052:041 Process Calculations Topic 2 Lesson 2 p. 11

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Topic 2 Lesson 2
Units and dimensions, systems of units, dimensional homogeneity

Intro to Temperature and Temperature Scales

The Fahrenheit Temperature Scale

D. G. Fahrenheit, from Phil. Trans. (London) 33, 78, 1724

"placing the thermometer in a mixture of sal ammoniac or sea salt, ice, and water a point on the scale will be found which is denoted as zero. A second point is obtained if the same mixture is used without salt. Denote this position as 30. A third point, designated as 96, is obtained if the thermometer is placed in the mouth so as to acquire the heat of a healthy man."

Other important temperatures:

Normal freezing point of water: 32°F
Normal boiling point of water: 212°F
Room temperature: approximately 72°F
Spontaneous ignition temperature of methane: 1170 °F

The Celsius (or centigrade) Temperature Scale

The value of zero degrees was set as the normal freezing point of water.

The value of 100 degrees was arbitrarily set as the boiling point of water.

Other important temperatures:

Normal freezing point of water: 0°C
Normal boiling point of water: 100°C
Room temperature: approximately 22°C
Spontaneous ignition temperature of methane: 633 °C

Conversion Factor for a Change in Temperature

The Fahrenheit degree corresponds to a smaller temperature change than a Celsius degree.

To convert a temperature change from one scale to another, the conversion factor is given below:

Conversion of a Temperature Measurement

The following equation can be used to convert a temperature measurement from one scale to another:

T(°F) = 1.8T(°C) + 32

This equation is readily derived by noting that 0°C corresponds to 32°F, and that each one Celsius degree change in temperature corresponds to 1.8 Fahrenheit degrees.

Relative Temperature Scales

For both the Fahrenheit and Celsius temperature scales, a value of zero does not correspond to a complete lack of thermal motion, and negative temperatures are possible.

	Fahrenheit Scale	Celsius Scale
Boiling point of mercury	674 degrees	357 degrees
Boiling point of water	212 degrees	100 degrees
Freezing point of water	32 degrees	0 degrees
Freezing point of water/ammonium chloride	0 degrees	-18 degrees
Boiling point of nitrogen	-321 degrees	-196 degrees

Absolute Zero Temperature

After the Fahrenheit and Celsius temperature scales were developed, it was recognized that temperature did indeed have an absolute zero (a lowest possible value) which corresponded to the complete lack of thermal motion.

	Fahrenheit Scale	Celsius Scale
Boiling point of mercury	674 degrees	357 degrees
Boiling point of water	212 degrees	100 degrees
Freezing point of water	32 degrees	0 degrees
Freezing point of water/ammonium chloride	0 degrees	-18 degrees
Boiling point of nitrogen	-321 degrees	-196 degrees
Absolute zero	-459.67 degrees	-273.15 degrees

Absolute Temperature Scales

For the Kelvin and Rankine scales, absolute zero is assigned the value of zero temperature. A Kelvin degree is the same size as a Celsius degree, while a Rankine degree is the same size as a Fahrenheit degree.

The following expressions can be used for conversions from one scale to the other:

T(K) = T(°C) +273.15

T(°R) = T(°F) + 459.67

T(°R) = 1.8T(K)