United States Customary System

Table 1-10 United States Customary System of Weights...

There are two major unit systems used in the world today. One is the United States Customary System (USCS, formerly called the British System of Units), used in the United States, primarily for nonscientific purposes. The other is the Systeme International (SI), which is used in most other nations. This system is also known as the International System of Units or as the metric system. The orderliness of this system makes it useful for scientific work, and it is used by scientists all over the world, including those in the United States. (And the International System is beginning to be used for nonscientific work in the United States, as Figure 1.14 shows.) This book uses the SI units given in Table 1.1. On occasion, USCS units are also used to help you make comparisons. 

This section will try to assist the reader in sizing SRVs. In the first part, sizing data are given in English units (United States Customary System, USCS) consistent with the requirements of ASME Section VIII, API Recommended Practice 520, for pressures above 1.03 barg (15psig). Metric units are given only for the most common sizes. 

Table 3.3 gives the more common conversion relationships between measurement units in the United States Customary System (USCS) and metric system. The USCS, formerly the British system of units, is the system used in the United States. All other countries of the world use the metric system. 

Units of pressure come from the definition. In the United States Customary System, if force is measured in pounds and area in square inches, the pressure unit is pounds per square inch (psi). The SI unit of pressure is the pascal (Pa), which is one newton per square meter. (The newton is the SI unit of force.) Although chemists generally follow SI guidelines, one pascal is a very small pressure the kilopascal (kPa) is a more practical unit. Many other pressure units are commonly used. One is the bar, which is 1 X 10 Pa or 100 kPa. The millimeter of mercury, or its equivalent, the torr, and the atmosphere are other common units of pressure. The millimeter of mercury is usually abbreviated mm Hg. (Hg is the elemental symbol for mercury.)... 

The equations in this chapter are based on SI units for consistency. They can be readl ly used in USCS (United States Customary System) units If the reader assumes the use of slugs and not pounds for mass units. There is considerable confusion about the use of slugs, which are units of mass, and pounds, which are units of force. The conversion factor between these is often called and is equal to 32.2 ft/sec. Many other references input in their equations to achieve such a conversion, but it was not deemed necessary in this book. The worked examples show how slugs should be used as a unit of mass. 

Now that you understand the importance of the length dimension, let us look at its divisions or units. There are several systems of units in use in engineering today. We will focus on two of thee systems the International System of Units (SI) and the United State Customary units. The unit of length in SI is the meter (m). We can use the mulriple and fracrions of this unit according to Table 6.2. Common multipliers of the meter are micrometer (/tm), millimeter (mm), centimeter (cm), and kilometer (km). Recall fiom our discussion of units and mulriplicarion... 

The egr system of units, based on the centimeter, gram, and second as units in mechanics, is a metric system which continues to be used in some branches of physics In daily life, the customary units in the United States are those based on the foot, pound-force, and second, but these units are... 

Following is a listing of typical nomenclature expressed in SI and U.S. Customary System units. Specific definitions and units are stated at the place of application in this section. 

As the United States moves toward acceptance of the International System of Units, or the so-called SI units, it is particularly important for the design engineer to be able to think in both the SI units and the U.S. customary units. From an international viewpoint, the United States is the last major country to accept SI, but it will be many years before the U.S. conversion will be sufficiently complete for the design engineer, who must deal with the general public, to think and write solely in SI units. For this reason, a mixture of SI and U.S. customary units will be found in this text. 

Long the language universally used in science, the SI has become the dominant language of international commerce and trade. The system is nearly universally employed, and most countries do not even maintain official definitions of any other units. A notable exception is the United States, which continues to use customary units in addition to SI. In the United Kingdom, conversion to metric units is government policy, but the transition is not quite complete. Those countries that still recognize non-SI units (e. g., the US and UK) have redefined their traditional non-SI units in SI units. 

In the United States, most engineers still use the U.S. Customary system of units. The unit of length is a foot (ft), which is equal to 0.3048 meter, the unit of mass is a pound mass (lb J, which is equal to 0.453592 and the unit of time is a second (s). In U.S. Customary system, the unit of force is pound force (Ib and 1 Ibf is defined as the weight of an object having a mass of 1 Ibj, at sea level and at a latitude of 45°, where acceleration due to gravity is 32.2 ft/s. One pound force is equal to 4.448 newton (N). Because the pound force is not defined formally... 

In other countries, such as the United States, the American democratic process has stalled conversion to the metric system. In 1800, the United States could have become the second nation in the world to adopt the SI because of its strong ties with France, where the SI was developed. It chose instead to adopt the U.S. customary system of measurement, which used units similar to the British Imperial system but sometimes varied in terms of quantities (an Imperial gallon held 160 fluid ounces, for example, while an American gallon held only 128 fluid ounces). The debate over adoption of the metric system in the United States continues, as many Americans oppose any official move to replace the... 

Historically, a number of units representing different amounts of the same properties have been used in various cultures around the world. For example, the United States traditionally uses the U.S. customary system (miles, cups, pints, ounces, and so on), while most other industrialized nations use the metric system (kilometer, milliliter, liters, grams, and so on). The metric system, developed in France in the late eighteenth century, represents the first true standard measurement system. The theory and physical practice of measurement is constant no matter what system of units is being used. 

As explained in Chapter 1 the engineering information provided in this book generally uses the SI (Le Systeme international d unites) metric units. However, much of the information to do with regulations and standards, particularly in the United States, is based on the English or customary units such as feet (ft.) and pounds (lb.). In most cases these English measurements have been converted to metric. 

We recognize that the International System of Units (SI) has become the fundamental basis of scientific measurement worldwide and is used for everyday commerce in virtually every country except the United States. As painM as it may be for those of us who have learned and practiced the British or US Customary system of units, we feel that it is time to put aside the units of the industrial revolution and adopt the SI system of measurement in all aspects of modern engineering and science. For this reason, SI units have been adopted as the primary system of units throughout this book. However, it is recognized that US customary or British units are stiU widely used in the United States and some use of them is provided herein for the benefit of those who still relate closely to them. Dimensional constants specific to the British system, such as gc, have been left out of the formulae. 

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