Other scales used throughout history include Rankine, Rømer, Newton, Delisle, Réaumur, Gas mark, Leiden, and Wedgwood.
[1][2][3] In practical terms, a temperature scale is always based on usually a single physical property of a simple thermodynamic system, called a thermometer, that defines a scaling function for mapping the temperature to the measurable thermometric parameter.
Different empirical scales may not be compatible with each other, except for small regions of temperature overlap.
ITS-90 is designed to represent the thermodynamic temperature scale (referencing absolute zero) as closely as possible throughout its range.
Accordingly, ITS–90 uses numerous defined points, all of which are based on various thermodynamic equilibrium states of fourteen pure chemical elements and one compound (water).
The standard even compensates for the pressure effect due to how deeply the temperature probe is immersed into the sample.
For instance, precise measurements show that the boiling point of VSMOW water under one standard atmosphere of pressure is actually 373.1339 K (99.9839 °C) when adhering strictly to the two-point definition of thermodynamic temperature.
The virtue of ITS–90 is that another lab in another part of the world will measure the very same temperature with ease due to the advantages of a comprehensive international calibration standard featuring many conveniently spaced, reproducible, defining points spanning a wide range of temperatures.
OV is a specialized scale used in Japan to measure female basal body temperature for fertility awareness.
[citation needed] Although these defining correlations are commonly taught in schools today, by international agreement, between 1954 and 2019 the unit degree Celsius and the Celsius scale were defined by absolute zero and the triple point of VSMOW (specially prepared water).
This definition also precisely related the Celsius scale to the Kelvin scale, which defines the SI base unit of thermodynamic temperature with symbol K. Absolute zero, the lowest temperature possible, is defined as being exactly 0 K and −273.15 °C.
On 20 May 2019, the kelvin was redefined so that its value is now determined by the definition of the Boltzmann constant rather than being defined by the triple point of VSMOW.
It is based on the fundamental laws of thermodynamics or statistical mechanics instead of some arbitrary chosen working material.
Besides it covers full range of temperature and has simple relation with microscopic quantities like the average kinetic energy of particles (see equipartition theorem).
Lord Kelvin devised the thermodynamic scale based on the efficiency of heat engines as shown below: The efficiency of an engine is the work divided by the heat introduced to the system or where wcy is the work done per cycle.
(Of course any reference temperature and any positive numerical value could be used—the choice here corresponds to the Kelvin scale.)