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Hydrometer method of grain size analysis

The hydrometer method of grain size(Sand, clay and silt content) analysis is based on the dispersion of soil aggregates using a sodium solution, called sodium hexametaphosphate.

The He(True height) in the pipette method is not constant here in the hydrometer method it keeps varying. Here there is no need to draw the sample like in the pipette method, we can directly take the specific gravity of the solids using a hydrometer. This device works on the principle of buoyancy.

Here, Vₕ - the volume of hydrometer, Aⱼ - Area of the jar

Hₑ = H + h/2 + Vₕ /2Aⱼ - Vₕ /Aⱼ = H + 0.5(h - Vₕ /Aⱼ)

1 + Rₕ /1000 = Specific gravity of soil suspension Hₑ = Density of soil suspension at Hₑ / Density of water

The density of soil suspension = Density of water + Immersed weight of solids per unit volume

1 + Rₕ /1000 = Wd / 𝛾ᵥᵥ (1 - 1/G), where Wd - concentration of soilds at Hₑ at time t

percentage finer than dHₑ = Wd / W x 100

Corrections to the hydrometer readings

  • Meniscus correction Cm (+ve)

  • Temperature correction Ct (+ or -): If temperature > 27°C then +ve correction and If temperature < 27°C -ve correction.

  • Deflocculating agent correction Cd (-ve)

Now, try this question out!

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