Temperature Scales
The temperature of a system is determined by bringing a second body, a thermometer, into contact with the system and allowing the thermal equilibrium to be reached. The value of the temperature is found by measuring some temperature dependent property of the thermometer. Any such property is called thermometric property. To assign numerical values to the thermal state of the system, it is necessary to establish a temperature scale on which the temperature of system can be read. This requires the selection of basic unit and reference state. Therefore, the temperature scale is established by assigning numerical values to certain easily reproducible states. For this purpose it is customary to use the following two fixed points:
(1) Ice Point: It is the equilibrium temperature of ice with air–saturated water at standard Atmospheric pressure.
(2) Steam Point: The equilibrium temperature of pure water with its own vapour of standard atmospheric pressure.
SCALE |
ICE POINT |
STEAM POINT |
TRIPLE POINT |
KELVIN |
273.15K |
373.15K |
273.15K |
RANKINE |
491.67R |
671.67R |
491.69R |
FAHRENHEIT |
32°F |
212°F |
32.02°F |
CENTIGRADE |
0°C |
100°C |
0.01°C |
Requirement of Temperature Scale
The temperature scale on which the temperature of the system can be read is required to assign the numerical values to the thermal state of the system. This requires the selection of basic unit & reference state.