I have been reading about how the SI kilogram has been defined throughout history.

From what I understand, the kilogram was initially defined in terms of the mass of a specific volume of water at a specific temperature. However, it is now (and has been for a while) defined as exactly the mass of the International Prototype Kilogram (IPK).

I have also read that there have been efforts to redefine the kilogram through fundamental constants, so that any changes to the mass of the IPK do not affect the value of the kilogram.

My question is: Why not redefine the kilogram in terms of the mass of the volume of water at a specific temperature once again - instead of through a Watt balance or other currently proposed re-definitions?

  • 2
    $\begingroup$ What, exactly, is the physics question here? $\endgroup$
    – ACuriousMind
    Dec 10, 2016 at 17:33
  • $\begingroup$ What is the advantage to using a Watt balance to define the Kg as opposed to defining the Kg in terms of the mass of a volume of water at a specific temperature (since the density of water is known)? $\endgroup$ Dec 10, 2016 at 17:34
  • $\begingroup$ @ACuriousMind It's a metrology question. Metrology is a branch of physics. $\endgroup$
    – gerrit
    Dec 13, 2016 at 16:14

1 Answer 1


If you use a known value for the density of water, then you have circular reasoning. You have to measure the mass in order to get the density.

You can define the density of water, but then you have the issue of creating a volume of water that you know sufficiently accurately. The water would have to be pure, absent of dissolved gasses, etc, and the container would have to be known to a very great precision, evaporation would have to be precluded, and the temperature and pressure would have to be specified and maintained. Then you would somehow have to eliminate from consideration the mass of the container itself without having any mass standard to compare it against.

I suppose it's easier to implement the new proposal rather than use water.


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