Extech Instruments AN340 User Manual page 16

Appendix: Useful Equations and Conversions
Area equation for rectangular or square ducts
Area equation for circular ducts
Radius
Cubic equations
NOTE: Measurements made in inches or centimeters must be converted to feet or meters before using these formulae.
Unit of Measure Conversion Table
1 m/s
1 ft/min
1 knot
1 km/h
1 MPH
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Area (A) = Length (L) x Width (W)
Area (A) = pi x r
Where pi = 3.14 and r
Diameter of circle = radius x 2
3
CFM (ft /min) = Air Velocity (ft/min) x Area (ft
3
CMM (m /min) = Air Velocity (m/sec) x Area (m
m/s ft/min
1 196.87
0.00508 1
0.5144 101.27
0.2778 54.69
0.4464 87.89
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2
2
= radius x radius
2
)
2
) x 60
knots
1.944
0.00987 0.01829
1
0.54
0.8679
16
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km/h MPH
3.6 2.24
0.01138
1.8519 1.1523
1 0.6222
1.6071 1
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