Experiment No. 4: Soil Particle Size Distribution (Hydrometer Analysis)
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Determination of the quantitative size distribution of particles of soil fraction finer than 75 micron.

Hydrometer (calibrated at 27°C, range of 0.995 to 1.030 g/cc), Two 1000 ml graduated glass cylinders, Dispersing agent solution containing sodium hexametaphosphate, Evaporating dish, Thermometer, Stop-watch, Mechanical stirrer.

1. Take 50 g of dry soil in an evaporating dish, add 100 ml of dispersing agent, and prepare a suspension.

2. Transfer the suspension into the cup of a mechanical stirrer, add more distilled water, and operate the stirrer for three minutes.

3. Wash the soil slurry into a cylinder, and add distilled water to bring up the level to the 1000 ml mark.

4. Cover the open end of the cylinder with a stopper and hold it securely with the palm of the hand. Then turn the cylinder upside down and back upright repeatedly for one minute.

5. Place the cylinder down and remove the stopper. Insert a hydrometer and start a stop-watch simultaneously. To minimize bobbing of the hydrometer, it should be released close to the reading depth. This requires some amount of rehearsal and practice.

6. Take hydrometer readings on the upper rim of the meniscus formed by the suspension and the hydrometer stem after time intervals of periods of 0.5, 1, 2 and 4 minutes, .

7. After the 4 minutes reading, remove the hydrometer slowly, and float it in a second cylinder containing 100 ml dispersing agent and distilled water up to 1000 ml mark.

8. Take further readings after elapsed time periods of 8, 15 and 30 minutes, and also after 1, 2, 4, 8 and 24 hours. Insert the hydrometer only just before the reading and withdraw immediately after the reading.

9. Observe and keep recording the temperature of the soil suspension.

10. Shake the solution in the second cylinder thoroughly. Insert the hydrometer and note the meniscus correction, which is the reading difference between the top of the meniscus and the level of the solution in the cylinder when observed along the hydrometer stem.

11. The composite correction is the difference between the top meniscus reading and value of 1.000 corresponding to the usual hydrometer calibration temperature of 27°C. This may be positive or negative.

12. Calibrate the hydrometer to establish a relation between any reading and its corresponding effective depth, and obtain a calibration plot. The effective depth is the distance from the surface of the soil suspension to the level at which the density of the suspension is being measured.

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