British System of Units

In the British systems of units, the pound, but never the slug, is taken as the unit of mass and temperature may be expressed either in degrees Centigrade or in degrees Fahrenheit. The units of heat are then, respectively, the pound-calorie and the British thermal unit (Btu). Where the Btu is too small for a given application, the therm (— 105 Btu) is normally used. 

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. 

The formation pressure gradient, expressed usually in pounds per square inch per foot (abbreviated by psi/ft) in the British system of units, is the ratio of the formation pressure, p, in psi to the depth, z, in feet. It is not the true gradient, dp/dz, but is strictly an engineering term. In general, the hydrostatic pressure gradient, Ph (in psi/ft), can be defined by... 

Imperial units The British system of units based on the pound and the yard. The former f.p.s. system was used in engineering and was loosely based on Imperial units for all scientific purposes SI units are now used. Imperial units are also being replaced for general purposes by metric units. 

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. 

In producing a third edition, we have taken the opportunity, not only of updating the material but also of expressing the values of all the physical properties and characteristics of the systems in the SI System of units, as has already been done in Volumes 1 and 3. The SI system, which is described in detail in Volume 1, is widely adopted in Europe and is now gaining support elsewhere in the world. However, because some readers will still be more familiar with the British system, based on the foot, pound and second, the old units have been retained as alternatives wherever this can be done without causing confusion. 

In the British system, the unit of work is called the foot-pound (ft-lb), because the pound is a unit of force and the foot is a unit of displacement. These units of work are all related by simple conversion factors. 

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. 

Eventually, ancient rules of thumb gave way to more carefully defined units. The metric system was adopted in France in 1799 and the British Imperial System of units was established in 1824. In 1893, the English units used in the United States were redefined in terms of their metric equivalents the yard was defined as 0.9144 meter, and so on. But English units continue to be used in the United States to this day, even though the Omnibus Trade and Competitiveness Act of 1988 stated that it is the declared policy of the United States...to designate the metric system of measurement as the preferred system of weights and measures for United States trade and commerce. ... 

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.)... 

With the current trend toward metrication, the question of using a consistent system of units has been a problem. Wherever possible, the authors of this Handbook of Environmental Engineering series have used the British system (fps) along with the metric equivalent (mks, cgs, or SIU) or vice versa. For the convenience of the readers around the world, this book provides a 55-page detailed Conversion Factors for Environmental Engineers. In addition, the basic and supplementary units, the derived units and quantities, important physical constants, the properties of water, and the Periodic Table of the Elements, are also presented in this document. 

Finally, the world literature on energy production and consumption is plagued by a proliferation of measurement units. Variously, data are presented in terms of the International System of Units (SI, e.g., metres, pascals, joules), traditional industry-based units e.g., barrels of oil, kilowatt hours of electricity, million tonnes of oil equivalent) and, especially in the USA, Imperial units e.g., miles, British thermal units of heat, quads of energy, cubic feet of natural gas, bars of pressure). For the expression of time, however, units of days and years are generally more appropriate than the SI unit (seconds) in this field. In order to assist readers in translating units into those with which they are familiar, a set of conversion factors has been included. 

Throughout the world, there are several systems of units in use today. The most common systems of units are International System (SI), British Gravitational (BG), and the U.S. Customary imits, which we will discuss next. 

As we explained the Btu (British thermal unit) in Chapter 11, one Btu is formally defined as the amount of thermal energy needed to raise the temperature of 1 lb of water by 1°R The calorie is defined as the amount of heat required to raise the temperature of 1 g of water by 1°C. And as you may also recall from our discussion in Chapter 11, in SI units no distinction is made between the units of thermal energy and mechanical eneigy, and therefore the units of thermal energy are defined in terms of fundamental dimensions of mass, length, and time. In the SI system of units, the joule is the unit of energy and is defined as... 

F.P.S. system n. The foot-pound-second system of units. The British system of physical units derived from the three fundamental units of length, mass, and time, i.e., the foot, pound, mass, and the second. 

Metre me-tor chiefly British variant of meter. The basic unit of length in the International System of Units (SI), equal to 39.37 in. SI spelling of meter. 

In the SI system of units the thermal conductivity is expressed in W/(m °K), and in the frequently used metric system of units the thermal conductivity is expressed in cal/(cm s °C). In British units (BU) the thermal conductivity is expressed as (BTU in)/(ft h °F). The thermal conductivity is usually expressed as k. 

Enthalpy It is a thermodynamic potential. Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy. The total enthalpy, H, of a system cannot be measured directly. Thus, change in enthalpy. Ml, is a more useful quantity than its absolute value. The change A/f is positive in endothermic reactions, and negative in exothermic processes. The unit of measurement for enthalpy in the International System of Units (SI) is the joule, but other historical, conventional units are still in use, such as the British thermal unit and the calorie. 

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