Conversions, unit English system

Beginning students often regard the metric system as difficult because it is new to them and because they think they must learn all the English-metric conversion factors (Table 2-3). Engineers do have to work in both systems in the United States, but scientists generally do not work in the English system at all. Once you familiarize yourself with the metric system, it is much easier to work with than the English system is. 

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 ... 

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. 

Units employed in diffusivity correlations commonly followed the cgs system. Similarly, correlations for mass transfer correlations used the cgs or English system. In both cases, only the most recent correlations employ SI units. Since most correlations involve other properties and physical parameters, often with mixed units, they are repeated here as originally stated. Common conversion factors are listed in Table 1-4. 

Some conversions between mettic and English system units are presented in Table 2.4. Engineers must know how to do such conversions because they still use some English system units. However, scientists rarely use English system units, and therefore, these conversions are less important for them. (The use of a metric unit that is slowly becoming familiar to the general public is shown in Figure 2.5. Note that 80 km/h is about 50 mph.)... 

Data, Results, and Units English and Metric Units Unit Conversion English and Metric Systems Conversion of Units within the Same System Conversion of Units from One System to Another... 

Clearly, operations such as the conversion of 1.62 yards to units of miles are not straightforward. In fact, the English "system" is not really a system at all. It is simply a collection of measures accumulated throughout English history. Because they have no fimctional relationship, it is not surprising that conversion from one unit to another is not straightforward. 

The conversion of a quantity expressed in units of one system to an equivalent quantity in the other system (English to metric or metric to English) requires a bridging conversion unit. Examples are shown in Table 1.3. 

Although the preferred SI unit of area is the square meter, land is often measured in the metric system in hectares (ha). One hectare is equal to 10,000 m. In the English system, land is often measured in acres (1 acre = 160 rod ). Use the exact conversions and those given in Exercise 43 to calculate the following. 

The metric and English systems are used interchangeably throughout most of the text. The choice of units in any particular case was governed by the application. However, a set of conversion factors is included in Appendix 1 for those who wish to utilize units other than the ones given in the tables, figures, and text. 

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. 

Conversion factors for some of the English system units commonly used in the United States for nonscientific measurements (for example, powds and inches) are provided inside the back cover of this book. 

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. 

Metric units have been generally used and conversion from the English system has been followed except for special cases particularly involving complex units. Where appropriate, we have cited the units used in the original papers, which has generally precluded use of the more recent and more appropriate S.I. system. 

It is often necessary to convert a measurement expressed in one unit to another unit in the same system or to convert a unit in the English system to one in the metric system. To do this we follow what is known as a conversion-factor approach or dimensional analysis. For example, to convert a volume of 536 cm to liters, the relation... 

Conversions between English and metric units can be made using Table 1.3. We will call these bridge conversions. They allow you to move from one system to another. 

Conversion between the SI and English systems can be achieved by realizing that the scale in English units is 9/5 times greater than that in SI. Hence,... 

Still other units encountered in the literature and workplace come from various other systems (absolute and otherwise). These include metric systems (c.g.s. and MKS), some of whose units overlap with SI units, and those (FPS) based on English units. The Fahrenheit and Rankine temperature scales correspond to the Celsius and Kelvin, respectively. We do not use these other units, but some conversion factors are given in Appendix A. Regardless of the units specified initially, our approach is to convert the input to SI units where necessary, to do the calculations in SI units, and to convert the output to whatever units are desired. 

Chemistry is full of calculations. Our basic goal is to help you develop the knowledge and strategies you need to solve these problems. In this chapter, you will review the Metric system and basic problem solving techniques, such as the Unit Conversion Method. Your textbook or instructor may call this problem solving method by a different name, such as the Factor-Label Method and Dimensional Analysis. Check with your instructor or textbook as to for which SI (Metric) prefixes and SI-English relationships will you be responsible. Finally, be familiar with the operation of your calculator. (A scientific calculator will be the best for chemistry purposes.) Be sure that you can correctly enter a number in scientific notation. It would also help if you set your calculator to display in scientific notation. Refer to your calculator s manual for information about your specific brand and model. Chemistry is not a spectator sport, so you will need to Practice, Practice, Practice. 

There are two important conversions that you should remember between English units and the SI system. They are ... 

Table 4-2 shows how some of the basic metric units are related to units commonly used in English-speaking countries for nonscientitle measurements. Although the United States, Great Britain, and Canada have officially resolved to convert to the metric system, it will be many years before the conversion is complete. In the meantime, you must learn to convert from one system to the other. The three conversion factors given in Table 4-2 (rounded off to 2.54... 

It is important to know that all the questions are based on real numbers. In terms of measurement, units of measure are used from both the English and metric systems. Although conversion will be given between English and metric systems when needed, simple conversions will not be given. (Examples of simple conversions are minutes to hours or centimeters to millimeters.)... 

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