A hydrometer is an instrument used for determining the specific gravity (or density) of the suspension, and this enables the percentage of particles of a certain equivalent particle diameter to be calculated. Hydrometer test is carried out to quantitatively determine the particle/grain size analysis (distribution ) for soil particles of size smaller than 75 micron. For soil particles of size above 75 micron and below 4.75 mm, wet sieve analysis is done and for soil particles of size 4.75 mm and bigger, dry sieve analysis is done.

  • IS: 2720 (Part 4): 1985

  • Hydrometer 151H–0.987 to 1.030.
  • Weighing balanceaccurate to 0.01 g.
  • 75micron IS sieve
  • Wire brush
  • Oven–to maintain the temperature at 105ºC to 115ºC.
  • Thermometer (0ºC -50ºC accurate to 0.5ºC).
  • Reagents: Hydrogen peroxide, Sodium hexametaphosphate & Sodium carbonate.
  • Mechanical high speed stirrer
  • 100ml and 1000ml capacity glass cylinder with ground glass or rubber stoppers
  • Water bath to maintain a constant temperature
  • Stop-watch
  • Porcelain evaporating dish
  • Conical flask or a beaker of 1000ml capacity
  • Glass rod
75 micron IS Sieve

  1. Pretreatment of soil (Required only for organic soil):- 50g of oven dry soil sample is taken in a clean dry beaker and add 150ml of Hydrogen peroxide and stir it. Allow it to stand overnight. The mixture in conical flask shall then be gently heated and stir it periodically. Boil it until the volume reduces to 50ml.
  2. Sodium hexametaphosphate solution :- 33g of Sodium hexametaphosphate and 7g of Sodium carbonate dissolved in 1000 ml of distilled water.
  3. Take 50g (in case of fines) and 100g (in case of sand) of oven dry sample and add 100ml Sodium hexametaphosphate solution and stir the mixture gently and keep it for soaking overnight.
  4. Mixture is stirred gently using mechanical high speed stirrer.
  5. After stirring the mixture is transferred to 1000 ml measuring cylinder. This mixture is made up to 1000 ml by adding distilled water.
  6. Shake the mixture in the cylinder properly so that the soil particles move freely.
  7. Place the hydrometer slowly in the cylinder containing mixture and start the stop-watch.
  8. Hydrometer readings shall be taken after period of ½, 1, 2, 4, 8, 15, 30 minutes and 1, 2, 4, 8, 24 hours.
  9. The temperature of the suspension shall be observed and noted once during the first 15 minutes and then after every subsequent reading.
Wire Brush

The percentage of soil retained on each sieve shall be calculated on the basis of the total mass of soil sample taken and from these results the percentage passing through each of the sieves shall be calculated.

The results of grain size analysis bu hydrometer shall be reported in a suitable form. A grain size distribution shall be drawn on a semi-logarithmic chart, plotting particle size on the log scale against percentage finer than the corresponding size on the ordinary scale.