Hydrometer test is carried out to quantitatively determine the Particle/Grain Size
Distribution for soil particles of size smaller than 75 micron.
IS2720 (Part 4):1985 (Reaffirmed May 2015) “Methods of test for soils: Grain size analysis”.
4.1 Calibration of Hydrometer

Determination of volume of the hydrometer bulb (V_{h}): 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 crosssection (A) of the measuring cylinder in which the hydrometer is be used,
measure the distance, in cm, between two graduations of the cylinder. The crosssectional 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 (R_{h}).

Calculate the effective depth (H_{e}) corresponding to each of the calibration marks (or hydrometer readings, R_{h})
by the following expressions:
H_{e} = H + 0.5 (h – V_{h}/A)

The readings may be recorded as illustrated in Table 1:
S. No. 
Hydrometer Reading, R_{h} 
H (cm) 
Effective depth, H_{e} (cm) 
1 



2 



3 



Table 1

Draw a calibration curve between H_{e} and R_{h} which may be used for finding the effective depth (H_{e}) corresponding
to hydrometer readings (R_{h}) during test.
Fig. 6 : Calibration Curve for Hydrometer

Meniscus correction:
Insert the hydrometer in the measuring cylinder containing about 700 ml of water. Take the readings of the hydrometer
at the top and bottom of the meniscus. The difference between two readings is taken as meniscus correction (C_{m}) which is a constant for a
hydrometer. During the actual sedimentation test, the readings should be taken at the bottom of the meniscus but since the soil suspension
is opaque, readings are taken at the top of meniscus, It is clear that readings decrease in the upward direction. Thus, the observed hydrometer
readings are always less than the true one. Hence the meniscus correction is always positive.

Pretreatment of soil:
The percentage of soluble salts shall be determined. In case it is more than one percent, the soil shall be washed
with water before further treatment.
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