Precision Measurement Fundacións of Engineering and Technology

PRECISION MEASUREMENT

“When measured quantities are reported, the last significant digit in a measurement

is somewhat uncertain.”

- Park (1996)

PRECISION MEASUREMENT

• Must take into consideration

• Significant digits• Units• What is being measured

• Energy

• Power

• Length

• Weight

ROUNDING• Look at the number to the immediate right of the place you

are rounding to:

• If it is greater than 5 (i.e. 6,7,8, or 9), round the place of interest up

• If it is less than 5 (i.e. 0,1,2,3, or 4), leave the place of interest unchanged

• If is if 5• Round up by one if there are any non-zero digits following

5

• If there are only zeros following the 5, round up by one if the place of interest is odd, if its even leave it unchanged

ROUNDING

• Let’s practice with (3.141592653589)

• Round to 4 decimal places• 3.1416

• Round to 2 decimal places• 3.14

• Round to 3 decimal places• 3.142

• Round to 7 decimal places • 3.1415927

SIGNIFICANT DIGITS• Non-zero digits are always significant

• 523.7 4 significant figures• Any zeros between two significant digits are

significant

• 23.07 4 significant figures• A final zero or trailing zeros in the decimal portion

ONLY are significant

• 3.200 4 significant figures• 200 1 significant figure• 2.00 x 102 3 significant figures

• Leading zeros in a number are not significant• 0.0075 2 significant figures

SIGNIFICANT DIGITS• Let’s take another look at rule number 3…

• Does 1000 ml mean exactly 1000?

• Not necessarily• 1000, 100, 10, and 1 all have one significant digit

• If the engineer wanted to express exactly 1000, what would they need to write?

• 1.000 x 103

SIGNIFICANT DIGITS

SIGNIFICANT DIGITS

UNITS OF MEASUREQuantity English Unit SI Unit Conversion

Length

Inch (in) Centimeter (cm) 1 in = 2.54 cm

Foot (ft) Meter (m) 1 ft = 0.3048 m

Mile (mi) Kilometer (km) 1 mi = 1.609 km

Mass Pound Mass (lbm) Kilogram (kg) 1 lbm = 0.4536 kg

Force Pound Force (lbf) Newton (N) 1 lbf = 4.448 N

TemperatureFahrenheit (TF) Kelvin (TK) TK = 5/9 x (TF – 32) +

273.15

Pressure

Pound per square inch (psi)

Pascal 1 psi = 6.895 x 103 Pa

Atmosphere (atm) Pascal 1 atm = 1.013 x 102 Pa

PowerBtu/hr Watts (W) 1 Btu/h = 0.2929 W

Horsepower (hp) Watts (W) 1 hp = 745.7 W

Magnetic fieldGauss (gauss) Telsa (telsa) 1 gauss = 1.000 x 10-4

telsa

UNITS OF MEASUREQuantity English Unit SI Unit Conversion

Time Seconds (sec)

Volume

Cubic Foot (ft3) Cubic Meter (m3) 1 ft3 = 0.02832 m3

Cubic Inch (in3) Cubic Meter (m3) 1 in3 = 1.6387 x 10-5 m3

Gallon (gal) Cubic Meter (m3) 1 gal = 3.785 x 10 -3 m3

Velocity Miles per hour (mph) Meters per second (m/s) 1 mph = 0.4470 m/s

Area

Square Inch (in2) Square Centimeter (cm2) 1 in2 = 6.452 cm2

Square Foot (ft2) Square Meter (m2) 1 ft2 = 0.0929 m2

Square Mile (mi2) Square Kilometer (km2) 1 mi 2 = 2.590 km2

Angle Degree () Radians (radian) 1 = /180 = 0.01745 rad

For more conversions visit http://www.digitaldutch.com/unitconverter/energy.htm

PRECISION VS. ACCURACY • Precision

• Refers to how close together a series of measurements are to each other

• Accuracy

• Refers to how close a measured value is to an accepted value

PRECISION AND ACCURACY??

PRECISION AND ACCURACY??

PRECISION AND ACCURACY??

FOR ADDITIONAL EXPLANATION

https://www.youtube.com/watch?v=b38hFWvEjwI