- Include a title page.
- In your introduction paragraph, provide a good overview of what you will discuss.
- Include a section header and discussion for each of the concepts below:
- distinguish the chemical elements that contributed to the fire,
- recognize measurements to understand the behavior of the fire,
- describe if enthalpy was a contributing factor in fire growth,
- define concepts and terms new to you, and
- discuss these concepts based on the scenario, the “Points-to-Ponder – Building on the Scenario”, and textbook information from chapter 1 and 3. Also, include tangible arguments about the concepts.
About Measurement
Measurement is the key to understanding fire phenomena and to translating that understanding into fire safety practice. To help understand the phenomena, it is important to ask when the fire started, how rapidly it grew, how hot it became, and how severe the threat to the population was. The answers to all of these (and many other) questions are rooted in an ability to quantify. The meaning of relative terms, such as “fast moving” or “big,” varies widely depending on people’s experiences and perceptions. To a gardener, a big fire may involve a large pile of leaves; in contrast, to an insurance company, a big fire may be one that destroys a house.
Given the many different languages of the world, it is not surprising that the early cultures made up their own methods to measure objects, frequently cast in terms of properties of the human body. (That way, you always had your “yardstick” with you.) The units of measurement varied from region to region and often from person to person. For example, the Chinese measured length using the bu (about 1.67 m), the Anglo-Saxons used the ell (about 1.14 m), and the Spaniards used the vara (about 0.86 m). As cultures expanded to the point of geographical contiguity, and as trade among multiple cultures began to prosper, the need for a common set of measurements grew.
The current international measurement system, also known as the metric system, was introduced in France by Napoleon at the beginning of the 19th century. It was refined further in the 1960s, and certain units, referred to as SI units, were agreed upon. SI comes from the French name Système International d’ Unités.
All industrialized countries, with the exception of the United States and to some extent the United Kingdom, have chosen SI units to express mass, length, time, electrical current, temperature, and other measures. Adoption of the SI system facilitates the following:
• Quantitative communication regarding nearly everything, from the weather to the multitudinous forms of life.
• Exchange of manufactured products among countries.
• Computations, due to the use of factors of 10 for each unit. Instead of 12 inches in 1 foot and 5280 feet in a mile, SI uses 1000 millimeters in 1 meter and 1000 meters in 1 kilometer.
In the United States, the primary users of SI units today are scientists and engineers. In other countries, both scientists and ordinary citizens primarily use SI units or are in the process of changing over to their use. Because the data compiled in different 1
When a quantity is measured, there is a limit to the precision of the value that is obtained. If a length measurement is performed with an inexpensive ruler, it may be possible to read only the nearest millimeter marking. A value might then be reported as 147 mm, in which case the length is represented by three significant figures. With a more meticulously marked ruler and a magnifying glass, it would be possible to estimate the length more precisely and report the value to four significant figures—for example, 147.3 mm. One should report a value to the number of significant figures. Using an electronic measuring device with a 10-digit display does not increase the number of significant figures. Similarly, entering a number into a computer spreadsheet in which the cells are set to display 10 digits does not increase the number of significant figures in the value.
When estimating a calculated value, it is acceptable to speed the calculation by using fewer than the actual number of significant figures. Thus, in estimating the total surface area of the Earth, one might assume that the planet is a perfect sphere with a radius of about 6000 km. The surface area is given by the formula 4πr2 (where r = the planet’s radius), and the value of π is close to 3. The magnitude of the surface area can then be estimated at approximately 500,000,000 km2, reported to one significant figure.
Length, Area, and Volume Units
The basic SI unit of length is the meter (m). Originally, the meter was selected as 1/10,000,000 of the distance from the Earth’s equator to the North Pole. Toward the end of the 19th century, however, it was redefined as the distance between two lines on a standard bar composed of an alloy of 90 percent platinum and 10 percent iridium, measured when the bar is at the melting temperature of ice. The meter is currently defined as the length of the path traveled by light in 1/299,792,458 of 1 second.
Table 1-1 shows various SI “meter” units as they relate to the English equivalents. References [1] and [2] at the end of the chapter contain more conversions among length (and other) units.
Table 1-1 SI Length Units as Related to the Meter, with English Equivalents
Table 1-2 shows the relationship of various mass units to the kilogram (with their English equivalents). To convert from metric units to English units, multiply the metric value by the number in the right column. To convert from English units to metric units, divide the English value by the number in the right column.
Table 1-2 SI Mass Units as Related to the Gram, with English Equivalents
The concepts of mass and weight are often confused. The mass of an object is a fundamental property of the object, representing the quantity of matter in the object. An object’s mass is invariant (except in a nuclear bomb explosion, when mass changes into energy). By comparison, weight refers to the force acting on an object because of gravity attraction and is a convenient way to measure mass on Earth at sea level. If an object were on the moon, its weight would be only about one-sixth of its weight on Earth, and if the same object were in an orbiting space station; it would be nearly weightless. However, its mass would be the same in all three cases.
Density is the mass of a substance in a unit volume. It is generally is expressed in grams per cubic centimeter (g/cm³), kilograms per cubic meter (kg/m³) or, in English units, pounds per cubic foot (lb/ft3). The term specific gravity refers to the ratio of the density of a substance to that of a reference substance. For liquids and solids, the reference substance is usually water; for gases, the reference substance is air. Especially for gases and liquids, the temperature and pressure must also be specified, because the densities of the substance of interest and the reference substance depend on the temperature and pressure (Table 1-3). The densities of most solids are less sensitive to temperature and pressure.
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Time Units
Units for time are the same in the SI system and the English system. The basic unit is the second (s). Table 1-4 shows abbreviations for related time units.
Speed is the rate at which an object is moving, with typical metric units being m/s or km/h. Velocity is speed in a chosen direction. Thus, if a train is moving to the northeast at a speed of 150 km/h, its velocity in the east direction is 106 km/h (150/√2). (An alternative wording is that the train is moving eastward at 106 km/h.) Colloquially, when the speaker and the audience both understand the direction of movement, the terms may be used synonymously.
Table 1-4 Time Units (SI and English)
|
Time Unit |
Abbreviation |
|
hour |
h |
|
minute |
min |
|
second |
s |
|
millisecond |
ms |
|
microsecond |
µs |
|
nanosecond |
ns |
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Force and Pressure Units
The basic unit of force in the SI system is the newton (N). A newton is the force needed to accelerate a mass of 1 kg at the rate of 1 m/s2. In the English system, 1 lb of force is the force that will accelerate 1 lb of mass at the rate of 32.2 ft/s2. This definition was selected so that 1 lb of mass at sea level would feel a gravitational attraction of 1 lb of force. (Note the use of the same term, lb, to denote two different types of units.)
From the relation between the pound of mass and the kilogram, and the relation between the foot and the meter, it is easy to show that 1 newton is equal to 0.224 pound of force. The gravitational force on 1 kg at sea level is 9.81 N.
Pressure is force per unit area. The basic SI unit of pressure is the pascal (Pa), which is 1 N/m². One Pa is a very low pressure, so a unit called the bar is also used. A bar is defined as 100,000 Pa or 100 kilopascals (kPa). One bar is only 1.3 percent greater than normal atmospheric pressure at sea level (101.3 kPa); therefore, for approximate calculations, 1 bar is often equated to 1 atmosphere (atm).
Several English units of pressure arose out of convenience in particular applications. The following describes the more common ones:
• Testing of the fracture or deformation condition for materials gave rise to the unit of pounds per square inch (psi). The pressure of compressed gases in their storage cylinders is commonly monitored in psig, where the “g” stands for “gauge.” This is the pressure above atmospheric pressure. Pressures in psig are 14.7 psi lower than pressures in psia, where “a” stands for “absolute.”
• Manometers (glass U-shaped tubes filled with a fluid) were frequently used to measure pressure differences or absolute atmospheric pressure. When using a manometer, the measured height of the liquid column is proportional to the gas pressure. The two commonly used fluids were mercury (Hg) and water (H2O). The conversion factors from SI units are as follows:
101 kPa = 760 mm Hg (also referred to as torr)
101 kPa = 4020 in. H2O
The latter units are often used to measure the small pressure differences that arise within buildings due to the heating and air conditioning systems. Manometers are no longer in general use: mercury is toxic and must be disposed as hazardous waste, while a water manometer can be very large.
SI pressure units are becoming more widely used in these applications, but they have not fully displaced these English units in engineering practice and reference tables.
Energy and Enthalpy Units
A fire in a closed, constant-volume system generates energy. The increase in energy transfers heat to the system, raising the pressure within the volume, and increasing the temperature of the gases, the combustibles, and the “box” itself. Of course, most fires occur in an environment of nearly constant pressure, as even a small pressure increase breaks windows and otherwise spreads the combustion products out of the room of fire origin. Additional energy release is needed to expand the gases to keep the system at the starting pressure. This augmented energy release Equation 1-1):
For example, 20 °C = 293 K, and −10 °C = 263 K.
The English system uses the Fahrenheit scale (°F), where 0 °F was chosen as the temperature at which a brine solution (reached by mixing water and salt) would freeze. On this scale (at sea level), water freezes Equation 1-2:
Conversions from °C to °F are computed using the formula in Equation 1-3:
For example, 86 °F = (86 – 32)/1.8 = 30 °C, and 25 °C = (1.8 × 25) + 32 = 77 °F.
Conversion Factors
References [1] and [2] at the end of this chapter contain extensive conversion factors among SI units and other units. Some pocket calculators and websites also offer conversion factors. Table 1-5 provides conversion factors for most of the quantities discussed in this text.
In practice, it is helpful to use a consistent set of denominations for units. This reduces the likelihood of error from mixing two units of mass (e.g., g and kg) in a calculation. Two sets of metric units are commonly used:
• The meter–kilogram–second system, known as the “MKS” system. Lengths are expressed in meters (m), mass in kilograms (kg), and time in seconds (s).
• The “cgs” system. Lengths are expressed in centimeters (cm), mass in grams (g), and time in seconds (s).
Temperature units are the same (K and °C) in both systems.
MKS units are generally preferred, but the magnitude of the calculated quantity can guide the choice of system. For example, automobile velocity (tens of meters per second) might be expressed in MKS units, while the dimensions of a human finger are conveniently expressed in cm.
Table 1-5 Conversion Factors among Common Units. To convert Column A to Column B, multiply Column A by Column C. To convert Column B to Column A, divide Column B by Column C.
1 The relationships among the units of these fundamental properties are quite precise. The conversion factors in this chapter have been rounded to a number of significant figures that provides sufficient precision while maintaining ease of computation. For sources of conversion factors of higher precision, type “SI units” in a web browser and visit the presented sites or see References [1] and [2] at the end of this chapter.
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