INTERNATIONAL PROTOTYPE KILOGRAM ? WHY THERE IS A RECENT PROPOSAL TO REDEFINE THIS INTERNATIONAL PROTOTYPE KILOGRAM

Kilogram, the base SI unit of mass is equal to the International Prototype Kilogram which is almost exactly equal to the mass of one litre of water.

International Prototype Kilogram (IPK) is a cylinder which is roughly the size of a golf ball and is made of 90%platinum and 10%iridium and is placed in the International Bureau of Weights and Measures at Sevres, France. Like other prototypes the edges of IPK have four angle chambers to minimize wear.

The IPK is in custody of International Bureau for Weights and Measures (BIPM) from 1889 that hold it on behalf of the General Conference on Weights and Measures.
After the IPK had been found to vary in mass over time, the International Committee on Weights and Measures recommended in 2005 that the kilogram be redefined in term of a fundamental constant of nature.

At its 2011 meeting, the General Conference on Weights and Measures (CGPM) agreed in principle that the kilogram should be redefined in term of Planck constant, but deferred a final decision until its next meeting, scheduled for 2014.

International Prototype Kilogram (IPK) is man-made (artifact) <made of platinum iridium alloy> stored in the vault of International Bureau of weight and measure (BIPM) on behalf of General conference on weight and measures (CGPM), which defines the magnitude of 1kg (since 1889).

-> In 2005, International Committee for weight and measurement (CIPM) recommended to redefine weight in terms of Fundamental constant of nature 

-> Then in 2011, CGPM accept to redefine it in terms of Planck's content - final decision to be made in next meet (2014)

Reason for the debate:
mass is invariant whereas the weight depends on strength of gravity. According to Einstein's Relativity theory, Mass of a body changes with velocity(although the change is very feeble, it is found to be varying over time)

Note:
In SI system:

 Mass -> Kg
In Avoirdupois:

Mass -> Pound = 16 Ounce = 0.45 Kg

In Troy weight: Mass -> Troy Ounce = 12th of Troy pound= 0.03 Kg (used to weigh Precious metal esp. gold)