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Wednesday, July 11, 2012

Temperature scales and units

Temperature is one of the major physical parameter, which characterises the condition of substances involved in processes. In order to measure this parameter we need to choose an appropriate temperature scale and the unit of temperature.

Anders Celsius, the Swedish astronomer, devised a scale for measuring temperature, which later was named after his name. This scale has the symbol °C. The Celsius scale was based on two fixed and easily reproducible points:

• the ice point, ie the temperature of a mixture of ice and water in equilibrium with saturated air at a pressure 101325 Pa. This temperature was numbered 0°C;

• the steam point, ie the temperature of the water and steam in equilibrium at a pressure 101325 Pa. This temperature was numbered 100 °C.

Later in 1954 this scale was redefined and was based on:

• a single fixed point - the triple point of water. This is the temperature at which solid, liquid and vapour phases of water exist together in equilibrium. The temperature of the triple point of water has the value of 0.01 °C;

• the ideal-gas temperature scale. On this scale the steam point was experimentally found to be equal to 100.00 °C.

The thermodynamic scale of temperature (or the absolute scale) was derived from the second law of thermodynamics. This scale is independent of any thermometric substance. The relation between the absolute scale and the Celsius scale is as follows:
                                 T = ϑ +273.15

where: T - temperature in the absolute scale, K;
ϑ - temperature in the Celsius scale, 

The unit for the absolute scale is K - Kelvin, named after Lord Kelvin (William Thomson). 1, K= 1/273.16 of the temperature at the triple point of water.

However, for practical purposes an International Practical Temperature Scale, IPTS-68 (adopted in 1968) has been used for 22 years. This scale was based on a number of fixed and easily reproducible points. Definite numerical values of temperature were assigned to these points. For interpolation purposes between the fixed points, specified formulas relating temperature to the readings on certain temperature-measuring instruments were used. Table 1 presents the primary fixed-point temperatures for IPTS-68. (Van Wylen G.J., Sonntag R.E. Fundamentals of Classical Thermodynamics, 1980).

Table 1. : Fixed points and corresponding temperatures for IPTS-68.

In this table: s - solid; l - liquid; v - vapour.

The International Committee of Weights and Measures (CIPM) adopted a new International Temperature Scale (ITS-90) at its meeting in September 1989. It became the official international temperature scale on January 1, 1990. Table 2 shows fixed point for ITS-90. (Burns G.W., Scroger M.G., Strouse G.F., Croarkin M.C., Guthrie W.F. Temperature-Electromotive Force Reference Functions and Tables for the Letter-Designated Thermocouple Types Based on the ITS-90).

Temperatures in ITS-90 are in closer agreement with thermodynamic values when compared with IPTS-68. The increased numbers of temperature subranges makes ITS-90 more flexible. There are certain differences between ITS-90 and IPTS-68 (similar fixed points are shown in italics in these tables).

Table 2.: Fixed points and corresponding temperatures for ITS-90.

Article Source:: Dr. Alexander Badalyan, University of South Australia


Basics of Instrumentation & Control

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