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

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? 21cm22cm23cm24cm25cm26cm27cm28cm

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 g, g and g 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 cm

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 = 7.74mm 0.5

Assignment Learning Activity 8.3

Precision Measurement 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 ± 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 ±

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!

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 =

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