Over the years, many of us at Morehouse have been asked what value one should use for Load Cell Reliability when a system is new.
You may think: “I need a load cell reliability number for my uncertainty budget. What should I use?”
The answer is variable as it depends on several factors.
One of the critical factors is figuring out how stable someone needs the load cell system to be to meet their measurement uncertainty requirements.
Is 89 % EOPR acceptable with 95 % Confidence, or is 95 % End of Period Reliability (EOPR) the goal?
The other reaction we often receive is, “No one does that because there are too many variables.”
Load cell reliability will depend on the complete system and its use.
The use would include anything that could influence the results.
Force is mechanical things such as using different adapters, different cables, changing thread engagement, overloading the load cell, the meter used, and the number of loading cycles.
So, after being asked numerous times, we decided to tackle the question, "What should I expect for stability with year-to-year annual calibrations?"
We started by finding enough samples to meet the 95 % Confidence Interval criteria, with 95 % End of Period Reliability, which seemed daunting.
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Force Calibration for Technicians eBook 2022 Edition, available for free by filling in the website form, 60 % more information on measurement uncertainty, calibration coefficients, and best measurement practices for force. Also contains guidance on factors that ensure repeatable results through the entire measurement chain. For force-measuring devices, there are various mechanical and electrical interfaces that matter. At the time of calibration, these consist of:
Force Calibration for Technicians Version One is also available on Amazon.
Recommended Compression and Tension Adapters for Force Calibration: We buy a great force standard, we demonstrate metrological traceability, & we make a force measurement. Without the proper compression and tension adapters, the errors can be over 10 times that of the specification. This article discusses the importance of compression and tension adapters and provides solutions for better calibrations using the appropriate compression and tension adapters. For more information on adapters, click here.
Portable Force Machine Solves Several Measurement Management Headaches. This paper examines several “bad” measurement practices, such as using multiple technicians to calibrate instruments (including hand-held dynamometers). Such practices include using a free-floating cable and stacking weights to calibrate a cable tensiometer. We discuss how a portable force calibration machine can be used to improve efficiency and solve several measurement challenges. Click Here to read our portable force calibration machine paper.
Measurement Bias: What can happen when we use an accuracy specification and assume all the measurements are centered to the specification limits?
Not correcting for Measurement Bias is a typical problem in the metrology community, where many papers assume a centered process or Measurement and that any known Measurement Bias has been corrected.
The Morehouse technical paper on Measurement Bias examines what happens if we fail to correct Measurement Bias at the bench level and gives examples based using Specific Risk. Specific Risk is the probability that an accepted item is non-conforming or that a rejected item does conform. This risk is based on measurements of a single item.
In general terms, Specific Risk is dependent on a single probability function and can be referred to as Probability of Conformance from the customer’s point of view.
Not correcting for Measurement Bias matters because when a known bias is ignored, meaning not corrected or not included in the Statement of Measurement Uncertainty on the Calibration Certificate, measurement traceability may not be fully achieved, and all subsequent measurements are suspect.
Metrology and the Consequences of Bad Measurement Decisions by Scott Mimbs
This paper was initially published in Test Magazine and is being shared as one of the essential documents we believe anyone making measurements should read.
In the simplest terms, measurements are made to support decisions.
Measurement data support decisions to:
• Establish research or investigative fact;
• Establish scientific or legal facts;
• Accept or reject a product;
• Rework or complete a design;
• Take corrective action or withhold it;
• Continue or stop a process (including a space launch).
The measurement is unnecessary if the data from measurements are not used in decision-making or establishing facts (including scientific research).
# Measurement Decisions
Scott Mimbs has written many papers, and one of our favorites discussing the importance of definitions can be found here.
Want to read more on Decision Rules? Check out our blog.
The new dimension to resolution paper. This paper stresses the importance of considering the unit under test resolution in a measurement uncertainty budget. Download our new dimension to resolution paper here.
Converting a mV/V Load Cell Signal into Engineering Units is a common need in the industry. This paper discusses the pros and cons of how meters handle Converting a mV/V Load Cell Signal into Engineering Units. The paper discusses significant error sources and best practices for using an indicator with a mV/V signal, and we offer guidance on some indicators that can convert that signal into engineering units.
A paper on the Common Measurement Errors in Weighing. Anyone calibrating or using crane scales, load cells, dynamometers, truck scales, and aircraft scales will benefit from learning about the various common errors in weighing. Download our Common Errors in Weighing paper here.
Why a 4 to 1 TUR is Not Enough: The Importance of Analyzing the Probability of False Accept Risk: Several organizations and publications reference or insist on maintaining a 4 to 1 TUR (Test Uncertainty Ratio) without understanding the level of risk that they may be subjecting themselves to. The general thought is if the lab performing the calibrations has standards at least four times better than what they are calibrating, everything is good. This paper discusses TUR, PFA Risk, and why the location of the measurement matters. We will discuss two managed risk guard banding methods (5 & 6) found in the ANSI/NCSL Z540.3 Handbook. We will show that a 4 to 1 TUR is not enough and can result in a 50 % risk. Check out our blog post on TUR here.
An Introduction to the Differences Between ASTM E74 and ISO 376, the Two Most Recognized Force Standards: The differences Between ASTM E74 and ISO 376 can be confusing. Morehouse has been performing both ASTM E74 and ISO 376 calibrations for more than fifteen years. We have been calibrating in accordance with the ASTM E74 standard since its introduction in 1974 and performing ISO 376 calibrations since sometime in early 2000. Until recently, we assumed that the rest of the world and the force community knew that the standards were completely different and that either standard could not be substituted for another. This paper explains those differences in more detail.
What is measurement risk? Measurement decision risk is the probability that an incorrect decision will result from a measurement. Are you telling your customers instrument passes without considering measurement uncertainty? If taken to court, are your measurement defensible? This paper examines the proper way to make statements of compliance. Download our Measurement Risk paper here.
Have trouble understanding Uncertainty Propagation for Force measurement and how to put together a budget? This paper examines all of the components required to put together a full calibration and measurement capability (CMC) reviewed by Accreditation Bodies for your scope. This Uncertainty Propagation guide to calculating force measurement uncertainties & was published in Cal Lab magazine. Download our Uncertainty Propagation for Force paper here.
Analyzing the Effects of Reducing the Ending Zero vs Ignoring the Trailing Zero on Measurement Instruments Used for Force Calibration (2014): ASTM E74 Method A and Method B—for the treatment of reducing the ending zero. Method A defines the deflection calculation as the difference between the deflection at an applied force and the initial reading at zero force. The article was written by Henry Zumbrun for Cal Lab Magazine 2014.
Load cell dual-range calibration errors happen when a load cell is not loaded to full capacity. The molecules inside the load cell do not react potentially causing an increased error. What you need to know about molecule excitement decline and dual-range calibrations. Article from Test Magazine May 2016 issue.
Potential Measurement Errors in Force Calibration: This article covers the most common errors one must know when making calibrating force instrumentation.
Potential Measurement Errors in Force Calibration in published test magazine from Oct-Nov 2015 issue.
3 Bar vs 2 Bar Universal Calibrating Machines Comparison Test: This paper describes the difference between these two designs. The paper compares the repeatability and reproducibility characteristics and examines if there is a significant difference in the performance and measurement uncertainty between the two designs.
The Design, Development, and Commissioning of a 2kNm Torque Standard Machine for Highly Accurate Torque Calibrations: This is a 2007 article detailing the world's second-most accurate torque machine at Morehouse. Here at Morehouse, we provide torque calibration from 1Nm to 2kNm. For more details on our torque calibration service, click here.
Aircraft Scale Calibration and Measurement Capability; Scale Calibration for several aircraft scales requires one to replicate the footprint of the tire. Not doing so may produce out-of-tolerance conditions or a very high bias. Information on our aircraft scale calibration services can be found here.
This paper will describe our calibration process, including the calculations of our Calibration and Measurement Capability (CMC), as it appears on our scope of accreditation.