Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

assigned the value 273.16 K. The Celsius scale was also redefined at this

conference in terms of the ideal-gas temperature scale and a single fixed

point, which is again the triple point of water with an assigned value of

0.01°C. The boiling temperature of water (the steam point) was experimen-

tally determined to be again 100.00°C, and thus the new and old Celsius

scales were in good agreement.

The International Temperature

Scale of 1990 (ITS-90)

The International Temperature Scale of 1990,which supersedes the Inter-

national Practical Temperature Scale of 1968 (IPTS-68), 1948 (ITPS-48),

and 1927 (ITS-27), was adopted by the International Committee of Weights

and Measures at its meeting in 1989 at the request of the Eighteenth Gen-

eral Conference on Weights and Measures. The ITS-90 is similar to its pre-

decessors except that it is more refined with updated values of fixed

temperatures, has an extended range, and conforms more closely to the

thermodynamic temperature scale. On this scale, the unit of thermody-

namic temperature T is again the kelvin (K), defined as the fraction

1/273.16 of the thermodynamic temperature of the triple point of water,

which is sole defining fixed point of both the ITS-90 and the Kelvin scale

and is the most important thermometric fixed point used in the calibration

of thermometers to ITS-90.

The unit of Celsius temperature is the degree Celsius (°C), which is by

definition equal in magnitude to the kelvin (K). A temperature difference

may be expressed in kelvins or degrees Celsius. The ice point remains the

same at 0°C (273.15°C) in both ITS-90 and ITPS-68, but the steam point

is 99.975°C in ITS-90 (with an uncertainly of 0.005°C) whereas it was

100.000°C in IPTS-68. The change is due to precise measurements made by

gas thermometry by paying particular attention to the effect of sorption (the

impurities in a gas absorbed by the walls of the bulb at the reference tem-

perature being desorbed at higher temperatures, causing the measured gas

pressure to increase).

The ITS-90 extends upward from 0.65 K to the highest temperature prac-

tically measurable in terms of the Planck radiation law using monochro-

matic radiation. It is based on specifying definite temperature values on a

number of fixed and easily reproducible points to serve as benchmarks and

expressing the variation of temperature in a number of ranges and subranges

in functional form.

In ITS-90, the temperature scale is considered in four ranges. In the

range of 0.65 to 5 K, the temperature scale is defined in terms of the vapor

pressure—temperature relations for^3 He and^4 He. Between 3 and 24.5561 K

(the triple point of neon), it is defined by means of a properly calibrated

helium gas thermometer. From 13.8033 K (the triple point of hydrogen) to

1234.93 K (the freezing point of silver), it is defined by means of platinum

resistance thermometers calibrated at specified sets of defining fixed

points. Above 1234.93 K, it is defined in terms of the Planck radiation law

and a suitable defining fixed point such as the freezing point of gold

(1337.33 K).

20 | Thermodynamics

• 273.15

°C

0

0.01 273.16

K

• 459.67

°F

0

32.02 491.69

R

Triple

point

of water

Absolute

zero

FIGURE 1–34

Comparison of temperature scales.