what is the science behind calibration weights ? How can we be so sure that the weight we’ve been using to calibrate is exact and not a few decimals off, which would make the concept of density completely wrong ?
In: Physics
Weight is a unit of force, the SI unit for it is Newton. A bit simplified it is the force of gravity on a mass on the surface of the earth. Because gravity depends on the location on earth, the difference is on the surface of Earth is 0.7% The result is the weight of an object depends on where you are on earth.
The mass has the SI unit of the kilogram, weight is mass * gravitational acceleration. Calibration weights that you use to calibrate a scale are made to have a specific mass, not a specific weight because weight is location-dependent.
The same way density is not weight/volume but mass/volume.
Between 1889 and 2019 the kilogram was defined as the mass of the [https://en.wikipedia.org/wiki/International_Prototype_of_the_Kilogram](https://en.wikipedia.org/wiki/International_Prototype_of_the_Kilogram) All other masses were derived as a comparison to it. There is always a level of accuracy so not two-kilogram prototype had the exact same mass. There was a small change in mass over time because if you use them they need to be clean and there was a drop of around 1micro gram per cleaning, which is a change of 1 in one billion.
Any good calibration weight will have documentation so you can trackback what is was compared to and when. That way any errors and inaccuracies can be tracked.
Because of problem like this the kilogram was redefined in 2019 as
>”The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015×10^−34 when expressed in the unit J s, which is equal to kg m^2 s −1, where the metre and the second are defined in terms of c and ∆ν_Cs.”[1]
Today mass is determined by a physical constant.
So any real physical object will not have a mass that is exact. The object made with higher accuracy will cost more.
Any density measurement of material will have one accuracy limit and I assume it is the volume that is harder to determine than the mass.
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