still a lot of confusion, but anyway, this is how it started:

history of kilogram (future is interesting too)

http://www.sizes.com/units/kilogram.htm

 

site seems to have a lot of information

http://www.sizes.com/units/base_units.htm

for instance, ”Vienna Standard Mean Ocean Water,”

http://www.sizes.com/units/temperature_kelvin.htm

As instrumentation improved, discrepancies between measurements at national labs revealed a problem with the definition of the kelvin. It turns out that the triple point of water depends significantly on the isotopic composition of the water. In 2005, at its 94th meeting, the CIPM formally recommended1 a specific isotopic composition for the water to be used, namely:

  • 0.000 155 76 mole of 2H per mole of 1H
  • 0.000 379 9 mole of 17O per mole of 16O, and
  • 0.002 005 2 mole of 18O per mole of 16O:

This composition is one used by the International Atomic Energy Agency, called “Vienna Standard Mean Ocean Water,” which the IAEA supplies as a reference standard. The use of this standard was recommended by the International Union of Pure and Applied Chemistry (IUPAC).2

 

also didn’t know about this: http://www.sizes.com/units/temperature_centigrade.htm

The problem was that the ice point, the “temperature of melting ice…at standard atmospheric pressure,” which was used to define zero degrees on the centigrade scale, cannot be measured with enough precision. Ideally one takes the temperature of a bath of pure, air-saturated water containing pure melting ice. But as ice melts it surrounds itself with a layer of insulating meltwater that is not air-saturated. The bath cannot be stirred because that would heat it.

there must be a book on this subject…

i suppose the wiki page shouldn’t be bad either, but it so happens i ran into the other one first…  http://en.wikipedia.org/wiki/SI_unit

 

different versions of calorie definition:

http://en.wikipedia.org/wiki/Calorie