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Saturday, January 17, 2009

Hydrometer Test

Description of Test

This method describes the quantitative determination of the distribution of particle sizes in soils. The distribution of particle sizes larger than 71 um is determined by a sedimentation process, using a hydrometer to secure the necessary data.

The hydrometer test is an application of Stokes Law, which in essence states that larger particles fall more quickly in a suspending fluid, while finer particles remain in suspension longer. The time at which the hydrometer readings are taken determines the size of particle remaining in suspension, while the reading on the hydrometer determines the amount of that size.

Several assumptions are made about particles shape and other test conditions, so the results are somewhat approximate. The sieve portion and hydrometer portion of the test may not exactly line up.

The method as presented, assumes a particle specific gravity of 2.65. For most purposes this will be sufficiently accurate even though S.G.'s may be somewhat lower or higher. If further refinement is required, additional corrections may be found in the reference.

Results are used to indicate whether the soil is frost susceptible and for comparing soils from different areas or strata.

Because the sample size is small, take extra care to obtain representative material. Considerable care should also be taken in all weighing and liquid volume measurements.

The sample must be completely dispersed and remain dispersed throughout the test.

Be sure the dispersing agent is not more than one month old. Also make sure the stirring paddle is not badly worn. Some soil (like heavy clays) tends to coagulate and form curds and then settle quickly giving false readings. If you see any evidence of coagulation, you must re-run the test. Reducing the sample size to 25 g sometimes helps this problem.

Basic objective of the test

Distribution of soil particles having sizes less than 75 micron (Fine Grained soils) is often determined by a sedimentation process using a hydrometer to obtain the necessary data such as the borderline between clay and silt. Using this test the GSD or grain size distribution for soils containing appreciable amount of fines is obtained.


Equipment Required

• Balance - sensitive to 0.01 g.

• Stirring apparatus - mechanically operated, with an electrical motor able to turn a vertical shaft at a speed of not less than 10,000 r.p.m. without load. The shaft shall have a stirring paddle made of metal, plastic, or hard rubber. The paddle shall be not less than 19.0 mm or more than 38.1 mm above the bottom of the dispersion cup. For details of the paddle and dispersion cup see Figure 205-10-1.

• Hydrometer - graduated to read in grams per litre of suspension and conforming to requirements for hydrometer 152 H in ASTM Specification E 100.

• Sedimentation Cylinder - glass, 457 mm in height, 63.5 mm in diameter and marked for 1000 ml volume.
• Thermometer - accurate to 0.5o C.

• Sieves – full set of Canadian metric sieves

• Water bath or constant room temperature – to maintain the soil suspension at or near 20°C during the analysis. If a room can be controlled at a constant temperature no water bath is necessary.

• Beaker - 250 ml.

• Timing device - a watch or clock with a second hand.

• Oven or hot plate.

Materials Required

Dispersing agent - prepare a solution of sodium hexametaphosphate (sometimes called sodium metaphosphate) in distilled or demineralized water. Use 40 g of sodium hexametaphosphate/litre of solution. Make new solutions at least once a month as it will slowly revert to the orthophosphate form causing a decrease in dispersive action.

Water - either distilled or demineralized water. It should be kept at the same temperature as the test is to be run. The basic temperature as the test is to be run. The basic temperature for the hydrometer test is 20°C.


Test Procedure

Step 1- Taking the weight of the beaker, Taking 50 g of oven dry soil in a beaker

Step 2- Making a 4% solution of dispersing agent, Mixing the solution thoroughly

Step 3-Take 125 cc of mixture prepared in step 2 and add it to the soil taken in step 1 ,This should be allowed to soak for about 8 to 12 hours

Step 4- Take a 1000cc graduated cylinder and add 875cc of distilled water plus 125 CC of dispersing mixture from step 3

Step 5- Record the temperature of the cylinder of step 4

Step 6- Put the hydrometer in the cylinder of step 5 and record the two corrections Fm and Fz

Step 7- mix the soil prepared in step 3 using a spatula Pour it into the mixer cup, wash the soils that stick to the sides of the beaker using distilled water

Step 8- add distilled water to the soil in mixer cup to make it about two-thirds full.

Step 8- Mix it for about 2 minutes using the mixer

Step 9- Pour the mix into the second graduated 1000-cc cylinder. Make sure that all of the soil solids are washed out of the mixer cup. Fill the graduated cylinder with distilled water to bring the water level to 1000 cc mark

Step 10- at the end of step 9, you should have these two cylinders

Step 11- After each reading the hydrometer is put into the transparent cylinder (R)

Sieve Analysis of 71 mm Material

After the final hydrometer reading, was the suspension through a 71 mm sieve with tap water. Dry the retained material in an oven at 110o ± 5oC.

Sieve the material on the following sieves: 71 mm, 160 mm, 250 mm, 400 mm, and 900 mm.

Enter the cumulative weights to the left side of the column marked "WEIGHT PASSING" on Form MR-8 (Figure 205-10-2).
Composite Correlation for Hydrometer

Hydrometer reading corrections are required to compensate for temperature and density changed in the dispersing fluid. Corrections are determined by doing a "blank" test without soil, as described below.

At the same time as the test is being run on the soil, prepare a second sedimentation cylinder with 125 ml of the dispersing solution but no soil. Fill the cylinder to the 100 ml mark with water and mix well by shaking.

Place the cylinder in the same area or in a water bath with the soil test cylinder.
Take hydrometer readings periodically in the same manner as for the soil test. Because the readings do not vary much, take only 3 or 4 readings over the sedimentation period.

Conditions. Enter the corrected readings in the column marked "CORRECTED FOR CALGON & TEMPERATURE."



Use the formulas given to calculate each of the following quantities:

Hydroscopic Moist. = Air Dry Wt. - Oven Dry Wt. x 100
Oven Dry Wt.

% Pass. 2.00 mm Sieve Total Sample Wt. - Wt. Ret. 2.00 x 100
Total Sample Wt.

Oven dry wt. of dispersed sample

Wd = Air Dry Wt. x 100
100 + % Hydroscopic Moist.

Wt. of Dispersed Sample Corrected to total sample basis = w

W = Oven Dry Wt. x 100
% Pass. 2.00 mm

Calculate grain diameter from each hydrometer reading using the formula following, and enter the data in the correct column on the form.

Grain Diameter = K L/T

All the values required for the calculations are taken from the data previously recorded in the various columns.

Calculated percentage (P) of total test sample in suspension for each hydrometer reading and enter the data in the final column on the form. Use the following formula to calculate P:

P = R/W x 100

Where R = corrected by hydrometer reading
W = dispersed weight of sample corrected to total sample

Calculate the sieve analysis portion of the test and enter the values in the column on the form marked "PERCENTAGE PASSING."

For the fraction retained on the 2.00 mm sieve, base calculations on the total air dry weight of sample.

For the fraction passing the 2.00 mm sieve, adjust the cumulative weights passing each sieve to the “weight dispersed basis” or Wd as previously calculated. First obtain the weight accumulated on the balance for the material from the 71mm sieve to the 2.00mm sieve. Subtract this weight from Wd and use this difference as an adjustment to each sieve size as shown in the following example

Assume weight dispersed or Wd = 49.5 g
Weight after washing over 71 mm and drying = 15
Adjustment = 34.5
Sieve Cumulated Wt. Adjusted Wt.
2.00 mm 15.0 g 49.5 g
400 mm 20.5 g 45.0 g
71 mm 1.0 g 35.5 g

Calculate percent passing each sieve using the formula:
% Pass. = Adjusted Wt. x 100

Reporting Results

Results of the test are attached in the next page.


We did the test for soil which was taken from the project “STOR UBAT JKN WILAYAH, KL” and the result which we obtain from the soil was 34% of clay, 40% of silt, 24% of sand and 2% of gravel. The classification of soil at different place is not same. We may not get the same result from another place.

ASTM D422-63 Standard is followed

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