Systems of Units:

The official international system of units (System International d'Units). Strong efforts are underway for its universal adoption as the exclusive system for all engineering and science, but older systems, particularly the cgs and fps engineering gravitational systems are still in use and probably will be around for some time. The chemical engineer finds many physiochemical data given in cgs units; that many calculations are most conveniently made in fps units; and that SI units are increasingly encountered in science and engineering. Thus it becomes necessary to be expert in the use of all three systems.

SI system:

Primary quantities:

Quantity

Unit

Mass in Kilogram

kg

Length in Meter

m

Time in Second

s or as sec

Temperature in Kelvin

K

Mole

gmol or simply as mol

Derived quantities:

Quantity

Unit

Force in Newton (1 N = 1 kg.m/s2)

N

Pressure in Pascal (1 Pa = 1 N/m2)

N/m2

Work, energy in Joule ( 1 J = 1 N.m)

J

Power in Watt (1 W = 1 J/s)

W

cgs Units:

The older centimeter-gram-second (cgs) system has the following units for derived quantities:

Quantity

Unit

Force in dyne (1 dyn = 1 g.cm/s2)

dyn

Work, energy in erg ( 1 erg = 1 dyn.cm = 1 x 10-7 J )

erg

Heat Energy in calorie ( 1 cal = 4.184 J)

cal

fps Units:

The foot-bound-second (fps) system has long been used in commerce and engineering in English-speaking countries.

Quantity

Unit

Mass in pound ( 1 lb = 0.454 kg)

lb

Length in foot (1 ft = 0.3048 m)

ft

Temperature in Rankine

oR

Force in lbf ( 1 lbf = 32.2 lb.ft/s2)

lbf

Conversion factors:

Mass:

1 lb = 0.454 kg

Length:

1 inch = 2.54 cm = 0.0254 m

1 ft = 12 inch = 0.3048 m

Energy:

1 BTU = 1055 J

1 cal = 4.184 J

Force:

1 kgf = 9.812 N

1 lbf = 4.448 N

1 dyn = 1 g.cm/s2

Power:

1 HP = 736 W

Pressure:

1 Pa = 1 N/m2

1 psi = 1 lbf/inch2

1 atm = 1.01325 x 105 N/m2 = 14.7 psi

1 Bar = 105 N/m2

Viscosity:

1 poise = 1 g/(cm.s)

1 cP = (1/100) poise = 0.001 kg/(m.s)

Kinematic viscosity:

1 Stoke = 1 St = 1 cm2/s

Volume:

1 ft3 = 7.481 U.S. gal

1 U.S. gal = 3.785 litre

Temperature:

ToF = 32 + 1.8oC

ToR = 1.8K

Gas Constant:

R = 8314 J / (kmol.K)

 Dimensions:

Dimensions of the primary quantities:

Fundamental dimension

Symbol

Length

L

Mass

M

Time

t

Temperature

T

Dimensions of derived quantities, can be expressed in terms of the fundamental dimensions.

Quantity

Representative symbol

Dimensions

Angular velocity

w

t-1

Area

A

L2

Density

r

M/L3

Force

F

ML/t2

Kinematic viscosity

n

L2/t

Linear velocity

v

L/t

Linear acceleration

a

L/t2

Mass flow rate

m.

M/t

Power

P

ML2/t3

Pressure

p

M/Lt2

Sonic velocity

c

L/t

Shear stress

t

M/Lt2

Surface tension

s

M/t2

Viscosity

m

M/Lt

Volume

V

L3



Last modified: Tuesday, 9 April 2024, 8:09 PM