When Professionals Run Into Problems With Sedimentation Analysis/ Using Hydrometer/calibration, This Is What They Do

Sedimentation analysis 

Objective: To determine the particle size distribution less than 75Pm by hydrometer 

method. 

Theory: Hydrometer analysis is a widely used method of obtaining an estimate of the 

distribution of particle sizes from the 0.075 mm to around 0.001 mm. It is generally used 

when material passing from 75 P is more than 10 %. 

It utilizes the relationship among the velocity (v, cm/s) of fall of sphere in liquid, the 

diameter (D, cm) of the sphere, specific weights of spheres and the fluid (Js and Jw,

g/cm3

) and the viscosity (K, g/cm-s) of the fluid as expressed by Stokes (1850) in the 

equation termed as Stoke’s law:

To obtain the velocity of fall of the particles, hydrometer is used. By mixing the given 

quantity of water with the soil and small amount of dispersing agent solution (33.0g 

Sodium-hexa-meta phosphate (NaPO3)6 and 7.0g Sodium carbonate (Na2CO3) to form a 

1000ml of suspension) the charge on the smaller grains are neutralized. 

The hydrometer displays the specific gravity of the soil-water dispersion at the center of 

the bulb. As time passes, soil grains larger than those still in suspension in the zone Zr

(i.e. distance between the center of the volume of the bulb and the water surface) fall 

below the center of volume, the constantly decreases the specific gravity of suspension at 

the center of volume of hydrometer. Since Zr represents the distance of the particles fall 

in some time interval t, it is evident that the velocity of fall in equation (1) and (2) is 

given as:

Correction to Hydrometer Readings:

a) Meniscus correction Cm: A hydrometer is calibrated to read correctly at the surface 

of the liquid in which it is immersed. Since soil suspension is not transparent enough 

to permit a reading to be taken at this level, the scale has to be read at the upper rim 

of meniscus. This correction is additive to the actual reading because density readings 

on the hydrometer stem increases downwards. It is constant for a given hydrometer 

and is determined by partly filling the measuring cylinder, inserting hydrometer and 

taking readings at the top and the bottom of meniscus. The difference between these 

two readings gives the meniscus correction.

b) Dispersing agent correction Cd: The addition of the dispersing agent results in the 

density of liquid in which sedimentation takes place, being greater than that of water. 

Generally 100ml Sodium-hexa-meta-phosphate solution is used as a dispersing agent 

in the form of its 4 % solution. To obtain the correction, 100ml of dispersing agent 

solution is taken in standard measuring cylinder and the volume is made equal to 

1000ml by adding distilled water. Hydrometer is inserted and reading is taken. 

Similarly hydrometer is inserted in standard measuring jar filled with water and 

reading is taken. The difference between the two readings gives the dispersing agent 

correction and it is always subtracted from the hydrometer reading after it is corrected 

for meniscus. 

c) Immersion correction: When the hydrometer is inserted into the suspension the 

surface of the suspension rises and therefore the immersion correction must be 

applied. The immersion correction is given below and must be applied to Zc value to 

obtain Zr.

Hydrometer Calibration:

 In a liquid of variable specific gravity, hydrometer reads the 

specific gravity at approximately the depth in the liquid where the center of volume of the 

hydrometer floats. Since it is this depth that must be used in the computation of particle 

diameter it is desirable to have a curve which gives the depth from the surface of 

suspension to the center of volume of hydrometer for any hydrometer reading. Such a 

curve is known as calibration curve, is essentially a straight line for a symmetrical bulb 

hydrometer. To calibrate the hydrometer, first locate the mid length of the bulb, which is 

very close to the center of the volume. Next measure the distance from a graduation mark 

on each end of the stem to the center of the bulb, because the curve is essentially a 

straight line, only two such measurements are needed. Plot a curve of hydrometer reading 

against depth.

Apparatus 

1. Soil hydrometer 

2. Dispersing agent 

3. Two graduated cylinders of 1000ml. 

4. Distilled water 

5. Thermometer 

6. Stop-watch 

Procedure 

1. Take 50-55g deaired soil passing through 4.75mm sieve. 

2. Determine the hygroscopic moisture content of the fraction passing through 4.75mm 

sieve. 

3. Add about 150ml of hydrogen per oxide (H2O2) and stir the mixture gently with a 

glass rod. 

4. Add enough water to soak the soil sample completely in a glass beaker and add 

100ml Sodium-hexameta-phosphate. Warm the mixture gently for about 10 minutes. 

5. Transfer the mixture to the top of the mechanical mixer using a jet of distilled water 

to wash all the traces of soil out of beaker. 

6. Stir the soil suspension about 15 minutes. 

7. Transfer the suspension on to 75 Pm sieve placed on a receiver and wash the soil on 

the sieve using a jet of distilled water. Particular care should be taken to wash off all 

the traces of suspension adhering to the dispersion cup. Do not exceed a total volume 

of about 800ml. 

8. Transfer the suspension to a measuring cylinder and make up the volume exactly to 

1000ml with distilled water. 

9. Start the stopwatch immediately after placing the measuring cylinder on the table and 

take the readings at the interval mentioned in record sheet. The reading shall 

correspond to the upper meniscus. Keep the hydrometer in the suspension for the first 

two minutes. Thereafter the hydrometer should be placed in separate cylinder filled 

with water and inserted in the suspension just before taking the reading.