We can directly measure the magnitude of the Buoyant force using
a spring scale. I carry a mass of 2 kg from the scale. In air the
scale reads about 20 N. When I lower the mass into water
the reading changes to about 14 N. The difference results from the
Buoyant force which simply equals the force of gravity on the volume of water displaced by the mass:
If we were to lower the mass into a light vegetable oil we would find
a smaller buoyant force while the buoyant force would be larger
if we were to lower the mass into mercury.