System of units English

The English system of units is complicated compared to the metric system. In the English system, the units of mass are pounds-mass (Ibm) and the units of weight are pounds-force (Ibf). By definition, a weight (i.e., force) of one Ibf equals the force produced by one Ibm under the acceleration of gravity. Therefore, the constant, g, which has the same numerical value as g (32.17) and units of Ibm-ft/lbf-sec, is used in the definition of weight ... 

The constant of proportionality in the English system of units, g, which causes one pound of mass produces one pound of force under the acceleration of gravity, equal to 32.17 Ibm-ft/lbf-sec. ... 

In all of the equations written here, the energy unit is presumed to be the joule, in accord with the SI system of units. For the English system of units, the kinetic- and potential-energy terms, wherever they appear, require division by the dimensional constant gc (see Secs. 1.3 and 1.8). However, in many applications, the kinetic- and potential-energy terms are omitted, because they are negligible compared with oth r terms. Exceptions are applications to nozzles, metering devices, wind tunnels, and hydroelectric power stations. 

In the English system of units heat flow is expressed in British thermal units per hour (Btu/h), area in square feet, and temperature in degrees Fahrenheit. Thermal conductivity will then have units of Btu/h ft °F. 

The units for K will depend on the units prescribed for the other terms. Thus, in the English system of units, if the pressure change is in psf (pounds per square foot), the distance is in feet, and the viscosity value is in Bvu s (British viscosity units of pounds/ft-sec), with the velocity in ft/sec, then the dimensions of K will be ft /sec. (It may be noted that the viscosity in poises has the dimensions of grams/cm-sec, and the viscosity in centipoises has the dimensions of centigrams/cm-sec. Accordingly, to convert a viscosity value in centipoises, multiply the value by 6.72 x 10 to yield the viscosity value in Bvu s.)... 

Within the English system of units, relationships between length, mass, and volume are exact ... 

It is recognized at this time that the English Engineering System of units cannot be completely replaced by the International System (SI). Transition from the English system of units to SI will proceed at a rational pace to accommodate the needs of the profession, industry, and the public. The transition period will be long and complex, and duality of units probably will... 

In the English system of units it is necessary to relate thermal and mechanical energy via the mechanical equivalent of heat J,. Thus... 

Engineering is about real physical things, which can be measured and described in terms of the units of measure. Most engineering calculations involve these units of measure. It would be simple if there were only one set of such units that the whole world agreed on and used, but that is not the case today. In the United States, most measurements use the English system of units, based on the foot, pound, and degree Fahrenheit, but most of the world uses the metric... 

Units are a fourth critically inportant area. The United States NASA program crashed a space craft into Mars because of failure to convert between the metric and English systems of units. Because conversion of units will remain necessary throughout your career, I have used data in the units in which they were originally presented. Thus, you must do conversions throughout the book. Although problem solutions and Appendix C show conversion factors, it is assumed that you are very familiar and proficient with unit conversions. This includes conversion from weight to mole fractions, and vice versa. 

Since the United States is the only major industrial nation that has not yet converted to metric units, some legal requirements in that direction are to be expected. It is now a contradiction to speak of the English system of units, and for some time to come U.S. engineers must accommodate to a wide use of conversions from one set of units to another. The extensive conversion tables that follow are offered with this expectation. 

The constant of proportionality (/r) is called viscosity. The dimensions of viscosity can be found from Eq. (5.3) to be [E TZ" ]. In the English system of units, the unit of viscosity is called the Rc5m (after Rc5molds) and equals one lb sec/in. In the metric system, the unit of viscosity is the Poise (after Poiseuille) and equals one dyne sec/cm. One Re m equals 68,950 Poise. Representative values of viscosity (p) and specific weight (/) are given in Table 5.1. 

Regulatory standards and most equipment are based upon the English system of units. The corresponding metric unit is the kilopascal (kPa). One p.s.i. is equal to 6.895 kPa. 

We are going to work this problem in lb s, ft- and lb-moles rather than g, mol, and m in order to give the reader more practice in working in both the English and metric systems. Many planes still use the English system of units. 

In one English system of units, lengths are measured in feet, masses are measured in pounds, abbreviated lb (1 lb = 0.4536 kg), and time is measured in seconds. The absolute temperature scale is the Rankine scale, such that 1.8° R corresponds to 1°C and to 1K. 

Most of the units used in tliis book are consistent with those adopted by the engineering profession in tlie United States. For engineering work, International SySterne (SI) and English units are most often employed in the United States, tlie English engineering units are generally used. Tliese systems of units are shown in Table 4.2.1. ... 

The density of a substance is its weight per unit of volume. The unit of volume in the English system of measurement is 1 cubic foot or 1 f. To find the density of a substance, you must know its weight and volume. You then divide its weight by its volume to find the weight per unit volume. 

Any Consistent System of Units may be Employed in Place of the English Units Specified... 

Sometimes it is necessary to convert from a measurement in the English system to a measurement in the SI system. (The English system is sometimes referred to as the U.S. Customary system of units.) There are numerous SI/English conversions. Consult your book and check with your instructor to see which they recommend. We will be using the following in many of our examples ... 

The system of units used in chemistry is the SI system (Systeme International), which is related to the metric system. There are base units for length, mass, etc. and decimal prefixes that modify the base unit. Since most of us do not tend to think in these units, it is important to be able to convert back and forth from the English system to the SI system. These three conversions are useful ones, although knowing the others might allow you to simplify your calculations ... 

Specific gravity appears to be nondimensional since the units of the density of the liquid are the same as the units of the density of water however, this is not strictly true. Actually, in the English system the units are... 

This system of units is the English-system equivalent of SI. Thus,... 

Other systems of units, such as the English engineering system, use units thai are related to SI units by fixed conversion factors. Thus, the foot (ft) is defined as 0.3048 m, the pound mass (lbm) as 0.45359237 kg, and the pound mole (lb mof as 453.59237 mol. 

In the English engineering system of units, force is treated as an additions independent dimension along with length, time, and mass. The pound force (lbr is defined as that force which accelerates 1 pound mass 32.1740 feet per seconc per second. Newton s law must here include a dimensional proportionality constant if it is to be reconciled with this definition. Thus, we write... 

This equation is the mathematical expression of the first law for a steady-state-flow process. All the terms are expressions for energy per unit mass of fluid in the SI system of units, energy is expressed in joules or in some multiple of the joule. For the English engineering system of units, this equation must be reexpressed to include the dimensional constant gc in the kinetic- and potential-energy terms ... 

The SI (metric) system of units is the primary one for the text. Because the Btu-ft-pound system is still in wide use, answers and intermediate steps to examples are occasionally stated in these units. A few examples and problems are completely in English units. Some figures have dual coordinates that show both systems of units. These displays will enable the student to develop a bilingual capability during the period before full metric conversion is achieved. 

In the literature, information is found using different systems of units metric, SI, and the English system. Quotations from the literature are presented in their original form. It would be difficult to change all these units in the book to one system. To assist the reader in converting these units, an appendix is provided with conversion factors for all units found in the text. 

Aerosol sizes are usually referred to in terms of the micrometer (pm) (previously called the micron p). One micrometer is equal to 10 4 centimeters (cm), 10-6 meters (m), or 104 angstrom units, abbreviated A. In working problems it is necessary to use a consistent set of units. Since most physical constants are available either in cgs or inks units (English units are too cumbersome to use), aerosol sizes given in micrometers very often must be converted to either centimeters or meters for computations (depending on the system of units chosen). When you are working problems involving ratios of particle size, this conversion is not necessary. 

FPS. The system of units based on the fundamental units of the English system foot, pound and second. 

Units. Labels which distinguish one type of measurable quantity from other types. Length, mass and time are distinctly different physical quantities, and therefore have different unit names, meters, kilograms and seconds. We use several systems of units, including the metric (SI) units, the English (orU.S. customary units), and a number of others of mainly historical interest. 

The metric system and its more modem counterpart SI (for Systeme International d Unites) are systems of units designed to make calculations as easy as possible. It was designed to make every word mean one and only one thing. Its subdivisions and multiples of units are powers of 10 times a primary unit. Each of its prefixes means the same thing, no matter what unit it is attached to. The abbreviations for the quantities and prefixes are easy to remember. All these features have been built into the metric system to make it easy. Contrast this lack of ambiguity with the ambiguity in the English system, as illustrated in Example 2.4. 

SI is the abbreviation of Systeme International d Unit , or in English, the International System of Units . The SI unit system was developed in I960 (Table I). 

The two most widely used systems of units are the English system used in the United States and the metric system used by most of the rest of the industrialized world. This duality obviously causes a good deal of trouble for... 

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