Soil Particle Size Distribution by Hydrometer Method

Reading time: 1 minute

Hydrometer method is used to determine the particle size distribution of fine-grained soils passing 75 µ sieve. 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.

Soil Particle Size Distribution by Hydrometer Method

Equipments Required

The equipment's required for the test procedure are:

1. Hydrometer
2. Glass measuring cylinder (jar), 1000ml
3. Rubber bung for the cylinder (jar)
4. Mechanical stirrer
5. Weighing balance, accuracy 0.01g
6. Oven
7. Deflocculating agent.
8. Desiccator
9. Evaporating dish
10. Conical flask or beaker, 1000ml
11. Stop watch
12. Wash bottle
13. Thermometer
14. Water bath
15. 75 µ Sieve
16. Scale

Theory of Hydrometer Test

The particle size (D) is given by: Where In which, n = viscosity of water in poise, G = specific gravity of solids, _Pw = density of water (gm/ml); , H_e= effective depth, t= time in minutes at which observation is taken, reckoned with respect to the beginning of sedimentation. The percentage finer than the size D is given by Where, R= corrected hydrometer reading, Ms= mass of dry soil in 1000ml suspension.

Procedure of Hydrometer Test

Part - 1: Calibration of Hydrometer

1. Take about 800ml of water in one measuring cylinder. Place the cylinder on a table and observe the initial reading.
2. Immerse the hydrometer in the cylinder. Take the reading after the immersion.
3. Determine the volume of the hydrometer (V_H) which is equal to the difference between the final and initial readings. Alternatively weigh the hydrometer to the nearest 0.1g. The volume of the hydrometer in ml is approximately equal to its mass in grams.
4. Determine the area of cross section (A) of the cylinder. It is equal to the volume indicated between any two graduations divided by the distance between them. The distance is measured with an accurate scale.
5. Measure the distance (H) between the neck and the bottom of the bulb. Record it as the height of the bulb (h).
6. Measure the distance (H) between the neck to each marks on the hydrometer (R_h).
7. Determine the effective depth (H_e), corresponding to each of the mark (R_h) as[Note: The factor V_H/A should not be considered when the hydrometer is not taken out when taking readings after the start of the sedimentation at ½, 1, 2, and 4 minutes.]
8. Draw a calibration curve between H_e and R_h. Alternatively, prepare a table between H_e  and R_h. The curve may be used for finding the effective depth H_e  corresponding to reading R_h.
   

   Fig 1: Hydrometer Method

Part - 2 : Meniscus Correction

1. Insert the hydrometer in the measuring cylinder containing about 700ml of water.
2. Take the readings of the hydrometer at the top and at the bottom of the meniscus.
3. Determine the meniscus correction, which is equal to the difference between the two readings.
4. The meniscus correction C_m is positive and is constant for the hydrometer.
5. The observed hydrometer reading R_h’ is corrected to obtain the corrected hydrometer reading R_h as

Part - 3 : Pretreatment and Dispersion

1. Weigh accurately, to the nearest 0.01g about 50g air-dried soil sample passing 2mm IS sieve, obtained by riffling from the air-dried sample passing 4.75mm IS sieve. Place the sample in a wide mouthed conical flask.
2. Add about 150ml of hydrogen peroxide to the soil sample in the flask. Stir it gently with a glass rod for a few minutes.
3. Cover the flask with a glass plate and leave it to stand overnight.
4. Heat the mixture in the conical flask gently after keeping it in an evaporating dish. Stir the contents periodically. When vigorous frothing subsides, the reaction is complete. Reduce the volume to 50ml by boiling. Stop heating and cool the contents.
5. If the soil contains insoluble calcium compounds, add about 50ml of hydrochloric acid to the cooled mixture. Stir the solution with a glass rod for a few minutes. Allow it to stand for one hour or so. The solution would have an acid reaction to litmus when the treatment is complete.
6. Filter the mixture and wash it with warm water until the filtrate shows no acid reaction.
7. Transfer the damp soil on the filter and funnel to an evaporating dish using a jet of distilled water. Use the minimum quantity of distilled water.
8. Place he evaporating dish and its contents in an oven and dry it at 105 to 110 degree C. Transfer the dish to a desiccator and allow it to cool.
9. Take the mass of the oven dried soil after pretreatment and find the loss of mass due to pretreatment.
10. Add 100ml of sodium hexa-metaphosphate solution to the oven – dried soil in the evaporating dish after pretreatment.
11. Warm the mixture gently for about 10minutes.
12. Transfer the mixture to the cup of a mechanical mixture. Use a jet of distilled water to wash all traces of the soil out of the evaporating dish. Use about 150ml of water. Stir the mixture for about 15minutes.
13. Transfer the soil suspension to a 75 µ IS sieve placed on a receiver (pan). Wash the soil on this sieve using a jet of distilled water. Use about 500ml of water.
14. Transfer the soil suspension passing 75 µ sieve to a 1000ml measuring cylinder. Add more water to make the volume exactly equal to 1000ml.
15. Collect the material retained on 75 µ sieve. Dry it in an oven. Determine its mass. If required, do the sieve analysis of this fraction.

Part - 4 : Sedimentation Test

1. Place the rubber bung on the open end of the measuring cylinder containing the soil suspension. Shake it vigorously end-over-end to mix the suspension thoroughly.
2. Remove the bung after the shaking is complete. Place the measuring cylinder on the table and start the stop watch.
3. Immerse the hydrometer gently to a depth slightly below the floating depth, and then allow it to float freely.
4. Take hydrometer reading (R_h’) after 1/2, 1, 2 and 4 minutes without removing the hydrometer from the cylinder.
5. Take out the hydrometer from the cylinder, rinse it with distilled water.
6. Float the hydrometer in another cylinder containing only distilled water at the same temperature as that of the test cylinder.
7. Take out the hydrometer from the distilled water cylinder and clean its stem. Insert it in the cylinder containing suspension to take the reading at the total elapsed time interval of 8minutes. About 10 seconds should be taken while taking the reading. Remove the hydrometer, rinse it and place it in the distilled water after reading.
8. Repeat the step (7) to take readings at 15, 30, 60, 120 and 240minutes elapsed time interval.
9. After 240 minutes (4 hours) reading, take readings twice within 24 hours. Exact time of reading should be noted.
10. Record the temperature of the suspension once during the first 15minutes and thereafter at the time of every subsequent reading.
11. After the final reading, pour the suspension in an evaporating dish, dry it in an oven and find its dry mass.
12. Determine the composite correction before the start of the test and also at 30min, 1, 2 and 4 hours. Thereafter just after each reading, composite correction is determined.
13. For the determination of composite correction (C), insert the hydrometer in the comparison cylinder containing 100ml of dispersing agent solution in 1000 ml of distilled water at the same temperature. Take the reading corresponding to the top of meniscus. The negative of the reading is the composite correction.

Data Sheet for Hydrometer Test

Mass of dry soil (Ms)=_______g Meniscus correction (Cm)= +_______ Specific gravity of solids (G)= ______ Table 1: Observation recording sheet 

Sl. No.	OBSERVATIONS				CALCULATIONS
	Elapsed time	Hydrometer reading	Temperature	Composite correction	Corrected reading Rh=Rh’+Cm	Height (cm) He	Reading R= Rh’+C	Factor M	Particle size D	% Finer
1	1/2min
2	1 “
3	2 “
4	4 “
5	8 “
6	15 “
7	30 “
8	1 hr.
9	2 hr.
10	4 hr.
11	8 hr.
12	12 hr.
13	24 hr.

Result of Hydrometer Test

Particle Size distribution curve can be plotted using particle size and percentage fineness.