Measurement Uncertainty | Metrology Matters

Summary

This script delves into the intricacies of measuring an aluminum part's diameter, highlighting the challenges of achieving exactness due to inherent measurement uncertainty. It explains the concepts of repeatability and reproducibility through GR&R studies, emphasizing the difference between precision and accuracy. The importance of adhering to standards, such as ASME B89.7, is underscored to manage uncertainty effectively. The script concludes with a practical example of how environmental factors and handling can significantly impact measurement outcomes, advocating for a deeper understanding of these uncertainties.

Takeaways

• 📏 The script discusses the challenge of measuring the exact diameter of an aluminum part due to inherent measurement uncertainty.
• 🔍 Two different tools provided measurements with different levels of precision, showcasing the variability in measurement results.
• 🤔 It's impossible to ascertain the true exact value of a measurement due to the limitations of measurement methods and tools.
• 🔬 The concept of measurement uncertainty is introduced as the gap between the true value and the measured value.
• 📈 The GR&R (Gauge Repeatability and Reproducibility) study is mentioned as a method to estimate measurement repeatability but not accuracy.
• 🎯 The goal of measurement is to achieve both high accuracy (closeness to the true value) and precision (consistency of repeated measurements).
• 🛠️ Sophisticated technology can help in reducing measurement uncertainty by allowing for more precise measurements.
• 📚 The importance of following strict standards in metrology is emphasized for managing uncertainty effectively.
• 📋 Standards like ASME B89.7 provide guidance on managing uncertainty, including measurement planning and estimating risks.
• 🔄 Factors contributing to measurement uncertainty include the environment, equipment, setup, and the object being measured, among others.
• 🧠 Understanding the sources of uncertainty is crucial for having confidence in the accuracy of reported measurements.

Q & A

What is the main challenge in measuring the diameter of an aluminum part accurately?

The main challenge is that no measurement method can find the exact true value due to the inherent measurement uncertainty. Even with precise tools, there's always room for even closer approximation.

What is meant by 'measurement uncertainty'?

Measurement uncertainty refers to the dispersion of measured values around the actual measure. It's the gap between the true value of a dimension and the measured value that can be obtained.

What does GR&R stand for and what is its purpose in measurement?

GR&R stands for Gauge Repeatability and Reproducibility. It is a study where different operators measure the same parts repeatedly to estimate the repeatability of the measurement process, but it doesn't necessarily indicate accuracy.

Why is it important to have both accuracy and precision in measurements?

Accuracy ensures that measurements are close to the true value, while precision ensures that repeated measurements are consistent and close together. Both are necessary to reduce measurement uncertainty.

How does technology contribute to reducing measurement uncertainty?

Sophisticated technology allows for more precise measurements, generally meaning that instruments capable of measuring smaller units are likely to have less deviation from the correct value.

What role do standards play in managing measurement uncertainty?

Standards, such as those created by the American Society of Mechanical Engineers, provide guidance for managing uncertainty by addressing measurement planning, estimating uncertainty, and traceability, among other considerations.

What are some factors that can cause measurement uncertainty?

Factors include the environment, reference elements, measurement equipment, setup, software, calculations, the object being measured, how the target characteristic is defined, and physical constants, among others.

Why is it useful to be certain about measurement uncertainty?

Being certain about measurement uncertainty gives more confidence in the accuracy of reported measurements, as it provides a concrete understanding of how and why measurements may be inaccurate.

How does the material's property, such as aluminum's expansion rate, affect measurement uncertainty?

Material properties like expansion can significantly affect measurement uncertainty. For example, aluminum's expansion rate can change its size with temperature, potentially increasing the uncertainty if not accounted for.

What additional measurement considerations are mentioned in the script that could affect uncertainty?

The script mentions the need to consider the object's roundness and the importance of reviewing the measurement process, which can also contribute to uncertainty if not properly addressed.

How does the script illustrate the concept of repeatability versus accuracy?

The script illustrates this by showing that even if a measurement method is highly repeatable (consistently hitting the same value), it can still be inaccurate if those values are not close to the true value.

Outlines

00:00

📏 Measurement Uncertainty and Precision

The script discusses the concept of measurement uncertainty, emphasizing that no measurement method can determine the exact true value of a physical object's dimension. It explains the difference between repeatability and accuracy, highlighting that high precision does not guarantee accuracy. The importance of gauge repeatability and reproducibility (GR&R) studies is mentioned as a way to estimate measurement uncertainty. The script also touches on the role of standards in managing uncertainty, referencing the ASME B89.7 standards for guidance.

Keywords

💡Diameter

Diameter refers to the length of a straight line that passes through the center of a circle or any other shape with rotational symmetry, reaching from one side to the opposite side. In the video, measuring the diameter of an aluminum part is the central activity, highlighting the challenges in obtaining an exact measurement due to inherent uncertainties in the process.

💡Measurement Uncertainty

Measurement uncertainty is the doubt or inexactness associated with a measured value. It is a fundamental concept in metrology, the science of measurement. The video emphasizes that no measurement can be perfectly accurate, and the gap between the true value and the measured value represents this uncertainty. The script discusses how uncertainty can be quantified and minimized through various means.

💡True Value

True value, in the context of measurement, is the actual size or quantity of a physical attribute that one is trying to determine. The video script mentions that while every object has an actual size, finding this exact true value is impossible due to measurement limitations and uncertainties.

💡Repeatability

Repeatability in measurement refers to the degree to which the same results are obtained when the same measurement is repeated under the same conditions. The video script uses the term to describe the consistency of measurements, noting that high repeatability does not necessarily equate to accuracy.

💡Reproducibility

Reproducibility is the ability of a measurement to be duplicated by different instruments, observers, or laboratories. The script mentions a gauge repeatability and reproducibility (GR&R) study, which is a method to estimate the measurement uncertainty by having different operators measure the same parts multiple times.

💡Precision

Precision in measurement is the closeness of agreement between independent measurements. The video script explains that precision is about being consistently close to the same measurement, but it does not guarantee that the measurements are close to the true value. It contrasts precision with accuracy, emphasizing the need for both to reduce measurement uncertainty.

💡Accuracy

Accuracy is the degree to which a measured value approaches the true value. The video script discusses the importance of accuracy in conjunction with precision, stating that while precision ensures consistent measurements, accuracy ensures those measurements are close to the true value.

💡Metrology

Metrology is the scientific study of measurement and its principles. The video script mentions metrology in the context of following strict standards to manage measurement uncertainty, emphasizing its importance in ensuring the reliability and traceability of measurements.

💡American Society of Mechanical Engineers (ASME)

ASME is a professional organization that develops various standards and codes to enhance the practice of mechanical engineering. The video script refers to ASME's B89.7 standards, which provide guidelines for managing measurement uncertainty, including planning, estimating, and decision-making processes.

💡Expansion

Expansion in the context of the video refers to the change in size of a material in response to temperature changes. The script uses the example of aluminum, which expands at a rate of 25 parts per million per degree Fahrenheit, to illustrate how environmental factors can significantly affect measurement outcomes and contribute to uncertainty.

💡Roundness

Roundness is a measure of how closely a two-dimensional figure approximates a circle. The video script briefly mentions the need to test the roundness of the aluminum part, indicating that the quality of a measurement can be influenced by the geometric properties of the object being measured.

Highlights

Measurement of an aluminum part's diameter with a specified uncertainty of ±408 micro inches.

Verification of the measurement using a different tool yielding a different result.

The impossibility of achieving an exact true value in measurements due to inherent uncertainties.

The concept of measurement uncertainty as the gap between the true value and the measured value.

Gauge repeatability and reproducibility (GR&R) study as a method to estimate measurement uncertainty.

The distinction between repeatability and accuracy in measurement results.

The potential for high precision measurements to be inaccurate if they consistently miss the true value.

The goal of increasing both accuracy and precision to reduce measurement uncertainty.

The role of sophisticated technology in achieving more precise and accurate measurements.

The importance of following strict standards in metrology to manage measurement uncertainty.

The American Society of Mechanical Engineers' B89.7 standards addressing uncertainty considerations.

Factors contributing to measurement uncertainty, including environmental conditions and equipment.

The significance of understanding the sources of measurement uncertainty for more reliable results.

The impact of physical properties, such as thermal expansion, on measurement uncertainty.

The need for comprehensive review of measurement techniques to ensure accuracy and precision.

Transcripts

I have an aluminum part and I want to

measure its diameter

0.2315 inches plus or minus 408 micro

inches let's check that with another

tool

this one says it's 0.2293 plus or minus

6.875 micro inches so what's the real

value

impossible to be certain

for every measure and the quantity we

intend to measure there's technically a

true exact value if we're talking

dimensional measurement any physical

object has an actual size we're just

trying to figure out what it is the

problem is that no measurement method

could ever find the exact True Value get

pretty close but that's about it no

matter how precise our tools may be

there will always be a little more room

to be even closer to the right answer

our goal is just to confidently get as

close as possible the gap between the

true value of a dimension and the

measured value we can obtain is the

measurement uncertainty the most

accepted way to describe measurement

uncertainty is through a gauge

repeatability and reproducibility study

typically shortened to grnr a number of

different operators take a number of

different parts that each repeatedly

measure those parts and then compare the

results this does a great job of

estimating our repeatability but it

doesn't necessarily speak at all to our

accuracy C measurement uncertainty

refers to the dispersion of measured

values around the actual measure Ram So

in theory if your aim is off by roughly

the same amount each time but in

slightly different directions the

average from your repeated trials should

lead you more or less at the actual

Target in the middle of that dispersion

but it's entirely possible to have

really great repeatability hitting very

close to the same measurement every time

and still be inaccurate High Precision

can just mean being repeatedly wrong

instead we want to get really close to

the true value of the measureand

directly that just kind of land

somewhere all around it extrapolate and

hope for the best and on top of that we

want reliable repeatability measurements

that consistently Land close together in

repeated trials we want to increase both

our accuracy and our Precision in other

words we want to shrink the measurement

uncertainty one of the ways we do this

is through increasingly sophisticated

technology that's up to the task of

taking such careful measurements

generally speaking the smaller the units

an instrument can reliably measure the

smaller the amount it's likely to to

stray from the correct value right I

mean you wouldn't say that you can

measure down to the micrometer if your

measurement tool could be off by a whole

dang millimeter good Precision helps

narrow the accuracy window another

crucial way that we handle measurement

uncertainty is you guessed it by

following strict standards here on

Metrology matters we can't overstate the

importance of Standards the guidance

from standards for managing uncertainty

really help make things less uncertain

for example the American Society of

mechanical engineers created the b89.7

theories of Standards six short

documents addressing several important

uncertainty considerations these include

measurement planning estimating

measurement uncertainty building

uncertainty statements estimating

measurement risk decision rules for

accepting and rejecting components and

traceability these standards also

address why any of this uncertainty

happens in the first place what causes

us to stray from the measure end turns

out basically everything some factors

include the environment you're working

in your reference element your

measurement equipment the measurement

setup software and calculations doing it

the object you're measuring how the

target characteristic is defined for

measuring procedure physical constants

and a whole lot more it's a lot to keep

track of but very important to

understand overall having reliable and

concrete knowledge of how inaccurate our

measurements are and why gives us more

confidence in the accuracy of what we

report in other words it's very useful

to be certain about our uncertainty

with all that in mind let's take one

more look at that aluminum part

now it says it's like 15 micro inches

bigger than it was before all right I've

had it in my pocket this whole time and

aluminum expands at 25 parts per million

per degree Fahrenheit so I probably like

quadrupled the uncertainty I haven't

even tested its roundness I guess I also

need to review more about the measure

end