This is defined as the Kelvin system which has zero as absolute zero and water melting at 273.45 (?) and boiling at 373.45 K.Has this value changed recently? It was *water melting at 273.16 K and boiling at 373.16 K*. When was it re-fixed, I seem to be out of date, here.
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(NOTE: In the following, I have typed the degree symbol as � , as in degrees Celsius ( �C ). It is possible that some of your systems will interpret this differently and print a different symbol. That is unfortunately the current and unavoidable situation in email transmission.)
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The kelvin is defined in terms of the triple point of water, not the freezing point (which depends on pressure). The triple point is the condition under which water exists in all three of its phases, solid, liquid and gas, simultaneously and in equilibrium. That occurs at just one specific pressure and just one specific temperature.
The "SI Bible", the BIPM booklet "The International System of Units (SI)", fixes the triple point of water at exactly 273.16 K. It does this by the official definition of the kelvin as "the fraction 1/273.16 of the thermodynamic temperature of the triple point of water" (page 96 of the English version.)
This definition was established by the CGPM (General Conference on Weights and Measures) in 1954. In 1968 the formal definition was reworded (without changing the basic facts of the definition) at the same time that the name of the unit was changed from the "degree Kelvin (symbol �K)" to the "kelvin (symbol K)". (See same page of the "SI Bible".)
Although the triple point is used rather than the freezing point, the triple point happens to be very close to the freezing point* (at ordinary atmospheric pressures). Therefore, it is not incorrect to say that the freezing point is "very close" to 273.16 K. With appropriate rounding for everyday uses, it would not be incorrect to say that the freezing point is equal to 273 K and that 0 �C equals 273 K. The reason for using the rather odd looking number "273.16" was (as is usually the case in refining definitions) so that the new definition of temperature units would leave the freezing point of water as close to the old definition (0 �C) as then-current techniques made possible.
Regards, Bill Hooper
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* The freezing point of water is 273.15 K, just slightly colder than the triple point. That temperature is zero on the Celsius scale (0 �C). Interestingly enough, one of the results of this is that the boiling point of water (under normal atmospheric pressure) is NOT exactly 100 �C; it is 99.975 �C. (I just discovered this myself, as I was looking up information for this message.) For everyday use, one can still use 100 �C for the boiling point.
