Temperature conversion between scales

At many other places in the text, linear relationships occur that were not shown in graphical terms. For example, the conversion between temperature scales in Chapter 1 can also be expressed in the form of a straight line ... 

Routine measurements of temperature are done with a thermometer. Thermometers found in chemistry laboratories may use either mercury or a colored fluid as the liquid, and degrees Celsius (°C) as the units of measurement. The fixed reference points on this scale are the freezing point of water, 0°C, and the boiling point of water, 100°C. Between these two reference points, the scale is divided into 100 units, with each unit equal to 1°C. Temperature can be estimated to 0.1°C. Other thermometers use either the Fahrenheit (°F) or the Kelvin (K) temperature scale and use the same reference points, that is, the freezing and boiling points of water. Conversion between the scales can be accomplished using the formulas below. 

Note that the lUPAC values refer to the lPTS-68 temperature scale, while the NIST values are based on the lTS-90 scale (where the normal boiling point is 99.974°C). The conversion between these scales can be found in Sec. 1. The difference between the scales leads to a difference in the density of water of about 20 ppm in the neighborhood of 100°C and much less at lower temperatures. 

A temperature scale that never quite caught on was formulated by the Austrian chemist Johann Sebastian Farblunget. The reference points on this scale were 0 FB, the temperature below which Farblunget s postnasal drip began to bother him, and 1000 FB, the boiling point of beer. Conversions between C and °FB can be accomplished with the expression... 

T. B. Douglas, Conversion of Existing Calorimetrically Determined Thermodynamic Properties to the Basis of the International Practical Temperature Scale of 1968, J. Res. Natl. Bur. Stand. (U.S.) Sect. A 73, 451 (1969). (Formulas for converting H, Cp, S, and G from one temperature scale to another. Differences between the two scales are tabulated from 90 to 10 000 K.)... 

In chemistry, you will have to use both the Celsius and Kelvin scales. At times, you will have to convert temperature values between these two scales. Conversion between these two scales simply requires an adjustment to account for their different zero points. 

When the temperature scale for the TAP measurements is adjusted to higher temperatures by approximately 100 K, a quite good correlation between the model gas and the TAP experiments is found for both the NOx conversion and the N2 selectivity and for both the stoichiometric and the lean gas mixture. The NOx conversion increases for the stoichiometric mixture with increasing temperature. The NOx conversion for the lean mixture shows a maximum in conversion at approximately 575 K (model gas temperature). The selectivity towards N2 shows for both gas mixtures a minimum around 575 K. This is of course equivalent to a maximum in N2O selectivity. In contrast to the conversion of NOx, the selectivity towards N2 is not strongly depending on the composition. From these figures it appears that the higher O2 concentration in the lean gas mixture compared to the... 

The conversion between °C and K is given on p. 18. In most calculations we will use 273 instead of 273.15 as the term relating K and °C. By convention, we use T to denote absolute (kelvin) temperature and t to indicate temperature on the Celsius scale. The dependence of the volume of a gas on temperature is given by... 

The conversion between the Fahrenheit and Celsius temperature scales requires two adjustments ... 

The three temperature scales are defined and compared in Fig. 1.9. Note that the size of the temperature unit (the degree) is the same for the Kelvin and Celsius scales. The fundamental difference between these two temperature scales is in their zero points. Conversion between these two scales simply requires an adjustment for the different zero points. 

The book contains both the metric and nonmetric measures of temperature (Celsius and Fahrenheit). However, it should be noted that exact conversion of the two scales is not often possible and, accordingly, the two temperature scales are interconverted to the nearest 5. At the high tan-peratures often quoted in the process sections, serious error will not arise from such a conversion. With regard to the remaining metric/nonmetric scales of measurement, there are also attempts to indicate the alternate scales. For the sake of simplicity and clarity, simple illustrations (often line drawings) are employed for the various process options, remembering, of course, that a line between two reactors may not only be a transfer pipe but also a myriad of valves and control equipment. 

Scientific uses of temperature require yet another temperature scale. The choice of the kelvin as the standard reflects mathematical convenience more than familiarity. The Kelvin scale is similar to the Celsius scale but draws its utility from the fact that the lowest temperature theoretically possible is zero kelvin. It violates the laws of namre to go below 0 K, as we will see in Chapter 10. The mathematical importance of this definition is that we are assured we will not divide by zero when we use a formula that has temperamre in the denominator of an expression. Conversions between Celsius degrees and kelvins are common in science and are also more straightforward. 

One of the early triumphs of the study of thermodynamics was the demonstration that there is an absolute zero of temperature. However, there are several different temperature scales, for historical reasons. All you need to know about this is that the kelvin scale (named after William Thompson, Lord Kelvin) has an absolute zero of OK and a temperature of 273.16K at the triple point where water, ice, and water vapor are at equilibrium together. The melting point of ice at one atmosphere pressure is 0.01 degrees less than this, at 273.15 K (Figure 3.1). The Celsius scale (named after Anders Celsius, a Swedish astronomer) has a temperature of 0 °C at the ice point (273.15 K) and absolute zero at -273.15°C. This gives almost exactly 100 C between the freezing and boiling points of water at one atmosphere, so water boils at 100 "C (373.15 K). Thus the numerical conversion between the two scales is... 

Perform simple temperature conversions between the Fahrenheit, Celsius, Kelvin, and Rankine scales. 

Conversions between these temperature scales are relatively straightforward and are achieved using the following formulas ... 

The conversion of p, V, T data published earlier on to the 1968 International Practical Temperature Scale will require a painstaking review of the whole literature a method of proceeding has been discussed by Angus. In this article we mainly deal with / , K, T data that relate to differences between real- and ideal-gas properties and as these differences are generally not established to an accuracy such that changes in definition of temperatures are relevant, we avoid the problem of revision here. The larger problem of converting measured thermodynamic properties that depend on temperature from IPTS 48 to IPTS 68 has been discussed by Rossini. ... 

Sample Problem 1.1 illustrates conversions between these two temperature scales. 

The solution to this problem lead to the creation of absolute temperature scales, ones on which 0° truly means the absence of any heat. The first absolute temperature scale was the Kelvin scale, named for the British physicist William Thomson (1824-1907), who went by the title Lord Kelvin. It is perhaps a technical point, but absolute temperature scales do not use the degree symbol hence, a temperature on the Kelvin scale is expressed in kelvins (k), not degrees Kelvin, and not capitalized. The size of a degree on the Kelvin scale is the same as the size of a degree on the Celsius scale, so the conversion between the two scales is relatively simple ... 

For equations of state such as equation (8.1) or (8.5), each of which is a dimensionless function of reduced density and temperature, the user can in principle select the desired units by suitable choice of the reducing parameters p and T. While for p this is straightforward, over the years there have been several different agreed international temperature scales, and there are no simple conversion factors between them. In 1990 a change was made from the then current scale, known as IPTS-68, to the present approved scale ITS-90. Some accurate equations of state which are recommended in Section 8.3 are based on IPTS-68 and others on ITS-90 conversion from one scale to the other can be made using the equations given by Rusby (1991). For the temperature range = 73.15 K to 903.89 K, which will cover most of the fluids of concern to readers of this book, the equation is... 

Fahrenheit scale A temperature scale invented in 1714 by German physicist and instrument-maker Gabriel Daniel Fahrenheit (1686-1736). Fahrenheit was the first person to make practical thermometers using mercury irrstead of spirits of wine (alcohol). He calibrated his thermometer between two fixed points for which he used the eutectic point of salt water as 0°, which was the lowest temperature he could obtain in the laboratory, and the temperature of a healthy human which he originally called 12° and later 96°. He found that on his scale, pure ice melted at 32°F and steam at normal atmospheric pressure was 212°F. The conversion to the Celsius scale is given by ... 

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