(UTED) Precision Measurement Lesson

Precision Measurement Lesson

Introduction

 

Evaluate the mechanisms used in the image of the clock mechanics shown. Consider how important it is to measure time accurately. Certainly in some situations it is acceptable for a time piece to be a few minutes off. However, accuracy in measurement and reporting of time is critical for Global Positioning Systems (GPS) satellite receivers and vital computing systems. Accuracy to the one hundredth of a second is also crucial in competitive sports events.

So how does this relate to the design of engineered products? Consider the selection of screws for a part. A #6 screw has a 0.13-inch diameter while a #8 screw has a 0.16-inch diameter. That difference of 0.03 inches may not be noticed by the human eye, but important for optimum part creation.

 

Explore

The study of precision measurement requires the understanding of several key terms as well as the correct use of precision measurement tools and instruments. This lesson will explain and demonstrate key terms and then explain and demonstrate a precision measurement tool.

What is the difference between precision and accuracy?

Accuracy is how close a measured data value is to the correct number. For example, if a known 2.5 kg weight measures 2.51 kg on Scale A and then 2.9 kg on Scale B then Scale A is far more accurate than Scale B. Validity is a term often used for accuracy.

Precision is the consistency of measured results from an instrument. If Tape Measure A has the following readings on the same object then it is a precise instrument: 3.0 in, 3.0 in, 3.01 in, 2.99 in. If Tape Measure B has the following readings on the same object then it is NOT a precise instrument: 2.75 in, 3.0 in, 3.25 in, 2.80 in. Reliability is a term often used for precision.

However, a precise instrument does NOT mean it is measuring correctly, but just providing consistent results. Realize it is possible for an instrument to be “precisely wrong”. The best instruments are both accurate and reliable. That is why calibration of instruments is important to ensure accuracy.

Applications for Precision Measurement

When measuring it is important to use the right tool for the job. In building a home, a floor joist can be off a quarter inch without concern but the same cannot be said for a machine screw. So when evaluating when to use precision measurement tools consider the application and the importance of accuracy and reliability in measuring tools and instruments.

Significant Figures

What is the relationship between significant figures to measurement?

The number of significant figures is the number of digits correctly measured. For example, a measurement of the pencil shown above might be 18.8 mm. This has 3 significant digits – the 1, 8, and the 8. It appears that this number was rounded. However, if the measurement was 18.73 mm, then the measurement would be considered precise. As mentioned earlier, precision is crucial in engineering design.

View the following video to continue understanding the application of significant figures in measurement.

 

 

 

 

 

Challenge

 

 

Take Away

Precision measurement tools are a critical component for the design and production of industrial products.

See this video of a professional industrial designer describing his role in creation of manufacturing tools for musical instruments. Watch as he uses measurement tools for design and verification. Listen and consider your career options.

 

 

 IMAGES CREATED BY GAVS