Hydrometer analysis ( stroke's law)

The grain size less than 75 micron cannot be obtained by seive analysis. To determined the the grain size less than 75 micron, hydrometer analysis performed. 

Hydrometer analysis is based on the strokes law. Which state that when the spherical is allowed to fall in infinite liquid medium the velocity increases at first but constant velocity is called terminal velocity is given by:
           
           

A hydrometer is used to determine the specific gravity of soil suspension. If the soil suspension created then its concentration changes with the time means the specific gravity changes with time. By measuring the specific gravity of a soil suspension at a known depth at a some elapse of the time after the prepation of suspension a poin can be obtain by grain size plot.

# Hydrometer correction
Meniscus correction : sometimes its difficult to take the lower reading in the hydrometer because the suspension  of the soil is opaque. There is applied correction called meniscus correction. For this correction put the hydrometer  in the 100 cc of measuring cylindrical and take the difference reading of upper and lower. That difference reading is added in the hydrometer reading. It is also noted that the meniscus reading is always positive.

Temperature correction :
The temperature correction is applied when the temperature of the spot is not equal to 20°c. Sometimes it may less than 20 °c and greater than 20 ° c.

• if the temperature is greater than 20°c then correction will be positive. 
• if the temperature is less than 20°c then the temperature correction will negative.

For this the different reading of hydrometer is taken from different temperature in distilled water. Plot the graph between the correction and range of temperature, it facilates the correction of the test.

Hydrometer immersion correction