Precision Measurement

Precision Measurement

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.

Describing Measurements• What is the precision of each

device

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.

Accuracy vs Precision

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?

Precision Measurement

Uncertainty

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

Uncertainty• How long is the pencil?

21cm

22cm

23cm

24cm

25cm

26cm

27cm

28cm

Uncertainty• How long is the crayon?

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.

Uncertainty• Calibration

– How a measuring device is adjusted or corrected.

– ‘reset’ a scale so it measure’s accurately

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.

Assignment• Learning Activity 8.2

Precision Measurement

Calipers and Micrometers

Measuring Devices• Manufacturing requires higher degree of

accuracy than standard rulers

• Two devices commonly used– Vernier Calipers

• Precise to the nearest one hundredth of a centimetre.

– Micrometers• Precise to the nearest one hundredth of a millimetre

Measuring Devices – Vernier Calipers

1. Outside jaws– Takes external measures of objects

2. Inside jaws– Takes internal measures of objects

3. Depth probe– Measure the depth of objects

Measuring Devices – Vernier Calipers

4. Main scale– #4 is metric & #5 is imperial

6. Vernier Scale – #6 Metric & #7 Imperial

8. Retainer– Used to block movable parts

Vernier Callipers

Callipers Applet

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

Measuring Devices - Micrometers

Micrometer

Micrometer Applet

Measurement Devices - Micrometers

1. Determine to the nearest whole mm using upper scale

2. Use the lower scale on the barrel to determine the ½ mm.

3. Determine the thimble reading

4. Add these values together.

7

0.24

7.5 + 0.24 = 7.74mm0.5

Assignment• Learning Activity 8.3

Precision Measurement

Tolerance

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”

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?

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

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!

0.30.152.6

upper tolerancelower toleranceNominal Value

00.524.5

iPad

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 =

0.40.35.3

0.0703.65

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

Example• A manufacturer is making bulletin

boards for classrooms. What is the tolerance for the area of the bulletin board?

Assignment

Tolerance WS