, August 14, 2018 - 12:03

Load cells are a combination of metal, strain gauges, glue, and more. Over time, fatigue ensures that there will be some instability in the system. Load cell stability or drift is usually assumed to be the amount of change in the entire cell system from one calibration cycle to the next. It is the relative standard uncertainty of a reference force transducer’s long-term instability. In an uncertainty budget, load cell drift can be referred to as either the reference standard instability or the reference standard stability.

Load cell stability can impact the following:
• Potentially consume your uncertainty budget
• Cause the force measuring device to be out of tolerance
• Cause all measurements between the last calibration and the current calibration to be recalled
• Raise the accuracy specification of the system

Calibrating load cells for more than 50 years, Morehouse has observed all kinds of instabilities from different manufacturers. Most load cells we see are categorized as either general purpose or those calibrated in accordance with more stringent standards, such as ASTM E74 or ISO 376. Here, we’re going to discuss general-purpose load cells and their typical instability characteristics. We’ll start with the general uncertainty contributors, then progress to what we normally observe and give recommendations for improvement.

The systems are each broken down into good, common, and bad. Systems that fall into the good category are usually those by reputable manufacturers that understand load cells and indicating systems. The common category might consist of suboptimal combinations, such as an excellent load cell and a middling indicator, or an excellent indicator and an OK load cell. In the bad category, one or both components are probably not suitable for the end user’s overall uncertainty needs.

Typically, general-purpose load cells are more inexpensive and are paired with indicating systems that also contribute toward drift. The requests we see on these systems are generally for a 10-pt. calibration. The accuracy specifications are usually 0.05 percent to 1 percent of full scale.


Figure 1: Typical instability numbers for various load cells

The long-term instability of the reference force transducer is determined either from previous calibrations or by estimations of similar systems until the actual values can be obtained. Figure 1 above shows the instability Morehouse typically observes on general-purpose load cells.

Next, we’ll discuss calculating expanded uncertainty and how reference standard stability (or instability) affects overall expanded uncertainty.

General-purpose load cells

Typical contributions for the CMC uncertainty of general-purpose load cells fall into two categories, as listed below.

Type A uncertainty contributions
1. Non-repeatability
2. Repeatability or non-repeatability of the reference standard
3. Repeatability of the best existing device (and technician)
4. Repeatability and reproducibility

Type B uncertainty contributions
5. Resolution of the best existing device
6. Reference standard resolution, if applicable*
7. Reference standard uncertainty
8. Reference standard stability (our topic in this article)
9. Environmental factors
10. Other error sources
11. Specified tolerance, if not listed, and you’re making ascending measurements only*
12. Hysteresis, only if the device is used to measure decreasing forces, and static error band (SEB) wasn’t used*

*Note: If the device is going to be used at points different from the points it was calibrated at, then SEB, nonlinearity, or hysteresis may must be used.

If making ascending and descending measurements, use static error band (SEB) or a combination of nonlinearity and hysteresis. If the force-measuring device is calibrated with an indicator and set up to have a tolerance, then it may not be necessary to include nonlinearity or SEB.

All uncertainty contributions should be combined, and the Welch-Satterthwaite equation should be used to determine the effective degrees of freedom for the appropriate coverage factor for a 95-percent confidence interval.


Figure 2: Expanded uncertainty budget with 0.2-percent instability

With general-purpose load cells, it is common to observe systems with accuracy specifications lower than the instability observed from one calibration to the next. If the accuracy requirement is for 0.1 percent of full scale, and the instability from one calibration to the next is 0.2 percent, it becomes nearly impossible to claim 0.1-percent accuracy as your tolerance. Figure 2 shows the uncertainty of 0.2-percent instability on a 10,000 lbf load cell. This accounts for approximately 95.90 percent of the uncertainty contribution.

When accounting for reference standard stability in an uncertainty budget, stability can be treated as type A or B. Most calibration laboratories claim instability as a type B uncertainty contributor with a rectangular distribution. This means that instability of 0.2 percent would be divided √3 (or 1.732), which is about 0.115 percent.

Now let’s think about that. A calibration laboratory is claiming 0.1-percent accuracy on its scope, and its device’s instability is accounting for 115 percent of the accuracy statement alone. I guess this is a case of accounting for more than the 100-percent allowable. The solution to this problem is often simple: Either shorten the calibration frequency, or purchase better equipment. This could mean upgrading the indicator, load cell, or both.

Now, let’s assume the end user decided it would be much less expensive to buy a better load cell than to shorten the calibration interval. A year after the purchase, the reference standard stability is observed to be 0.05 percent, or 5 lbf, on a 10,000 lbf load cell.


Figure 3: Expanded uncertainty budget with 0.05-percent instability

In this example, shown in figure 3, the reference standard stability still is the largest contribution to the expanded uncertainty. However, the end-user can now actually claim 0.1 percent of full scale and have a bit of room to maintain the accuracy from one calibration to the next. In fact, the instability can go as high as 0.077 percent, and they could still be within the 0.1 percent of full scale.

For general-purpose calibration, Morehouse recommends our Calibration Grade Load Cells and either a PSD battery powered or Morehouse/Admet High Stability Indicator.


Figure 4: Morehouse calibration-grade load cells

The Morehouse calibration-grade load cell with PSD hand-held indicator will typically maintain an accuracy of 0.1 percent of full scale year over year, with instability accounting for about 0.05 percent of overall accuracy. If an accuracy of 0.05 percent or better is required, then we recommend a different meter, most likely either the Gauge Buster 2 or the Morehouse 4215.

The purpose of this article is to show the effect of stability on a load cell system and how that should impact your decisions when purchasing a load cell system. If stability is bad, the system will probably not meet your accuracy requirements. In general, I would say repeatability, reproducibility, and stability are the most important characteristics when evaluating a load cell system. Adapters play a huge role in actual results, and careful attention to purchasing the right adapters must also be considered. For more information on the proper adapters, Morehouse has a technical paper that can be downloaded here.

Conclusion
If I can stress anything, its that reference standard stability must be considered in any uncertainty budget, and overall system accuracy should be adjusted accordingly. Manufacturers often highlight short-term accuracy and discount stability from their accuracy specifications. We know that instability largely depends on how equipment is used, the number of load cycles, age of the material, the strain gauge bond, electronics, and more. Instability is a number best quantified when comparing one calibration against another with all other things being equal. Load cell stability is often overlooked and should not be discounted by the end-user when considering system accuracy.

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About The Author

Henry Zumbrun’s picture

Henry Zumbrun

Henry Zumbrun is president of Morehouse Instrument Co. where he has managed the force and torque calibration lab and services in the family-owned business since the 1990s. Morehouse helps labs lower their force measurement uncertainties and torque, resultin in more accurate measurements, which lowers costs, reduces risk, and increases quality. Morehouse designs and manufactures products in line with customer requirements, lean, Six Sigma, and best practices guidelines.