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My teachers lied to me about Celsius!

Posted By: Taper_Mike
Date: Saturday, 7 December 2013, at 10:03 a.m.

In Response To: Ranting. Metric System. (Igor)

My teachers lied to me! Sounds they got to you too!

From Wikipedia:

## Melting and boiling points of water

One effect of defining the Celsius scale at the triple point of Vienna Standard Mean Ocean Water (VSMOW, 273.16 K and 0.01 °C), and at absolute zero (0 K and −273.15 °C), is that neither the melting nor boiling point of water under one standard atmosphere (101.325 kPa) remains a defining point for the Celsius scale. In 1948 when the 9th General Conference on Weights and Measures (CGPM) in Resolution 3 first considered using the triple point of water as a defining point, the triple point was so close to being 0.01 °C greater than water's known melting point, it was simply defined as precisely 0.01 °C.[28] However, current measurements show that the difference between the triple and melting points of VSMOW is actually very slightly (<0.001 °C) greater than 0.01 °C. Thus, the actual melting point of ice is very slightly (less than a thousandth of a degree) below 0 °C. Also, defining water's triple point at 273.16 K precisely defined the magnitude of each 1 °C increment in terms of the absolute thermodynamic temperature scale (referencing absolute zero). Now decoupled from the actual boiling point of water, the value "100 °C" is hotter than 0 °C—in absolute terms—by a factor of precisely (approximately 36.61% thermodynamically hotter). When adhering strictly to the two-point definition for calibration, the boiling point of VSMOW under one standard atmosphere of pressure is actually 373.1339 K (99.9839 °C). When calibrated to ITS-90 (a calibration standard comprising many definition points and commonly used for high-precision instrumentation), the boiling point of VSMOW is slightly less, about 99.974 °C.

Now they are planning to get rid of the water altogether:

## Proposed redefinition

In 2005 the CIPM embarked on a program to redefine, amongst others, the kelvin using a more rigorous basis than was in use. The current (2010) definition is unsatisfactory for temperatures below 20 K and above 1300 K.[11] The committee proposes defining the kelvin as the temperature scale for which Boltzmann's constant is 1.3806505×10−23 J/K exactly.[12] The committee hoped that the program will be completed in time for its adoption by the CGPM at its 2011 meeting, but at to 2011 meeting the decision was postponed to the 2014 meeting when it would be considered as part of a larger program.[13]

From a scientific point of view, this will link temperature to the rest of SI and result in a stable definition that is independent of any particular substance.

Mike

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