Scales, temperature

In chemistry, temperatures are usually expressed in metric units of degrees Celsius (°C), in which water freezes at 0°C and boils at 100°C. The Fahrenheit scale, still used for some non-sdentific temperature measurements in the United States, defines the freezing temperature of water at 32 degrees Fahrenheit (°F) and boiling at 212°F, a range of 180°F. Therefore, each span of 100°C is equivalent to one of 180°F and each °C is equivalent to 1.8°F. 

Unit of Volume Abbreviation Number of Liters Example of Use for Measuremmt 

Kiloliter or cubic meter kL 103 Volumes of air in air pollution studies 

Liter L 1 Basic metde unit of volume (1 liter = 1 dm3 = 1.057 quarts 1 cubic foot = 28.32 L) 

Milliliter ml. 10-3 Equal to 1 cm3. Convenient unit for laboratory volume measurranraits 

Obviously the empirical temperature scale we have defined has some defects from a practical point of view. The first defect is that although we may feel that Fahrenheit made the only wise choice of reference states in choosing the temperature of a mixture of ice, NH4CI, and water and the temperature of the human body, it may happen that someone else will feel that the temperature of the dew at daybreak on the first day of summer and the temperature of the flame of a wax candle would be much better reference states for a temperature scale, and that 666 would be a good number of divisions to have between them. It is even conceivable that others might find the 

A second major defect of the empirical temperature is that any property of any substance that varies appreciably as the substance warms and cools can be used, and all are assumed to change in a linear fashion between and beyond the two reference temperatures. But in fact observation shows that in general, substances are all different in the way they respond to temperature changes. The result is that even if one particular set of reference temperatures were chosen., different thermometric substances would give different temperatures for states in between the two reference states. This state of affairs makes the empirical temperature seem a rather dubious sort of fundamental property. 

There are several temperature scales in existence. Canadian readers are accustomed to using the Celsius scale (°C). Readers who live in the United States are more familiar with the 

Fahrenheit scale, (°F). Equivalent temperatures on the two scales include the following Boiling point of water (at 1 atm) 100°C 212°F 

A little arithmetic shows that the difference between the boihng point and the freezing point of water is 100° on the Celsius scale and 180° on the Fahrenheit scale. Therefore, the magnitude of a Celsius degree is 180 100 = 1.8 times the size of a Fahrenheit degree. In 

For example, if T = 21°C (a pleasant spring or fall day), on the Fahrenheit scale the temperature would be 70°F. If T = -10°F (a cold northern winter day), on the Celsius scale the temperature would be -23°C. 

The general concept of temperature scales is discussed briefly in Exp. 1. The thermodynamic temperature scale, based on the second law of thermodynamics, embraces the Kelvin (absolute) scale and the Celsins scale, the latter being defined by the equation 

The size of the kelvin, the SI temperature unit with symbol K, is defined by the statement that the triple point of pure water is exactly 273.16 K. The practical usefulness of the thermodynamic scale suffers from the lack of convenient instruments with which to measure absolute temperatures routinely to high precision. Absolute temperatures can be measured over a wide range with the helium-gas thermometer (appropriate corrections being made for gas imperfections), but the apparatus is much too complex and the procedure much too cumbersome to be practical for routine use. 

TABLE 1 Fixed points of the International Temperature Scale of 1990 (ITS-90) 

The International Practical Temperature Scale of 1968 (IPTS-68) has been replaced by the International Temperature Scale of 1990 (ITS-90). The ITS-90 scale is basically arbitraiy in its definition but is intended to approximate closely the thermodynamic temperature scale. It is based on assigned values of the temperatures of a number of defining fixed points and on interpolation formulas for standard instruments (practical thermometers) that have been cahbrated at those fixed points. The fixed points of ITS-90 are given in Table 1. 

The major change from IPTS-68 to ITS-90 has been the elimination of the normal (i.e., 1-atm) boiling points that were previously used as fixed points. This change was made because the temperature of a boiling point is much more sensitive to the ambient pressure than that of a freezing point. The latter is defined as the equilibrium temperature of coexisting pure solid and liquid at one standard atmosphere (101 325 Pa), and corrections can be made for small pressure deviations (the effect is only about 5 mK per atm).  

Since 1 degree on the Celsius scale is the same as 1 unit on the Kelvin scale, the relationship between the two scales is a simple one. 

FIGURE 2.7 A comparison of three common temperature scales, Fahrenheit, Celsius, and Kelvin. 

On the Kelvin scale, the freezing point of water is 273.15 K and the boiling point of water is 373.15 K. 

To convert from Celsius to Fahrenheit, multiply by, then add 32.0°F. 

The Kelvin temperature scale is defined to have the freezing point of pure water as 273.15 K and the normal boiling point of pure water as 373.15 K. The unit of the Kelvin temperature scale is the kelvin. (We do not use degrees with kelvins.) Thus, its temperatures are essentially 273° higher than the same temperatures on the Celsius scale. To convert from degrees Celsius to kelvins, merely add 273° to the Celsius temperature. To convert in the opposite direction, subtract 273° from the Kelvin temperature to get the Celsius equivalent. (See Fig. 2-2.) 

Note that a change in temperature in kelvins is the same as the equivalent change in temperature in degrees Celsius. 

EXAMPLE 2.39. Convert 0°C and 30°C to kelvins. Calculate the change in temperature from 0°C to 30°C on both temperature scales. 

(a) Write the reciprocal for the following factor label 4.0 mi/h. (b) Which of these—the reciprocal or 

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Now cascade any surplus heat down the temperature scale from interval to interval. This is possible because any excess heat available from the hot streams in an interval is hot enough to supply a deficit in the cold streams in the next interval down. Figure 6.18 shows the cascade for the problem. First, assume that no heat is supplied to the first interval from a hot utility (Fig. 6.18a). The first interval has a surplus of 1.5 MW, which is cascaded to the next interval. This second interval has a deficit of 6 MW, which reduces the heat cascaded from this interval to -4.5 MW. In the third interval the process has a surplus of 1 MW, which leaves -3.5 MW to be cascaded to the next interval, and so on. 

Looking at the heat flows in Fig. 6.18a, some are negative, which is infeasible. Heat cannot be transferred up the temperature scale. To... 

Figure 15.1a shows a single-stage evaporator represented on both actual and shifted temperature scales. Note that in shifted temperature scale, the evaporation and condensjftion duties are shown at different temperatures even though they are at the same actual temperature. Figure 15.16 shows a similar plot for a three-stage evaporator. 

To generate characteristic velocities and bring a molecular system toequillbrium at th e sim illation temperature, atom s are allowed to in teract W ith each other through the equation s of motion. For isothermal simulations, a temperature bath" scales velocities to drive the system towards the simulation temperature,. Scaling occurs at each step of a simulation, according to equation 2S. 

The new international temperature scale, known as ITS-90, was adopted in September 1989. However, neither the definition of thermodynamic temperature nor the definition of the kelvin or the Celsius temperature scales has changed it is the way in which we are to realize these definitions that has changed. The changes concern the recommended thermometers to be used in different regions of the temperature scale and the list of secondary standard fixed points. The changes in temperature determined using ITS-90 from the previous IPTS-68 are always less than 0.4 K, and almost always less than 0.2 K, over the range 0-1300 K. 

Defining fixed points of the International Temperature Scale of 1990 (ITS-90). Except for the triple points, the assigned values of temperature are for equilibrium states at a pressure of one standard atmosphere (101 325 Pa). 

Much more information can be obtained by examining the mechanical properties of a viscoelastic material over an extensive temperature range. A convenient nondestmctive method is the measurement of torsional modulus. A number of instmments are available (13—18). More details on use and interpretation of these measurements may be found in references 8 and 19—25. An increase in modulus value means an increase in polymer hardness or stiffness. The various regions of elastic behavior are shown in Figure 1. Curve A of Figure 1 is that of a soft polymer, curve B of a hard polymer. To a close approximation both are transpositions of each other on the temperature scale. A copolymer curve would fall between those of the homopolymers, with the displacement depending on the amount of hard monomer in the copolymer (26—28). 

The absolute temperature scale that corresponds to the Celsius scale is the Kelvin scale for the Fahrenheit scale, the absolute scale is called the Rankine scale. The Celsius scale reads 0 when the Kelvin scale reads 273 the Fahrenheit scale reads 0 when the Rankine scale reads 460. These relationships are shown in Figure 1. 

Accurate temperature measurements in real-life situations are difficult to make using the KTTS. Most easily used thermometers are not thermodynamic that is, they do not operate on principles of the first and second laws. Most practicable thermometers depend upon some principle that is a repeatable and single-valued analogue of temperature, and they are used as interpolation devices of practical and utilitarian temperature scales which are themselves... 

The KTTS depends upon an absolute 2ero and one fixed point through which a straight line is projected. Because they are not ideally linear, practicable interpolation thermometers require additional fixed points to describe their individual characteristics. Thus a suitable number of fixed points, ie, temperatures at which pure substances in nature can exist in two- or three-phase equiUbrium, together with specification of an interpolation instmment and appropriate algorithms, define a temperature scale. The temperature values of the fixed points are assigned values based on adjustments of data obtained by thermodynamic measurements such as gas thermometry. 

Many special-purpose electrical thermometers have been developed, either for use in practical temperature measurement, or as research devices for the study of temperature and temperature scales. Among the latter are thermometers which respond to thermal noise (Johnson noise) and thermometers based on the temperature dependence of the speed of sound. 

Some of tfie physical piopeities of tungsten ate given in Table 3 fuithei property data ate available (12—14). For thermodynamic values. References 5,15, and 16 should be consulted. Two values are given for the melting point. The value of 3660 K was selected as a secondary reference for the 1968 international practical temperature scale. However, since 1961, the four values that have been reported ranged from 3680 to 3695 and averaged 3688 K. 

The option most promising today iavolves use of a new family of polymers, the so-called high temperature scale-control chemicals. These are compounds that, added ia 3—8 ppm, lead to lattice distortioa and the formation of a nonadhering scale. Belgard (CIBA-GEIGY) was the first compound... 

Vapor pressure data for soHd carbon dioxide are given in Table 2 (10). The sublimation temperature of soHd carbon dioxide, 194.5 K at 101 kPa (1 atm), was selected as one of the secondary fixed points for the International Temperature Scale of 1948. 

They are basically temperature monitoring devices and can indicate the temperature on a temperature scale. They may be one of the following types. 

As Fig. 20.7 shows, if DS eutectics ( DSEs ) prove successful, they will allow the metal temperature to be increased by =100°C above conventional DS nickel alloys, and the inlet temperature by =200°C (because of a temperature scaling effect caused by the blade cooling). Further improvements in alloy design are under way in which existing nickel alloys and DS eutectics are being blended to give a fibre-reinforced structure with precipitates in the matrix. 

It must be modified to incorporate velocity-dependent forces or temperature scaling. 

No. Chemical Temperature Scale No. Chemical Temperature Scale No. Chemical Temperature Scale... 

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