Conversion of Units and SI Prefixes | Engineering Calculations

Conversion of Units and SI Prefixes:

As an Engineer, it must be his skill to handle all kind of units and be able to convert one set of units to another without finding any difficulty. If you are an Engineer or a student of Engineer you may probably know that conversion of units is carried by simply multiplying any number and its associated units by ratios termed Conversion Factors to arrive at the desired answer with its associated units. Conversion factors are statements of equivalent values of different units in the form of ratios. Note that conversion of units is carried out by multiplying it just change the unit, not the Physical Quantity. For Example, to convert kg into g we have to use conversion factor but it doesn't change the Physical Quantity which is actually mass.

Conversion of Units of some Physical Quantities:

Mass Unit Conversions:

1 kg = 1000 g

1 kg = 2.2 lbm

Length Unit Conversions:

1 ft = 12 in

1 in = 2.54 cm

1 km = 1000 m

1 m = 100 cm

1 mi = 5280 ft

Pressure Unit Conversions:

1 atm = 760 mmHg

1 atm = 14.69 psi

1 atm = 101325 Pa

1 atm = 29.92 in.Hg

1 atm = 1.013 bar

1 atm = 101.325 kPa

Time Unit Conversions:

1 hour = 60 min

1 min = 60 sec

Temperature Unit Conversions:

K = C+273

deg.C = 5/9*(F-32)

deg.F = (C*9/5)+32

deg.C = 1.8R
R = 460+deg.F

Energy Unit Conversions:

1 joule = 0.7376 (ft)(lbf)

1 joule = 2.773*10^-7 kWh

1 joule = 3.725*10^-7 (hp)(hr)

1 joule = 9.484*10^-4 Btu

1 joule = 0.2390 calorie*

As an Example, consider the following problem. 

If a plane travels at the speed of sound (assume that the speed of sound is 1100ft/s, how fast is it going in miles/hr?

To solve this problem, let us convert ft into miles first and then seconds into hr.

As mentioned above 1 mile = 5280 ft

1100ft/s*1/5280 mi/ft = 1100/5280 mi/s = 0.2083 mi/s

Now as we applied conversion factor of length to convert feet into miles, now unit ft is eliminated. As for the time required unit is hour, now we have to convert seconds into hour. Let's do it

0.2083 mi/s * 3600/1 s/hr = 750 mi/hr

So the plane travels at the speed of 750 in mi/hr.

Prefixes:

Prefixes are used in Engineering Calculations and can make the calculations easier. Prefixes are used to reduce zeros in the calculations and make the unit simpler. For Example, 1000 or 10^3g can be written as 1kg because 1k = 1000 or 10^3. Here below are given some prefixes with symbols and factors that you should keep in mind so it will be easier for you to solve problems.

deka (da) = 10^1

hecto (h) = 10^2

kilo (k) = 10^3

mega (M) = 10^6

giga (G) = 10^9

deci (d) = 10^-1

centi (c) = 10^-2

milli (m) = 10^-3

micro (u) = 10^-6

nano (n) = 10^-9