Grain drying is a battle between two forces of vapour pressure, that of the air and that of the grain. The water in the air has a force or pressure trying to push its water into everything it comes in contact with. Likewise grain has a vapour pressure in which water is being pushed out of the seed. When air vapour pressure is greater than grain vapour pressure, water is pushed into the grain, and the grain is wetted down. However when the vapour pressure of the grain exceeds that of the air; water is transferred from the grain to the air and we have drying. The vapour pressure (Vp) of the air is determined by its temperature and by the amount of water it is carrying. At any given temperature the greatest vapour pressure occurs when the air is holding its maximum capacity and this is called the saturated vapour pressure (Vps). Compared to atmospheric pressure (14.7 psia), vapour pressure is typically much smaller (1psia).
Grain vapour pressure is determined by the temperature of the grain, the moisture content (MC) and the type of grain. It is measured indirectly by leaving it equalize in vapour pressure with its surrounding air in a sealed environment. This has resulted in the creation of Equilibrium Moisture Content equations which will be discussed later.