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A significant amount of a practicing engineer’s time is spent in converting data and equations from one set of units to another. As long as scientists provide data and information in grams and centimeters while engineers calculate in terms of pounds and feet, this confusion in terminology will continue to exist. For the most part, the units in this text are consistent with those adopted by the engineering profession in the United States.

The physical, biological, and chemical properties of materials are important considerations in any study of desalination. To promote a better understanding of such properties, many of which are covered in Chapter 04, some definitions are briefly introduced in this section. Equations used in engineering as well as in the physical sciences are dimensional equations. The choice of the fundamental system of units determines the magnitude of the quantity described. Since the choice is optional, it is usually made to suit the convenience of a certain segment of the scientific community. For example, astronomers measure distance in light years, nuclear physicists employ angstroms, engineers use feet, etc. Engineers in the United States generally employ English units, but technical individuals need to be able to convert from one system of units to another.

Information on conversion constants (sometimes referred to as conversion factors) for length, mass, time, force, pressure, volume, energy, power, concentration, viscosity, and heat capacity are provided in Table 3.2. Commonly used water and water-related conversion constants, as they relate to volume and volumetric flowrate, are provided in Tables 3.3 and Table 3.4, respectively.

Table 3.2 Conversion constants.

To convert from	To	Multiply by
Length
m	cm	100
m	mm	1000
m	microns (µm)	106
m	angstroms (Å)	1010
m	in	39.37
m	ft	3.281
m	mi	6.214 x 10-4
ft	in	12
ft	m	0.3048
ft	cm	30.48
ft	mi	1.894 x 10-4
Mass
kg	g	1000
kg	lb	2.205
kg	oz	35.24
	ton	2.268 x 10-4
kg	grains	1.543 x 104
lb	oz	16
lb	ton	5 x 10-4
lb	g	453.6
lb	kg	0.4536
lb	grains	7,000
Time
s	min	0.01667
s	hr	2.78 x 10-4
s	day	1.157 x 10-7
s	week	1.653 x 10-6
s	yr	3.171 x 10-8
Force
N	kg-m/s2	1
N	dynes	105
N	g-cm/s2	105
N	lbf	0.2248
N	lb-ft/s2	7.233
lbf	N	4.448
lbf	dynes	4.448 x 105
lbf	g-cm/s2	4.448 x 105
lbf	lb-ft/s2	32.17
Pressure
atm	N/m2 = Pa	1.013 x 105
atm	kPa	101.3
atm	bars	1.013
atm	dynes/cm2	1.013 x 106
atm	lbf/in2 = psi	14.696
atm	mm Hg at 0°C (torr)	760
atm	in Hg at 0°C	29.92
atm	ft H2O at 4°C	33.9
atm	in H2O at 4°C	406.8
psi	atm	6.80 x 10-2
psi	mm Hg at 0°C (torr)	51.71
psi	in H2O at 4°C	27.70
in H2O at 4°C	atm	2.458 x 10-3
in H2O at 4°C	psi	0.0361
in H2O at 4°C	mm Hg at 0°C (torr)	1.868
Volume
m3	L	1,000
m3	cm3 (cc, mL)	106
m3	ft3	35.31
m3	gal (U.S.)	264.2
m3	qt	1,057
ft3	in3	1,728
ft3	gal (U.S.)	7.48
ft3	m3	0.02832
ft3	L	28.32
Energy
J	N-m	1
J	erg	107
J	dyne-cm	107
J	kW-hr	2.778 x 10-7
J	cal	0.2390
J	ft-lbf	0.7376
J	Btu	9.486 x 10-4
cal	J	4.186
cal	Btu	3.974 x 10-3
cal	ft-lbf	3.088
Btu	ft-lbf	778
Btu	hp-hr	3.929 x 10-4
Btu	cal	252
Btu	kW-hr	2.93 x 10-4
ft-lbf	cal	0.3239
ft-lbf	J	1.356
Power
W	J/s	1
W	cal/s	0.2390
W	ft-lbf/s	0.7376
W	kW	10-3
kW	Btu/s	0.949
kW	hp	1.341
hp	ft-lbf/s	550
hp	kW	0.7457
hp	cal/s	178.2
hp	Btu/s	0.707
Concentration
µg/m3	lb/ft3	6.243 x 10-11
µg/m3	lb/gal	8.346 x 10-12
µg/m3	gr/ft3	4.370 x 10-7
gr/ft3	µg/m3	2.288 x 106
gr/ft3	g/m3	2.288
lb/ft3	µg/m3	1.602 x 1010
lb/ft3	µg/L	1.602 x 107
lb/ft3	lb/gal	7.48
Viscosity
P (poise)	g/cm-s	1
P	cP (centipoise)	100
P	kg/m-hrv	360
P	lb/ft-s	6.72 x 10-2
P	lb/ft-hr	241.9
P	lb/m-s	5.6 x 10-3
lb/ft-s	P	14.88
lb/ft-s	g/cm-s	14.88
lb/ft-s	kg/m-hr	5.357 x 103
lb/ft-s	lb/ft-hr	3,600
Volume
m3	L	1,000
m3	cm3 (cc, mL)	106
lb/ft-s	kg/m-hr	5.357 x 103
lb/ft-s	lb/ft-hr	3,600
Heat Capacity
cal/g-°C	Btu/lb-°F	l
cal/g-°C	kcal/kg-°C	l
cal/g-°C	cal/gmol-°C	molecular weight
cal/gmol-°C	Btu/lbmol-°F	l
J/g-°C	Btu/lb-°F	0.2389
Btu/lb-°F	cal/g-°C	1
Btu/lb-°F	J/g-°C	4.186
Btu/lb-°F	Btu/lbmol-°F	molecular weight

Table 3.3 Conversion constants for volume.

Units	Conversion constants
L	gal	ft3	m3	ac-ft
Liters (L)	1.0	0.2642	3.531 x 10-2	0.0001	8.106 x 10-7
Gallons (gal)	3.785	1.0	0.1337	3.785 x 10-3	3.068 x 10-6
Cubic feet (ft3)	28.32	7.481	1.0	2.832 x 10-3	2.296 x 10-5
Cubic meters (m3)	1,000	264.2	35.31	1.0	8.106 x 10-4
Acre feet (ac-ft)	1.223 x 106	3.259 x 105	4.356 x 104	1,233	1.0

Table 3.4 Conversion constants for volumetric flowrate.

Units	Conversion constant
gpm	L/s	ac-ft/d	cfs	m3/d
Gallons/min (gpm)	1.0	6.309 x 10-2	4.419 x 10-3	2.228 x 10-3	5.45
Liters/sec (L/s)	15.85	1.0	7.005 x 10-2	3.351 x 10-2	86.4
Acre feet/day (ac-ft/d)	226.3	14.28	1.0	0.5042	1,234
Cubic feet/sec (ft3/s)	448.8	28.32	1.983	1.0	2.447
Cubic meters/day (m3/d)	1.369 x 109	8.64 x 107	6.051 x 106	3.051 x 106	1.0