Harjai Rapid Moisture Meter by Harjai And Company

Water Content of soil is the quantity of soil contained in a sample of soil. Generally this is expressed in ratio.

Here : s-soil (dry), v-void (pores filled with water or air), w-water, a-air. V is volume, M is mass.
Volumetric Water Content is defined by 
 
where V_w is the volume of water and V_T = V_s + V_v = V_s + V_w + V_a is the total volume (that is soil volume + water volume + air space).
Gravimetric water content is expressed by mass (weight) as follows: 
 
where m_w is the mass of water and m_t is the bulk mass. The bulk mass is taken as the total mass, except for geotechnical and soil science applications where oven-dried soil (m_s, see the diagram) is conventionally used as m_t
   We can determine the water content in soil by calcium carbide method as per IS: 2720 (Part II) - 1973. 

PRINCIPLE 
     It is a method for rapid determination of water content from the gas pressure developed by the reaction of calcium carbide with the free water of the soil. From the calibrated scale of the pressure gauge the percentage of water on total mass of wet soil is obtained and the same is converted to water content on dry mass of soil.