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Mass, force and weighing- a general discussion.

Mass is a physical quantity. It is a measure of the amount of matter in an object. The unit of mass is the kilogram(kg). Mass describes a property of an object. This property is revealed not only in the inertia of an object when changing its momentum (acceleration or deceleration), but also in the force of attraction between two objects: In the earth's field of gravity each object is subject to a force W:
W = m . g
m = mass
g = local gravitational acceleration (on earth about 9.8 m/sec/sec; ,depending on exact location and altitude)
The force W is commonly referred to as weight. Because gravitational acceleration depends on a particular location, weight also depends on location. For instance, if an object is transported to the moon -with its lower field of gravity- the mass of this object remains constant even though its weight changes.
Force (or weight), and mass must not be confused: Force is the effect on an object that deforms or accelerates it (e.g. during free fall). Mass is a property of an object, describing its quantity. There is an additional difference between force and mass: To describe a force, its size and direction must be defined. For defining mass the size suffices. The units for mass and force are likewise different: Mass is measured in kilogram (abbreviated kg), force is measured in newton (abbreviated N). where 1 N = 1 kg . 1 m/sec/sec. The kilogram is a base unit of the International System of Units. It is the only base unit that is not derived from natural constants: The kilogram is defined by the mass of a prototype. This international kilogram prototype is kept at the "Bureau International des Poids et Mesures", BIPM, at Sèvres near Paris. The National kilogram prototypes are compared with the international kilogram prototype by weighing at the BIPM. The primary standards of national metrological institutions are tested against the national prototype, and are used, in turn, to test the secondary standards of verification boards. The result is a hierarchy of mass standards in which the accuracy is lower on each level, but the frequency of use is greater than that of the preceding level. (see table below)
 
Mass standards
Mass comparison
International kilogram prototype. 
Material: Pt-Ir 
Density: 21.5 g/cm3;
Primary standards of the BIPM. 
Material: Pt-Ir 
At the BIPM, when necessary. 
National kilogram prototypes. 
Material: Pt-Ir
At the BIPM, when necessary. 
e.g. every 12 years. 
Primary standards of the national metrological institution. 
Material: steel (density 8.0 g/cm3;) 
or brass (density 8.4 g/cm3;) 
At the national metrological institution, 
when necessary, e.g. every 5 years. 
Secondary standards of the metrological institution, 
standards of verification boards, 
standards of companies. 
Material: steel or brass.
At the national metrological institution, 
every 10 years or less. 
Control standards. 
At the national metrological institution, 
or at the place of use, 
every 5 years or less. 
Working standards. 
At the national metrological institution, 
or at the place of use, 
every 1 year or less. 
The standards are generally made of steel that has a density of 8.0 g/cm3;, unlike the prototypes which are made of a platinum-iridium alloy that has a density of 21.5 g/cm3;. When two weights of different density are compared, there must be a correction for air buoyancy (see also "errors in weighing").
At present the air buoyancy yields a greater uncertainty (about 1.5) than the uncertainty originating from the weighing instrument used (about 8).

National metrological institutions: In Holland the NMI (Nederlands MeetInstituut), in Germany the PTB (Physikalisch-Technische Bundesanstalt), in the U.S. the N.I.S.T. (Bureau of Standards) , in the U.K. the N.P.L. (National Physical Laboratory).
More links and addresses are to be found at the site of  B.I.P.M.