Understanding Equilibrium Moisture Content
Understand Equilibrium Moisture Content (EMC) and you will understand the drying principle for aeration. EMC is the maximum drying potential air has with a given temperature and relative humidity.
The table below gives the equilibrium moisture content for wheat and canola with drying air at 50 degrees F and 77 degrees F. Various relative humidities are also listed.
| Relative Air Humidity |
Equilibrium Moisture Content (EMC) | |||
| Wheat | Canola | |||
| 50° F | 77° F | 50° F | 77° F | |
| 58% | 12% | 12% | 8.6% | 7.5% |
| 64% | 14% | 13% | 9.4% | 8.2% |
| 70% | 15% | 14% | 10.3% | 9.0% |
| 75% | 16% | 15% | 11.1% | 9.8% |
| 79% | 17% | 16% | 12% | 10.8% |
| 83% | 18% | 17% | 13.2% | 12% |
| 86% | 19% | 18% | 14.5% | 13.4% |
For example, as shown in this chart, 50 degrees F air with a relative humidity of 58% can dry wheat down to about 12% and canola down to 8.6%. It does not matter how long you run the fan, grain will never get any dryer than the EMC of the air blowing through the grain. The lower the EMC the faster the grain will dry. EMC changes throughout the day.
Normal Equilibrium Moisture Content During Harvest
| August | September | October | |||||||
| Temp. | Hum. | Wheat EMC |
Temp. | Hum. | Wheat EMC |
Temp. | Hum. | Wheat EMC |
|
| 1 PM | 73° F | 54% | 12% | 61° F | 56% | 12.5% | 50° F | 59% | 13% |
| 7 PM | 70° F | 62% | 12.5% | 67° F | 67% | 14% | 45° F | 69% | 15% |
| 1 AM | 59° F | 82% | 16% | 50° F | 80% | 17% | 40° F | 78% | 18% |
| 7 AM | 58° F | 84% | 16.5% | 47° F | 84% | 17.5% | 37° F | 82% | 17.5% |
As you can see in the above table, during August and early September normal temperature and humidity levels give you an EMC that is low enough to create good drying conditions for at least half the day. But, as it gets towards the end of September and into October, temperature and humidity levels are such that you only get an EMC that is low enough to dry grain for a few hours each day.
As you study the EMC tables, think about your own aeration experiences and recall the temperature and humidity levels. Sometimes you get dry grain with aeration and sometimes you do not. It all depends on the EMC.
Aeration can dry a bin of grain if it is harvested early enough. Aeration can also increase the storage life of late harvested grain by cooling, but nobody can guarantee dry grain every year with just aeration.
Add a Desert Air supplemental heater to your aeration to help ensure dry grain regardless of when weather lets you harvest.
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