What is the Hydrometer Test of Soil?

Hydrometer Test for Soil, Measuring the particle size distribution of fine-grained soils like clay and silt is best performed using the soil hydrometer test. This sedimentation method is familiar to all geotechnical laboratories.

The Hydrometer method is used to determine the particle size distribution of fine-grained soils passing 75 µ sieves. The hydrometer measures the specific gravity of the soil suspension at the center of its bulb. The specific gravity depends upon the mass of solids present, which in turn depends upon the particle size.

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Equipment for Hydrometer Test for Soil

  • Balance

  • Mixer (blender)

  • Hydrometer (152H model preferably,

  • Sedimentation cylinder (1000 mL cylinder)

  • Graduated 1000 mL cylinder for control jar

  • Dispersing agent [sodium hexametaphosphate (NaPO3) or sodium silicate (NaSiO3)]

  • Control cylinder

  • Thermometer

  • Beaker

  • Timing device

What Is the Theory Behind the Hydrometer Analysis Test of Soil?

Hydrometer analysis is based on Stokes's law. According to this law, the velocity at which grains settle out of suspension, all other factors being equal, is dependent upon the shape, weight, and size of the grain.

In the case of soil, it is assumed that the soil particles are spherical and have the same specific gravity. Therefore we can say that in a soil water suspension, the coarser particles will settle more quickly than the finer ones.

If V is the terminal velocity of the sinking of a spherical particle, it is given by;

V = 1/18 [(Gs-Gw)/n)]*D2


V = Terminal velocity of soil particle (cm/s)

D = Diameter of soil particle (cm)

Gs = Specific gravity of soil particle

Gw = specific gravity of water

n = viscosity of water (g-s/cm2)

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Practical Application of Hydrometer Test for Soil

Hydrometer analysis is essential for obtaining the complete particle size distribution of such soils. Particle size distribution obtained from sieve analysis may be combined with the data from a hydrometer analysis to produce a complete gradation curve. It is possible to approximate the percentage of silt and clay particles present in the finer portion from the hydrometer analysis.

Particle size is one of the criteria used to determine whether a soil is suitable for building roads, embankments, dams, etc.
Information obtained from a particle size analysis can be used to predict soil-water movement if a permeability test is not available.

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Hydrometer Test for Soil