(¦in·tər¦nash·ən·əl ′tem·prə·chər ′skāl)

(thermodynamics) A standard temperature scale, adopted in 1990, that approximates the thermodynamic scale, based on assigned temperature values of 17 thermodynamic equilibrium fixed points and prescribed thermometers for interpolation between them. Abbreviated ITS-90.

---

IPTS, ITS

temperature (Metric) Introduced in 1927 to provide a common reproducible basis for scientific and industrial measurement of temperatures over a large range of values, this scale has been progressively refined and redefined as technologies have improved.
[Preston-Thomas H., Quinn T. J., Hudson R. P. Metrologia Vol. 21, 75-9 (1985)] Whereas the familiar everyday scale uses just two points (originally the freezing and boiling points of water, more recently ‘absolute zero’ and the triple point of water, i.e. the point of equilibrium between solid, liquid, and vapour), the compatible ITS's, to meet practical necessity, use multiple points between which it interpolates (all within defined experimental conditions).
[Hall J. A. Proc. Roy. Soc. London Ser. A Vol. 186, 179-84 (1946)]

The ITS-27 was succeeded in 1948 by ITS-48, and re-labelled in 1960 as IPTS-48 when amended but without numeric change. A greatly augmented scale, with some changes to common points too, was introduced in 1968 as IPTS-68.
[Barber C. R. Metrologia Vol. 5, 35-44 (1968)] In 1975 it was also amended without numeric change. An adjustment to the measured value of the triple point of water, and hence to the size of the kelvin, was agreed in 1988, to take effect in 1990, with ITS-90.
[Preston-Thomas H. Metrologia Vol. 27, 3-10 and 27 (1990)] The chosen points and their cited values for the three later versions are shown in Table 29.

IPTS-48	IPTS-68	ITS-90
°C	°C	K	K
	-259.34	13.81	13.803 3	triple point of hydrogen
	-256.108	17.042		intermediate equilibrium
				point of hydrogen
	-252.87	20.28		liquid hydrogen and its vapour
			24.556 1	triple point of neon
	-218.789	54.361	54.358 4	triple point of oxygen
			83.805 8	triple point of argon
-182.970	-182.962	90.188		liquid oxygen and its vapour
			234.315 6	triple point of mercury
0			‘ice point’	ice and air-saturated water
	0.01	273.16	273.16	triple point of water
			302.914 6	freezing point of gallium
100	100	373.15	‘steam	liquid water and its vapour
			point’
			429.748 5	melting point of indium
444.600				liquid sulphur and its vapour
			505.078	melting point of tin
	419.58	692.73	692.677	melting point of zinc
			933.473	melting point of aluminium
960.8	961.93	1 235.08	1 234.93	melting point of silver
1 063.0	1 064.43	1 337.58	1 337.33	melting point of gold
			1 357.77	melting point of copper
The element-specific points are often called the ‘sulphur point’, etc.

Above the uppermost points, temperatures are measured by means of the Planck radiation formula.
[Jones T. P., Topping J. Metrologia Vol. 8, 4-11 (1972)] Below 13.81 K the scale was undefined prior to ITS-90; the helium scale and Curie temperature scale providing in part for this ultra-low zone. Using helium, ITS-90 extends the scale to 0.65 K; for 0.65 to 3.2 K this uses the relation of vapour pressure to temperature, from 3.2 to 24.556 1 K it uses the constant-volume gas thermometer. Work is active in extending the range down to 0.001 K, based on the melting pressure of helium.
[http://www.bipm.org/en/committees/]