hydrometer analysis
General
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Fine-grained soil. The hydrometer test
uses Stokes equation (for the velocity of a free falling sphere in
suspension) to determine grain size distribution smaller than #200
sieve.
The grain size distributions of soils are commonly determined by sieve (smallest being #200) and hydrometer procedures. In the hydrometer analysis the soil smaller than #200 sieve is placed in suspension and by use of Stokes' equation for the velocity of a free falling sphere the equivalent particle size and percent of soil in suspension are computed. For soils with both fine and coarse grained materials a combined analysis is made using both the sieve and hydrometer procedures. Stokes Law:A sphere falling freely through a liquid of infinite extent will accelerate rapidly to a certain maximum velocity and will continue at that velocity as long as conditions remain the same. The relationship of the terminal velocity to the physical properties of the sphere and the liquid are expressed by Stokes' Equation as follows: Equation Set 1.1: Stokes Equation
Stokes' Law is applicable to spheres varying from 0.02 mm to 0.0002 mm in diameter. As applied to soil particles falling through water, inaccuracies occur due to the following:
By proper sample and laboratory technique all except item 1 can be controlled or minimized so that the resulting inaccuracies can be ignored in normal testing. The shape of soil particles will vary from cubes to flakes with each of the shapes between these limits having different influence. The results of the hydrometer analysis are valid if they are considered equivalent grain diameter rather than actual grain diameter. |
Apparatus
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Procedure
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References
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