International Practical Temperature Scale

international practical temperature scale [¦in·tər¦nash·ən·əl ¦prak·tə·kəl ′tem·prə·chər ‚skāl]

(thermodynamics)

Temperature scale based on six points: the water triple point, the boiling points of oxygen, water, sulfur, and the solidification points of silver and gold; designated as °C, degrees Celsius, ort_int; replaced in 1990 by the international temperature scale.

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International Practical Temperature Scale 

(IPTS-68), a temperature scale established in 1968 by the International Committee on Weights and Measures on the basis of 11 primary reproducible temperature points (see Table 1), each of which has been assigned a fixed temperature value. IPTS-68 distinguishes the international practical Kelvin temperature (symbol T₆₈} and the international practical Centigrade temperature (symbol t₆₈. The relationship between T₆₈ and t₆₈ is t₆₈ = T₆₈ - 273.15°K.

Table 1. Main fixed (constant) points of the International Practical Temperature Scale (1968)
Equiliburium states	Accepted value of international practical temperature1
	T68 (°K)	t68 (°C)
′With the exception of the triple points and one point of hydrogen at equilibrium (17.042° K), the accepted values for temperature are valid for a state of equilibrium at a pressure of 101,325 N/m2 (1 normal atmosphere)
Triple point of hydrogen at equilibrium ........	13.81	-259.34
Equilibrium of liquid and gaseous phases of hydrogen at equilibrium at a pressure of 33,330.6 N/m2 (25/76 of a normal atmosphere)	17.042	-256.108
Boiling point of hydrogen at equilibrium	20.28	-252.87
Boiling point of neon............	27.102	-246.048
Triple point of oxygen	54.361	-218.789
Boiling point of oxygen	90.188	-182.962
Triple point of water	273.16	0.01
Boiling point of water	373.15	100
Solidification point of zinc	692.73	419.58
Solidification point of silver	1235.08	961 .93
Solidification point of gold	1337.58	1064.43

The intermediate points of IPTS-68 are reproduced according to interpolation formulas, which establish the relation between temperature and the thermometric properties of instruments calibrated according to these points.

In the range between 13.8°K and 630.74°C, a platinum resistance thermometer is used as the standard instrument; in the range from 630.74°C to 1064.43°C, the instrument used is a thermocouple with platinum-10 percent rhodium/platinum electrodes. Above 1337.58°K (1064.43°C), IPTS-68 is determined by means of Planck’s law of radiation. In the region of low temperatures, IPTS-68 has been extended to 13.8TK. Temperatures in the range from 0.3° to 5.2°K are determined from the vapor pressure of liquid ⁴He (1958 scale) and liquid ³He (1962 scale). Even lower temperatures are determined by resistance thermometers made from carbon, germanium, or superconducting alloys and by magnetic methods.

Temperatures determined by IPTS-68 correspond to the temperatures on the thermodynamic temperature scale (to the accuracy of modern measurements), which has been accepted as the basic scale in physics.