…the degree to which two or more attributes or measurements on the same group of elements show a tendency to vary together. Dictionary.com Unabridged. Random House, Inc. 13 Oct. 2013.
The moisture content (MC) of stored grain equalizes (equilibrates, comes into equilibrium, balances, or settles without further change) with the combination of temperature and relative humidity (RH) of the air that surrounds the grain.
The inverse is also true; the temperature and RH of air will change when exposed to grain of a different temperature and MC. These three elements change together somewhat like a triangle. Changing any one of them impacts the others like changing any side in a triangle impacts either the length of the other sides or the angles thereof.
Once grain in a bin has reached equilibrium, any air movement with a different combination of temperature and RH disrupts this balance and the grain temperature and MC starts to move toward the new conditions. This may occur naturally by convection currents (chimney effect) from the outside temperature (average outside compared to the grain temperature) being different or sunshine on the bin wall. This can result in moisture migration which is a principal cause of spoilage. Crusts on the top of stored grain are a common result of this.
This phenomenon also opens the door to better manage the temperature and MC of grain by means of precisely controlled aeration.
An example of thermal (heat) equilibrium is to place a frozen turkey (0° F) and a warm watermelon (80° F) into a refrigerator set at 40° F. Right away heat starts to flow from the watermelon and toward the turkey. This continues at a rapid rate when the differences are great. As the temperature of each approaches 40° F, the rate of exchange slows down. Experience tells us that one hour is not enough time for these to equalize to 40° F. The center of the turkey is probably still frozen after a day.
A fan to re-circulate the air in the refrigerator will speed this up. After three days we would expect that the items will have equalized to the same temperature. However, the watermelon will still be a little bit warmer and the turkey a little bit colder because the exchange slows down more and more with less difference and virtually stops. This little difference is called hysteresis — we might think of it as lagging behind. A similar effect occurs in grain — the same air that will dry to a set point will not quite re-wet to that set-point and vice a versa. However, repeated cycles above and below the TMC tend to reduce the amount of hysteresis.
The MC level of grain is interdependent with two outside parameters (factors), not just one as in the example above. This means there is a more complex (and difficult to measure) set of information to work with in order to successfully store grain. It is not possible to change any one of these three elements without changing at least one of the others. Temperature and relative humidity are interdependent in their relationship with the MC of grain.
The relationship among temperature, relative humidity and grain moisture content is shown mathematically in the following historical document. Some factors that affect how different grains interact with these relationships include: variety, fertility, climate and season. EMC formulas that are improved with fresh data and analytics year by year can provide improved outcomes.