What is EMC (Equilibrium Moisture Content)?

For years farmers have been advised to use EMC as a guide to determine when conditions are right for running their fans in order to dry their grain.  But, how does this work?  What is EMC anyway?  Why aren’t more people using it as a guide?  Well, let’s see if we can’t answer these questions.

First let’s define EMC.  If one takes a grain, or any biological material for that matter, at a certain moisture content (MC) and put it into a sealed container, and left it for sometime such that the temperature of the grain and that of the air  inside become the same. They have reached equal temperature (T) and are thus in equilibrium.  Depending on the moisture content of the grain and the grain type, the air will reach a certain relative humidity (RH).

This experiment of putting a specific grain, with a specific MC and T into a sealed container; waiting for it to reach equilibrium, and then recording the RH was done thousands of times.  All these points were fitted into an equation to obtain  EMC equations  in which the MC is a function of the T and RH.  Or we can get RH as a function of the T and MC.  This has been done by many researchers such as Henderson,  Chung, Pfost, and Hasley; each with a slightly different nasty looking equation.  They involve natural logarithms and exponents and I will spare you the details.  Each grain has a different set of coefficients.  The American Society of Ag Engineers (ASAE) have published these equations as “Moisture Relationships of Plant-Based Agricultural Products” ASAE D245.5 Oct95. To avoid the ugly equations, EMC is usually presented as a table, but the tables were generated from these equations.

How can this be used by a farmer?  If you have a bin of wheat, in which the fan has been off for some time — at least an hour — and for all intense purposes there is no air entering or leaving the bin, (consider it sealed).  We can take the T and RH, plug it into the appropriate EMC equation with coefficients for that particular grain and we can get the MC of that grain.  The accuracy of the MC is sometimes in question, but usually it’s within one percentage point of the MC. There are many things that erode the accuracy. RH sensors have error, the density of the grain varies, and the variety of the grain will affect the accuracy as well as the amount and kind of dockage and foreign material.

What is EMC of the air?  The recommendations for using EMC to determine if one has good drying conditions goes something like this:

If ambient air has a T of 10 C and RH of 60%, we plug this into the EMC equation for wheat and get 14.2% MC. The EMC of the air is 14.2%. That means if the air conditions stay constant at 10 C and 60% RH, wheat would eventually equilibrate to 14.2%.  Whether the wheat started with a moisture content lower or higher than 14.2% — it would eventually end up with a MC of 14.2%.  If the EMC of the air is less than the MC of the grain, then drying conditions exist.

The above statements are true, but we have some assumptions that are ridiculous to realize.  The outside T and RH are not constants, they are changing hourly.  And the temperature of the wheat is assumed to be the temperature of the air when doing the EMC calculation, and in fact the temperature of the grain is never the same as that of the air.  The grain temperature is always following the air T.  Maybe in a lab you can produce a constant T and RH for hours and hours and eventually the wheat will become the same temperature, but one has no control over the T and RH of the outside air.  And what happens in the mean time — when the grain is at a much different temperature than the air?

Let’s use the above example, with the wheat being 14.4% and at 5 C.  The example says that the EMC of the air is 14.2%, and since this is less than the MC of the grain, we would think that drying would occur.  But it does not, in fact wetting or hydration occurs.  Why?  When the outside air, at 10 C, hits the grain at 5 C, it instantly becomes the same temperature as the grain, 5C (because the specific heat of grain is so much greater than the air).  The amount of water in the air, or absolute humidity, remains the same, and therefore a reduction in temperature will result in a higher RH.  The RH of the air will increase , as the T decreases to 5 C.  In using the grain drying calculator we see that the outside RH must be less than 45.1% to have drying.  Since the outside RH is well above 45; indeed by 15%, we will get some pretty serious wetting.  The calculator takes into account the grain temperature, and the fact that the outside air will become the same as the grain temperature as soon as it hits it.  It predicts the drying conditions right now, not what eventually will happen.  The calculator does not assume a constant air T and RH.  Outside conditions change, and sometimes rapidly.  The calculation should be done every hour to determine the current drying conditions.   EMC of the air is an absurd concept; the air is not in equilibrium with the grain, as the name EMC would suggest.  EMC equations can be used to predict drying conditions, but they must be used correctly and not rely on unrealistic assumptions.

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