American Engineering System

The base units of the American engineering system are the foot (ft) for length, the pound-mass (Ibm) for mass, and the second (s) for time. This system has two principal difficulties. The first is the occurrence of conversion factors (such as 1 ft/12 in), which, unlike those in the metric systems, ate not multiples of 10 the second, which has to do with the unit of force, is discussed in the next section. 

What is the derived SI unit for velocity The velocity unit in the CGS system In the American engineering system ... 

In the American engineering system, the derived force unit—called a pound-force (Ibf)—is defined as the product of a unit mass (1 Ibm) and the acceleration of gravity at sea level and 45° latitude, which is 32.174 ft/s ... 

The pressure at a point in a fluid (gas or liquid) is the force per unit area that the fluid would exert on a plane surface passing through the point. Standard units of fluid pressure are N/m, (pascal, or Pa) in the SI system, dyne/cm in the CGS system, and Ibf/ft in the American engineering system. The unit lb /in. (psi) is also common in the American engineering system. The pressure at the base of a vertical column of fluid of density p and height h is given by the expression... 

Specify the basic and derived units in the SI and American engineering systems for mass, length, volume, density, and time, and their equivalences. 

Only the American engineering system has four basically defined units. Consequently, in the American engineering system you have to use a conversion factor, gc, a constant whose numerical value is not unity, to make the units come out properly. We can use Newton s law to see what the situation is with regard to conversion of units ... 

However, in the American engineering system we ask that the numerical value of the force and the mass be essentially the same at the earth s surface. Hence, if a mass of 1 Ibm is accelerated at g ft/s where g is the acceleration of gravity (about 32.2 ft/s depending on the location of the mass), we can make the force be 1 Ibf by choosing the proper numerical value and units for C ... 

Division by gc achieves exactly the same result as multiplication by C in Newton s law. You can see, therefore, that in the American engineering system we have the convenience that the numerical value of a pound mass is also that of a pound force if the numerical value of the ratio gigc is equal to 1, as it is approximately in most cases. A nonstandard usage sometimes encountered analogous to Ibf and Ibm is kgf... 

You should develop some facility in converting units from the SI system into the American engineering system, and vice versa, since these are the two sets of units in this text. Certainly you are familiar with the common conversions in both the American engineering and the SI systems. If you have forgotten, Table 1.3 lists a short selection of essential conversion factors. Memorize them. Common abbreviations and symbols also appear in this table. 

Prepare a table in which the rows are length, area, volume, mass, and time. Make two columns, one for the SI and the other for the American engineering systems of units. Fill in each row with the name of the unit, and in a third column, show the numerical equivalency (i.e., 1 ft = 0.3048 m). 

If we had started with units in the American engineering system, the pressure would be computed as... 

List the standard conditions for a gas in the SI, universal scientific, and American engineering systems of units. 

Now for a word of warning Be certain you use consistent units for all terms in the American engineering system the use of foot-pound, for example, and Btu in different places in Eq. (4.23) is a common error for the beginner. 

Throughout this book we will use primarily the SI system of units with occasional use of the American Engineering system. The main quantities of interests are pressure, temperature, and energy. These are briefly reviewed below. Various physical constants that are commonly used in thermodynamics are shown in Table i-i. 

An older unit of pressure, still in use, is the Torr, or mm Hg, representing the hydrostatic pressure exerted by a column of mercury i mm high. In the American Engineering system of units, pressure is measured in pounds of force per square inch, or psi. The relationship between the various units can be expressed through their relationship to the standard atmospheric pressure ... 

Temperature is a fundamental property in thermodynamics. It is a measure of the kinetic energy of molecules and gives rise the sensation of hot and cold. It is measured using a thermometer, a device that obtains temperature indirectly by measuring some property that is a sensitive function of temperature, for example, the volume of mercury inside a capillary (mercury thermometer), the electric current between two different metallic wires (thermocouple), etc. In the SI system, the absolute temperature is a fundamental quantity (dimension) and its unit is the kelvin (K). In the American Engineering system, absolute temperature is measured in rankine (R), whose relationship to the kelvin is. 

The pound-mol (Ib-mol) is the analogous unit in the American Engineering system and represents an amount of matter equal to the molecular weight expressed in Ibm. The relationship between the mol and Ib-mol is... 

A. Klinkenberg, The American engineering system of units and its dimensional constant gc,... 

The code for clicking the Calculate BMI ( ) push button will be the same as that shown in Fig. 11.23. except that it is for calculating BMI as a function of input parameters, expressed in the American (engineering) system of units fFig. 11.24T... 

This chapter presents two systems of units so that you can follow the examples ahead. These two systems of units are the metric SI and what is termed by the American Society of Mechanical Engineers (ASME) as the U.S. Customary system of units, namely in the ASME Section II Part D [I], This system is also termed the American Engineering System (AES) by the U.S. government. I mentioned the latter term in my book Piping and Pipelines Assessment Guide [2], in how to use the two systems of units. In this book, we will discuss briefly the other variants of the metric SI system, but it is the prevailing metric system of units. Likewise, we will concentrate on the U.S. Customary system versus the British Imperial system. Even though the latter two are similar, there are some differences. 

The American engineering system The unit force is 1 Ibf, where 1 Ibm is accelerated at g ft/s, where g is the acceleration of gravity. Equation 1.1 is rewritten as... 

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