THE MARTIN TECH LEARNING CENTER
Air can dry grain, add moisture, or help it stay near balance. Learn what EMC tells you—and why the hours you choose can matter to grain condition, weight, and electricity costs.
01 / START HERE
Grain is not sealed off from the air around it. Under some conditions it releases water; under others it absorbs water. The useful question is: which way would this air move my grain’s moisture?
Equilibrium Moisture Content
For a particular crop, temperature, and relative humidity, EMC is the moisture percentage the grain tends toward if those conditions last long enough.
A 12% EMC means a tendency toward 12% grain moisture. It does not mean the air is 12% humid, or that all the grain has already reached 12%.
Different grains respond differently.
Temperature changes the balance.
How humid the air is at that temperature.
Compare this with your grain moisture.
SAME HUMIDITY. DIFFERENT BALANCE.
These rounded corn EMC examples keep relative humidity at 70%. Switch the air temperature and compare the EMC.
At 60°F and 70% RH, corn EMC is about 15% (14.8% in the published table).
Rounded for illustration: 14.8% at 60°F and 17.6% at 20°F, both at 70% RH. These are equilibrium values, not a forecast or drying-time calculation. Source: UMN Extension—corn EMC table 6. For other crops and conditions, calculate EMC with Fan Buddy.
Researchers measure how grain balances with controlled air, then fit equations to those measurements. Here is one common approach, simplified. “Isotherm” means at a constant temperature.
Teaching illustration only: the samples, curves, and animation timing below are not laboratory data or a crop calculator.
Grain samples sit above, without touching, a humidity-control solution in sealed jars. Different solutions establish different relative humidities; temperature stays steady. This illustrates a static laboratory method.
As water moves in or out, weight changes. Once the sample meets an equilibrium criterion, researchers determine its moisture content using its water and dry-matter masses.
Each humidity and equilibrium-moisture pair becomes a data point. Researchers fit a mathematical model across many measurements and temperatures to estimate EMC between tested conditions.
The coefficients belong to that equation, crop dataset, and unit system. The fit has uncertainty and a tested range.
One curve describes one temperature. Tests at other temperatures reveal how the relationship changes. For grains, EMC generally decreases as temperature rises at the same relative humidity.
The points and fitted curve appear as the illustration progresses. Only three sample points are drawn to keep the illustration simple.
Why this matters on the farm: EMC equations turn measured grain behavior into a practical way to interpret temperature and humidity readings.
A little more science: Grain gaining moisture (adsorption) and grain losing moisture (desorption) can follow different curves. This difference is called hysteresis. Researchers test both paths; the simple illustration above does not show that effect. Read a soybean sorption and hysteresis study.
02 / COMPARE THE NUMBERS
Try an example or adjust the two values to see the moisture direction change.
EMC is 4.0 percentage points below grain moisture. Moisture tends to move from grain to air.
Direction tells you what the air tends to do. Your target determines whether that helps.
Below grain moisture: drying potential. Above it: moisture-gain potential. Matching values: no net moisture exchange at equilibrium. The ±1-point near-balance band is a teaching guide; moisture can still move within it.
Green means a difference of up to 1 percentage point, yellow over 1 to 4, and red over 4. These are illustrative difference ranges, not safety limits or fan commands. Storage suitability also depends on grain moisture and temperature.
EMC indicates a balance point, not drying speed or moisture throughout the bin. Airflow, temperature, grain depth, and time affect progress. Cooling also requires comparing air and grain temperatures. Values to a tenth illustrate the relationship; field EMC is an estimate.
For EMC calculated from your crop, temperature, and humidity, use Fan Buddy.
03 / WHY IT MATTERS TO YOU
Moving air is only part of the job. The same conditions can help one bin and work against another. Start with the grain you have and the result you want.
Lower-EMC air has drying potential. Useful progress also depends on temperature, airflow, and time. Watch the moisture target as the grain dries.
Why care? Fan time should make drying progress.
Air near the grain’s moisture has less tendency to change it. Much lower EMC can continue drying grain after it reaches your goal, removing saleable water weight.
Why care? Extra drying can mean less weight to sell.
Higher-EMC air tends to add moisture. Added water increases weight, but rewetting can be uneven and can damage grain. A favorable EMC alone does not establish a safe reconditioning plan.
Why care? Moisture and weight are connected.
Compare air temperature with grain temperature, too. EMC explains moisture movement; cooling decisions also need temperature, adequate airflow, and enough time for the cooling front to pass through the grain. Explore airflow and storage.
04 / PUT IT ON A TIMELINE
Explore a hypothetical daily EMC curve for one crop. Switching goals changes the starting moisture, target, and example window—not the crop or the curve. Green marks readings inside the stated EMC window; amber marks moisture movement in the desired direction outside that window. These are teaching examples, not recommended control settings.
Blue: EMC for the cropGold dashed: targetWhite dashed: starting grain moistureGreen shading: inside example EMC windowAmber shading: direction only, outside window
The windows are illustrative selections, not Fan Buddy’s algorithm or universal crop limits. Different crops have different EMC under the same weather. Reassess the window as grain changes: 15% EMC can dry 16% grain, but sustained conditions at 15% EMC will not bring it to a 14% target.
Air exchanges heat and moisture as it passes through grain, so lower and upper layers respond at different times. Waiting for preferred EMC conditions must fit the time available before deterioration; wet grain may need continued fan operation. UMN explains drying progress and when to operate fans.
| Hour | Air EMC | Moisture direction |
|---|
Select a location to follow a typical upward-airflow system. This is a schematic, not a sensor-placement or installation plan.
The fan moves air into the bin. Outside temperature and humidity help establish its starting EMC. Heating changes the air conditions, so ambient EMC may differ from plenum EMC.
GrainSight combines grain temperature readings with EMC calculated from temperature and relative-humidity measurements at the ambient air, plenum, and roof sensor to guide automated fan control. Together, these readings help track how the air and grain are interacting as you work toward your selected storage goal.
Explore GrainSight components05 / WHAT IT CAN MEAN FOR YOUR OPERATION
Try the two examples below to see why those moisture decisions matter. One connects fan hours to electricity; the other connects moisture to soybean weight. They illustrate the arithmetic, not guaranteed field results.
Running 24/7 vs. running when EMC favors corn drying. Compare the same fan over your selected operating period. Start with an illustrative estimate of 45% favorable air, then adjust it to see how runtime and electricity costs change.
Choose 1–90 days for the cost comparison. Both schedules cover the same period; this is not a prediction of time to reach your moisture target.
The share of hours when the air’s EMC suits your corn-drying goal. The 45% starting value is an example, not a measured seasonal average; weather and moisture targets change this percentage.
Fewer operating hours—not proof of the same drying result. This compares electricity for two schedules over the same period. Well-chosen fan operation can help avoid unwanted moisture changes, but humid-weather operation can also benefit some layers. Restricting fan hours may extend calendar drying time. Total cost to reach the target depends on drying progress, airflow, temperature, and grain condition; this example does not calculate that cost or predict an earlier finish.
30 days × 24 hours = 720 available hours. Favorable-air runtime = 720 × 45% = 324 fan hours, compared with 720 hours running continuously.
Cost = electrical input kW × fan hours × $/kWh. This simple comparison assumes the fan runs during every favorable hour and stays off otherwise, at constant electrical input. It compares electricity over the same calendar period, not the time needed to reach the same final corn moisture. Cooling and other fan needs are not included.
Use measured electrical power when available. Motor horsepower × 0.746 is rated mechanical power, not actual electrical input; efficiency and loading affect consumption.
What happens to soybean weight when moisture increases? Start with 60,000 bushels at 10% moisture and compare that same soybean dry matter at 12%. A change from 10% to 12% is two percentage points of moisture—not 2% more grain weight. The added weight comes from water. Change the bushels or either moisture value to explore your own example.
Uses 60 lb per soybean bushel to calculate starting weight. Enter bushels at the starting moisture, before any moisture shrink adjustment.
Reconditioning dry soybeans is achievable with suitable airflow, favorable air conditions, and careful management. This example shows the potential weight change from 10% to 12% moisture. A practical plan may involve smaller batches or periodically unloading some grain to accommodate expansion, while checking moisture through the grain depth. The approach should suit your bin and equipment so swelling and uneven moisture are managed. Read NDSU’s reconditioning considerations.
Starting weight
Weight at comparison moisture
Starting weight = entered bushels × 60 lb/bu. Bushels here are weight equivalents, not a measurement of bin volume or test weight.
Final weight = starting weight × (100 − starting moisture) ÷ (100 − comparison moisture). All moisture values are wet-basis percentages. Dry matter is held constant, with no handling or spoilage losses. Added weight is water, not added grain dry matter.
This is not a prediction of sale proceeds or a reconditioning recommendation. Buyer shrink schedules, discounts, costs, grain condition, and the feasibility of reaching the comparison moisture are not modeled. Rewetting can be uneven and cause damage; EMC timing does not eliminate those risks.
06 / TAKE IT TO YOUR BINS
Start with your crop, a representative grain-moisture measurement, and a goal. Use temperature and humidity to find EMC, compare it with the grain, and account for airflow and storage condition. Check moisture and temperature as you go—conditions and useful fan windows change.
Use Fan Buddy for crop-specific EMC and planning tools.
Explore grain depth, airflow, and storage considerations.
Browse the university and industry charts on the page.
Combine temperature monitoring and EMC-based fan control for your bins.
For the underlying science, read the University of Arkansas drying guide and the linked university explanations above. The daily curve and cost/weight comparisons are illustrative, not measured customer results. More reference material is available on our downloads page.
University of Arkansas: On-Farm Grain Drying Methods (PDF)