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GRAIN SIZE ANALYSIS BY HYDROMETER
1. Objective
Hydrometer test is carried out to quantitatively determine the Particle/Grain Size Distribution for soil particles of size smaller than 75 micron.
2. Apparatus Required
Fig. 1: Hydrometer
Calibrated at 27°C, range of 0.995 to 1.030 g/cc, graduation lines at interval of 0.0005; in conformity with IS-3104: 1965. The maximum permissible scale error on the hydrometer is plus or minus one scale division.
Fig. 2: Laboratory setup for Hydrometer Test
Two 1000 ml graduated cylinders, Dispersing agent solution containing sodium hexa-metaphosphate, Mechanical stirrer, Water Bath, Balance, Sieve, Dessicator and Centimetre Scale.
Fig. 3: Porcelain Evaporating Dish
Fig. 4: Thermometer
For measuring Temperature required for Temperature corrections- with accuracy of 0.5^0 C
Fig. 5: Stop Watch
For recording Time interval.
IS-2720 (Part 4):1985 (Reaffirmed- May 2015) “Methods of test for soils: Grain size analysis”.
4. Procedure
4.1 Calibration Of Hydrometer
- Determination of volume of the hydrometer bulb (Vh): Pour about 800 ml of water in the 1000 ml measuring cylinder and note the reading at the water level. Immerse the hydrometer in water and note the water reading. The difference between the two readings is recorded as the volume of the Hydrometer bulb plus the volume of that part of the stem which is submerged. For practical purpose the error due to the inclusion of this stem volume may be neglected. Alternatively, weigh the hydrometer to the nearest 0.1g. This mass in grams is recorded as the volume of the hydrometer in ml. This includes the volume of the bulb plus the volume of the stem.
- In order to find the area of cross-section (A) of the measuring cylinder in which the hydrometer is be used, measure the distance, in cm, between two graduations of the cylinder. The cross-sectional area (A) is then equal to the volume included between the two graduations divided by the distance between them.
- Measure the distance (h) from the neck to the bottom of the bulb, and record it as the height of the bulb. The distance from the neck of the bulb to the nearest calibration mark shall be measured and recorded.
- With the help of an accurate scale, measure the height between the neck of the hydrometer to each of the other major calibration marks (Rh).
- Calculate the effective depth (He) corresponding to each of the calibration marks (or hydrometer readings, Rh) by the following expressions: He = H + 0.5 (h – Vh/A)
- The readings may be recorded as illustrated in Table 1:
S.
No.
Hydrometer Reading, Rh
H
(cm)
Effective depth, He (cm) 1 2 3
Table 1
- Draw a calibration curve between He and Rh which may be used for finding the effective depth (He) corresponding to hydrometer readings (Rh) during test.
Fig. 6 : Calibration Curve for Hydrometer
3. Reference
- Composite Correction : In order to determine the composite correction, put 100 ml of dispersing agent solution in another 1000 ml measuring cylinder and make it to 1000 ml by adding distilled water. The cylinder should be maintained at the same temperature as that of the test cylinder containing soil specimen. Insert the hydrometer in this comparison cylinder containing distilled water and the dispersing agent and take the reading corresponding to the top of the meniscus. The negative of the hydrometer reading so obtained gives the composite correction (C). The composite correction is found before the start of the test, and also at every time intervals of 30 minutes, 1 hour, 2 hours and 4 hours after the beginning of the test, and afterwards, just after each hydrometer reading is taken in test cylinder.
- The temperature of the suspension should be observed and recorded once during the first 15 minutes and then after every subsequent reading.
5. Observation And Recording
Elapsed Time, t (in minute)
Hydrometer Reading, Rh
**R'h
Rh
Cm**
R
**R'h
Ct- Cd**
Effective Depth, He (cm)
K He/t
√He/ √t
Particle Size, D = K√He/ √t (mm)
Percent Finer (N')%
Percent Finer on the total weight N
Table 2 : Determination of Grain Size by Hydrometer
The results of the Table 2 are plotted to get a particle size distribution curve with “percentage finer” (N) as the ordinate and the “particle diameter” (D) on logarithmic scale as abscissa as shown in Fig. 7.
Fig. 7 : Grain Size Distribution Curve
6. Calculations
- The loss in mass in pre-treatment of the soil in percentage is calculated from the following expression:
P = (1 – Mb/Md) * 100
Where,P = loss in mass in percentage Md = mass of dry soil sample taken from the soil passing 2 mm sieve Mb = mass of the soil after pre-treatment
- The diameter of the particles in suspension at any sampling time ‘t’ is calculated from equation below:
D = {√(30 μ)/980(G-G 1 ) * √(He/√t)} D = K *√(He/√t) Where, K = {√(30 μ)/980(G-G 1 ) D = diameter of particle in suspension, in mm. μ = coefficient of viscosity of water at temperature of the suspension, at the time of taking the hydrometer reading, in poises; G = specific gravity of the soil fraction used in the sedimentation analysis; G 1 = specific gravity of water; He = effective depth corresponding to Rh in cm t = time elapsed between the beginning of sedimentation and takin of hydrometer reading in minutes.
7. General Remarks
- Sodium hexametaphosphate has been found to ineffective when dealing with certain highly flocculated soils. In such cases dispersion may be carried out by adding N-sodium hydroxide solution at the rate of 4 ml per 10 g of soil.
- The suspension should be kept out of direct sunlight and away from any local source of heat. Evaporation should be retarded by keeping a cover on the measuring cylinder between the readings.
- The specific gravity should he determined for the fraction of the sample passing 75 micron sieve.
- This method shall not applicable if less than 10% of the material passes the 75 micron IS Sieve.
8. Video
Grain Size Analysis by Hydrometer
