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 |
o R |
|
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 on: 12-Sep-2014
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