G&G GmbH | Density determination (Hydrostatic weighing)
13
Example:
A piece of metal in the air weight 100 g and weighs 80 g in water. Therefore, the object has 20 g (= 20 cm³)
water displaced. Its volume is therefore 20 cm³. Now it is easy to determine the density of the metal piece.
10.1
Density determination of solids
For the density determination of solid material, the solid is weighed first in air [ A ] and then in the
measuring liquid [ B ]. If the density of the buoyancy medium [ ρ
o
] is known, the density of the solid [ ρ ] is
calculated as follows:
𝜌 =
𝐴
𝐴 − 𝐵
𝜌
0
𝜌
= Density of the sample
𝐴
= Weight of the sample in air
𝐵
= Weight of the sample in the measuring liquid
𝜌
0
= Density of the measuring liquid
10.2
Determining density of liquids
For density determination of liquids, a glass sinker is used, whose volume ([ V ] see embossing) is known.
The glass sinker is weighed first in air [ A ] and then in the liquid [ B ] whose density is to be determined.
According to the Archimedes’ principle, any object, wholly or partially immersed in a fluid, is buoyed up by
a force equal to the weight of the fluid displaced by the object.
The volume [ V ] of the glass sinker is same as the volume of the displaced liquid.
Buoyant force of the glass sinker = weight of the glass sinker in air [ A ] - weight of the glass sinker in the
measuring liquid [ B ]
That implies that:
𝜌 =
𝐴 − 𝐵
𝑉
+ 𝜌
𝐴
𝜌
= Density of the sample liquid
𝐴
= Weight of the glass sinker in air
𝐵
= Weight of the glass sinker in the measuring liquid
𝑉
= Volume of the glass sinker
𝜌
𝐴
= Density of air (0.0012 g/cm
3
)
10.3
Influencing magnitudes and error sources
Air Pressure
Temperature
Volume deviation of the sinking object (± 0.005 cm
3
)
Surface tension of the liquid
Air bubbles
Immersion depth of the sinking object
Porosity of the object
