# Determination Of Grain Size Analysis (Hydrometer Method)

STANDARD

• IS: 2720 (Part 4) 1985.

OBJECTIVE

• To determine the grain size analysis of soils by sedimentation method.

APPARATUS

• Glass measuring cylinders 2 nos of 1000 ml capacity with ground glass or rubber stoppers about 7cms diameter and 33 cms high marked at l 000 ml volume.
• Thermometer capable of reading temperature up to 90°C and sensitivity 0.5°C.
• Water bath or constant temperature bath.
• 4.75 mm and 75 microns IS sieves.
• Balance of accuracy 0.01 gram.
• Thermostatically controlled oven of capacity 250°C.
• Stopwatch.
• Centimeter scale.
• Porcelain evaporating dishes.
• Wide mouth conical flask of 1000 ml capacity.
• Measuring cylinder of capacity of 100 ml.
• Wash bottle.
• Glass rod of about 15 to 20cm long and 4 to 5mm in diameter.

Reagents

• Sodium Hexametaphosphate (In solution – Dissolve 33 grains of Sodium Hexameta phospate and 7 gram of Sodium Carbonate in distilled water to make one liter of solution).

PROCEDURE

Calibration of Hydrometer
i.  Measure and record the distance from the lowest calibration mark on the stem of a hydrometer to each of the major calibration marks (Rh).
ii.   Measure and record the distance from the neck of the bulb to the nearest calibration mark.
iii.  The distance H1 corresponding to a reading Rh is the sum of the distance measured in (i) and (ii).
iv.   Measure and record the distance (h) from the neck to the bottom of the bulb as the height of the bulb.

Find out the Volume (Vh) of the bulb and area of the jar(A).

• Calculate the effective depth (HR) corresponding to each of the major calibration marks (Rh) from the
equation

HR =H1+0.5 * (h-Vh /A)
HR = Effective depth.
H1 = Length from the neck of bulb to graduation Rh in crns.
h = Twice the length from the neck of the bulb, to its center of volume in cm
Vh = Vol. of the bulb
A= Area of the jar

• Plot a smooth curve through the points corresponding to HR and Rh.
• Obtain the effective depth (HR) corresponding to hydrometer reading (Rh) from the curve.
Meniscus Correction
• Insert the hydrometer in a 1000 ml measuring cylinder containing about 1000 ml of water.
• Place the eye slightly below the plane of the surface of the liquid and note the reading at which the lowemeniscus   intersects the hydrometer scale.
• Again place the eye slightly above the plane of the surface of the liquid and note the reading at which the upper meniscus intersects the hydrometer scale.
• Record the difference between the two readings as the meniscus correction Cm.

Pretreatment

• Take exactly oven dried  50 grams of sample for clayey soils and 100 grams for sandy soils passed through 75 microns.

Dispersion of Soil

• Add 100 ml of sodium hexametaphosphate solution to the soil in the evaporating dish.
• Gently heat the mixture for about 10 minutes.
• Stir the whole mixture for 15 minutes.
• Transfer the suspension into the 1000 ml measuring cylinder.
• Make up exactly 1000 ml by adding distilled water.
• Use this soil suspension for the sedimentation analysis.

Sedimentation

• Insert a rubber hung into the mouth of the cylinder containing soil suspension.
• Shake it vigorously and finally invert it end to end.
• Immediately keep the cylinder in the constant temperature bath.
• Insert the hydrometer into the soil suspension.
• Start the stopwatch and take readings at periods of intervals half, one, two and four minutes.
• Remove the hydrometer from the soil suspension cylinder and insert in to the control jar.
• Reinsert the hydrometer into the soil suspension and take readings after periods of 8, 15 and 30 minutes, one, two and four hours.
• After four hours take hydrometer readings once or twice within 24 hours.
• Finally, take the reading at the end of 24 hours.
• It is usual to leave the hydrometer in soil water suspension for the first four readings and then removed and kept in the control jar and reinserted into the soil water suspension for each of other readings.
• Allow ten seconds for each operation.
• Record the temperature in the suspension once during the first 15 minutes and then after every subsequent reading.
• Take also hydrometer readings in control jar at the corresponding temperatures.
• Calculate the temperature correction (Mt) as the difference between this reading and the reading corresponding to the density of water.
• Take care, to see that the temperature difference between the soil water suspension and solution in the control jar shall be within 1°C.

CALCULATIONS

• Calculate the diameter of the particle in soil suspension at any time (t) of taking a reading from the following formula

D= √ ((30µ) / (980* (Gs-G)) ) × (√ HR/t)

D= Diameter of the particle in suspension, in mm

µ = Co-efficient of the viscosity of water at the temperature of the suspension at the time of taking the hydrometer reading

Gs = Specific gravity of the soil.

G = Specific gravity of water.

HR = effective depth corresponding to Rh in cms.
= time elapsed between the beginning of the sedimentation and taking of hydrometer reading in minutes.
Properties of distilled water

 Temperature (°C) Unit weight of water (g/cm3) Viscosity of water (poises) 26 0.9968 0.00875 27 0.9965 0.00855 28 0.9963 0.00836 29 0.9960 0.00818 30 0.9957 0.00800 31 0.9954 0.00783 32 0.9951 0.00751 33 0.9947 0.00736 34 0.9944 0.00721

• Correct the hydrometer reading for meniscus (Rh) using the following formula
Rh =R h + Cm
Rh =
Actual hydrometer reading.
Cm          =Meniscus correction.
Calculate the percentage of particles finer than D  from the following formula
100 G,
N =          —————–X ( Rh +/- Mt )
Ws (G,—1)

GS           =Specific gravity of soil particles.
Ws           =Weight of soil taken for sedimentation.
Rh       =Hydrometer reading corrected for meniscus.

N            = Percentage finer

Mt =temperature correction.

• Calculate the values of N for all values of D obtained and expressed as the percentage of particles finer than the corresponding values of D.
• Express these percentages as combined percentages of the total soil sample taken for analysis.

PRECAUTIONS

• The temperature of the suspension over the period of the test shall not differ from the mean temperature by more than ± 2°C.
• Take care, during insertion and withdraw of hydrometer before and after taking readings to avoid disturbing in the suspension.

CALIBRATION OF HYDROME’IER
IS: 2720 (Part 4)1985

Hydrometer no.:

 Description Unit 1 2 3 4 5 6 7 Distance from the neck of the bulb to the nearest calibration mark ( A ) Distance from the lowest calibration mark to each the major calibration marks ( Rh ),h1 Distance  H1 corresponding to the bottom of the bulb  and corresponding Rh = h1+A Distance from the neck to the bottom of the bulb ( h ) Effective depth He corresponding to each major calibration mark (Rh), HR=H1+0.5*( h-Vh/A) Remark

Graph

DETERMINATION OF GRAIN SIZE ANALYSIS (HYDROMETER METHOD)
15:2720 (Part 4)1985

 Time of reading Elapsed time (t) min. Temp. C Cefficient of the viscosity of water Actual Hyd. Reading R’h Meniscus correction Rh=R’h+Cm Effective depth, Hr Specific gravity of water (G) The diameter of particle, D(mm) Percentage of particle finer than D, W% 