Capítulo 4. Dotación contra incendios del establecimiento industrial
4.2 Requisitos de las instalaciones de protección contra incendios
This is a complete system of measurement units including names and symbols for base units from which derived units may be formed so that any physical quantity may be expressed. It includes a system of prefixes by which the base and derived units may be made any convenient size from very small to very large. Finally, the precise basis for its units, and the symbols for expressing them, have received worldwide agreement. It is officially recognized by all industrial nations, is referenced by SAE, ASTM, ASME, and many other soci-eties, is required by ISO in all documents, and is the official basis of our U.S.
units (the inch and pound are defined in terms of the metre and kilogram).
Fortunately, there is much about this system that is old and familiar. There is also much that is new.
Base Units There are seven base units of SI. These units and the quantities for which they are used are listed in Table 7.3.
TABLE 7.3 Seven Base Units of SI
Quantity Unit Symbol
Length metre m
Mass kilogram kg
Time second s
Electric current ampere A
Thermodynamic temperature kelvin K
Amount of substance mole mol
Luminous intensity candela cd
All of these are defined in terms of readily reproducible natural phenomena except for the kilogram. This is based on a prototype kept at the International Bureau of Weights and Measures, copies of which are in use all over the world.
Supplementary Units Two more essential units are included in the system, but agreement could not be reached whether these were base or derived.
Accordingly, they were designated as supplementary (see Table 7.4).
Derived Units The nine preceding units may be combined mathematically (by multiplication or division) to provide new units as required, for measure-ment of any physical quantity. Fifteen of these have been given special names and symbols, and the rest are created at will to suit the need.
Those derived units with special names, including their formulas for defi-nition, are listed in Table 7.5.
All other necessary derived units are produced by multiplying or dividing the base and derived units according to the definition of the quantity to be
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TABLE 7.5 Derived Units
Quantity Unit Symbol Formula
Frequency hertz Hz s1
Force newton N kg m/s2
Pressure, stress pascal Pa N/m2
Energy, work joule J N m
Power watt W J/s
Quantity of electricity coulomb C A s
Electric potential volt V W/A
Capacitance farad F C/V
Electric resistance ohm Ω V/A
Conductance siemens S A/V
Magnetic flux weber Wb V s
Magnetic flux density tesla T Wb/m2
Inductance henry H Wb/A
Luminous flux lumen lm cd sr
Illuminance lux lx lm/m2
TABLE 7.4 Supplementary Units
Quantity Unit Symbol
Plane angle radian rad
Solid angle steradian sr
measured. A few are shown here as examples, but of course no list could be complete (see Table 7.6).
Multiplying Prefixes Any of these units are then made larger or smaller if desired by the addition of one of a list of prefixes provided for this purpose (see Table 7.7).
This is the new system. Much is common with the old metric system, but SI differs sharply from customary European metric use. Most of these differences are related to “coherence.”
TABLE 7.6 Other Derived Units
Quantity Unit Symbol and Formula
Area square metre m2
Volume cubic metre m3
Velocity metre per second m/s
Acceleration metre per second squared m/s2
Density kilogram per cubic metre kg/m3
Specific volume cubic metre per kilogram m3/kg
Entropy joule per kelvin J/K
Radiant intensity watt per steradian W/sr
Bending moment, torque newton-metre N m
Heat capacity joule per kilogram-kelvin J/kg K
TABLE 7.7 SI Prefixes
Multiplication Factor Prefix Symbol
1012 tera T
109 giga G
106 mega M
103 kilo k
102 hecto h
101 deka da
101 deci d
102 centi c
103 milli m
106 micro μ
109 nano n
1012 pico p
1015 femto f
1018 atto a
Coherence In SI every derived unit is the result of the equation expressing some physical law. The unit of area is the square of the unit of length, or square metre, because the area of a rectangle is the product of the length of its two sides. Similarly, the unit of velocity is metre per second, since velocity is defined as distance traveled divided by time. When all derived and funda-mental units are related in this way, a system is called “coherent,” and each of its units is a coherent unit. The factor relating all units to each other is always one. The coherence of SI is one of its most important characteristics: A force of one newton operating through a length of one metre produces energy of one joule. If this energy is produced in one second, the power is one watt.
Two other major characteristics of SI are directly related to coherence, but merit separate discussion.
An Absolute System The relationship between force and mass is tangled in a complicated history of differing customs. In a gravitational system the units of mechanics are all derived from the three fundamentals, length, force, and time, and the unit of mass is a derived unit. In an absolute system, on the other hand, the units of mechanics are derived from the three fundamentals, length, mass, and time, and the unit of force is a derived unit. In either of these two systems, the units of mass and force are always related by Newton’s law F ma, and in a coherent system these cannot be the same unit. Properly, the units in the world’s measurement systems should be as shown in Table 7.8.
Unfortunately, common use all over the world has ignored the existence of these two kinds of systems and has casually used the same units for force and mass. We speak of stress in pounds, pressure in pounds per square inch, mass in pounds, density in pounds per cubic inch, using the pound for both force and mass. The European treats the kilogram in the same fashion.
SI is an absolute system, and must be used as such. A most difficult prob-lem in transition to SI will be to learn to use different units for force and mass.
Mass—unit, the kilogram Force—unit, the newton
An absolute system has several advantages, the greatest being simplicity of calculations. A force of a newton accelerates a mass of one kilogram one metre per second squared. In contrast, a force of one kilogram accelerates the same mass 9.80665 metres per second squared.
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Complicating this change is the fact that the term weight has traditionally been badly misused. Its formal definition has always been that of a force—
loosely the force of gravity acting on a mass—and the unit of weight has been the force of standard gravity on a unit of mass. Early in the development of measurement systems, the word was incorrectly used to mean mass; thus we have a Bureau of Weights and Measures, and we use “weights” on a scale to measure mass. As a result the term has been widely used to mean either a force or a mass.
The future of the term is somewhat uncertain. By far the most common use has been for mass, and strong pressure exists to redefine the term to this mean-ing. The urgent need, however, is to separate force and mass and use the proper SI units for each, as just shown. The ASTM Metric Practice Guide (ASTM E 380-74) states, “In commercial and everyday use on the other hand, the term weight nearly always means mass. Thus when one speaks of a person’s weight, the quantity referred to is mass. Because of this dual use, the term weight should be avoided except under circumstances in which its meaning is com-pletely clear.”
Uniqueness SI is a unique system in which there is only one unit for each kind of physical quantity, regardless of whether it is mechanical, electrical, or thermal. Power in engines or air conditioners is measured in watts. Of course, this rule does not prevent the use of either a special name or the derived name for a unit—pressure may be expressed either in pascals or in newtons per square metre.
Vector Influence Some units, such as torque, involve vector quantities, but in common with other systems of measurement units, the symbols and names do not indicate this fact. As a result, an apparent anomaly exists in the use of newton-metre for torque or bending moment, and the use of joule for work (1 J 1 Nm). These are entirely different units, since the unit of work results from unit force moving through unit distance, while the unit of bending moment involves a force applied normal to a lever of unit length. This would be readily seen if vectors were incorporated in the unit symbols. For this rea-son it is important not to express moments in joules.