Precision Measurement

1. Precision Measurement

2. Describing Measurements • Accuracy • How close a measurement is to the true value or quantity. • Precision • The degree of exactness of a measurement • The degree to which measurements agree • How close together multiple measurements are. • Precision of a measuring instrument is stated as the smallest scale division of measurements.

3. Describing Measurements • What is the precision of each device

4. Accuracy vs Precision • If a measuring device gives consistent measurements it is considered precise… this does not mean it is accurate! • If your speedometer is always 15km/hr off, it is very precise, but completely inaccurate.

5. Accuracy vs Precision

6. Example • A homeowner is landscaping her backyard. The shape of her property is a rectangle, and the dimensions are 65m by 32 m. • How precise are the dimensions? • How much fence would she need to purchase? • What area of sod would she need to purchase?

7. Precision Measurement Uncertainty

8. Uncertainty • Indicates how close we think our answer is • Within what range is our measurement accurate? • Determined by the smallest scale division on the measuring device. • We should be accurate within 1 scale mark… halfway on either side. • Measured value ± uncertainty

9. Uncertainty • How long is the pencil? 21cm 22cm 23cm 24cm 25cm 26cm 27cm 28cm

10. Uncertainty • How long is the crayon?

11. Uncertainty • Brendon measures his height and finds it to be 72.0cm. • State the precision of the measurement • State the uncertainty of the measurement • Correctly state Brendon’s height.

12. Uncertainty • Calibration • How a measuring device is adjusted or corrected. • ‘reset’ a scale so it measure’s accurately

13. Calibration • Example • A 100g baseball is weighed three times on very precise scale . The three weights are 84.523g, 84.585g and 84.531g • Comment on the degree of precision of the scale • Comment on the degree of accuracy of the measur4ements • What may cause the measurements to be inaccurate.

14. Assignment • Learning Activity 8.2

15. Precision Measurement Calipers and Micrometers

16. Measuring Devices • Manufacturing requires higher degree of accuracy than standard rulers • Two devices commonly used • VernierCalipers • Precise to the nearest one hundredth of a centimetre. • Micrometers • Precise to the nearest one hundredth of a millimetre

17. Measuring Devices – Vernier Calipers • Outside jaws • Takes external measures of objects • Inside jaws • Takes internal measures of objects • Depth probe • Measure the depth of objects

18. Measuring Devices – VernierCalipers • Main scale • #4 is metric & #5 is imperial • Vernier Scale  • #6 Metric & #7 Imperial • Retainer • Used to block movable parts

19. Vernier Callipers Callipers Applet

20. Measurement Devices - Vernier Callipers • To read a vernier scale • Find where the first line of the moving scale aligns with the fixed scale. • This determines the first two digits. • Find where the sliding scale and the fixed scale match the best. This is your last digit. 3.7 0.02 3.72cm

21. Measuring Devices - Micrometers

22. Micrometer Micrometer Applet

23. Measurement Devices - Micrometers • Determine to the nearest whole mm using upper scale • Use the lower scale on the barrel to determine the ½ mm. • Determine the thimble reading • Add these values together. 7 0.24 0.5 7.5 + 0.24 = 7.74mm

24. Assignment • Learning Activity 8.3

25. Precision Measurement Tolerance

26. Tolerance • Nominal Value • The targeted value, the measurement you want. • Tolerance Level • Limit of error you are willing to accept • Example: • You are building a flat for the play, that you want to be 15’ tall, but you are happy if it is anywhere between 14’ 6” and 15’ 6”

27. Tolerance Limits • Allowable limits of the size of an object in a specific design. • Example • A measurement calls for 8.231 ± 0.002. What are the measurements upper and lower limit?

28. Ways of Stating Tolerance • Limiting Dimensioning • Gives maximum and minimum limits • 7.75 – 7.79 or • Tolerance = max limit – min limit • Nominal value is the middle (mean) • Plus and Minus Notation • Nominal Value ± tolerance • 8.25 ± 0.5 • 7.75 • 7.79

29. Ways of Stating Tolerance cont’d • Bilateral Tolerance • Different levels of tolerance on either side of a nominal value • Unilateral tolerance • has one limit of 0 • ie) don’t go over!

30. iPad

31. Examples • Given the following measures with tolerance levels, state the • Nominal Value • Upper limit • Lower limit • A = 5.6 ± 0.3 • B = 5.23 – 6.34 • C = • D =

32. Tolerance: Area and Volume • Tolerance in measurements of length has a direct effect on Area and volume • To determine tolerance of area and volume, determine the difference between the upper and lower limits

33. Example • A manufacturer is making bulletin boards for classrooms. What is the tolerance for the area of the bulletin board?

35. Assignment Tolerance WS