Grain Drying Calculator

Natural air can be used for drying grain if the conditions are right. This calculator will determine if the ambient outside air is suitable for drying. The inputs are grain moisture content, %, grain temperature, C, and the outside air temperature, C. The output is the threshold relative humidity for several different types of grain. If the outside air’s relative humidity is less than this threshold drying will occur. A larger difference is indicative of better drying conditions.

The calculator grain drying calculator is found at

When grain at a specific moisture content is allowed to equalize with the surrounding air, it will approach a relative humidity as determined by its moisture content. Equations relating the relative humidity, the temperature and the Moisture Content at Equilibrium have been developed (EMC) and can be found: ASAE D245.5 ‘Moisture Relationships of Plant-based Agricultrual Products’. These equations will give the relative humidity of a specific grain, that is at a specific moisture content and temperature. If we blow air into the grain that is at the grain temperature; but has a relative humidity below this EMC relative humidity, then drying will occur.

However the outside air temperature is not the same as the grain temperature as it swings up and down in its daily cycle and the grain temperature reluctantly chases. We must find this threshold relative humidity for the air that is at ambient temperature, not grain temperature. We will use pyschrometric saturation charts and equations to do this.

The EMC equations gave us the threshold relative humidity for air at the temperature of the grain. So, first calculate the maximum amount of water (saturation) that water could hold at this temperature (grams of water per cubic meter of air). Multiply this by the EMC threshold relative humidity as determined by the EMC equation, and this then will be the amount of water that is in the air for the grain at that temperature.

When the outside air hits the grain in question it will become the same temperature as the grain because it is much, much denser. It will have changed temperature, but will contain the same amount of water (absolute humidity). And we just calculated this absolute humidity for the air at EMC. Now calculate the saturation absolute humidity for air that is at ambient outside temperature. The ratio of the EMC absolute humidity over the saturation absolute humidity is the Threshold Relative Humidity for drying for air at ambient temperature.

If the outside relative humidity is same as this threshold relative humidity, then no drying or wetting will occur. If the outside relative humidity is greater, than wetting will occur. And if the outside relative humidity is less than this calculated threshold relative humidity, then drying will occur.

This calculator can also be used to determine when condensation will occur on the interior of the walls and roof. This is the case if the calculated threshold relative humidity is greater than 100. This happens when the air temperature is much less than the grain temperature. The cold outside air goes through the grain, is warmed and moisture is added but when it hits the cold exterior walls of the bin that are the same temperature as the outside air, this discharge air becomes over-saturated and water is expelled in the form of condensation. This condensation can run down and form pockets of grain. It is not recommended to run the fans if conditions have a threshold humidity > 100.

Roof Drips


Hi Ron,

The situation is this:

–        His flax was harvested at 8 % moisture roughly a week ago

–        It was put in the bin right away, with no drying, or fans

–        Two days ago he decided to put the fan on because the flax was at 27 C. He ran it for a day and night.

–        When he checked back, there was moisture on the inside roof of the bin, the grain has not changed temperatures


I suggested he start running the fan 9 pm to 9 am, he could also open the hatch on good weather days overnight.. And could consider augering out half the flax and then back into the bin to force air through it.

Did he actually add moisture to the grain by running the fan during the day?

Any thoughts, comments are appreciated.


Good question:
I don’t have all the facts here, but I will run with what you have given me.  The flax is 27 C with a moisture content of 8%.  It’s been standing like this for a while so it will have reached equilibrium with the air in the bin  at 27 C and a relative humidity of 65% ( I used the Henderson EMC equations for canola, I didn’t have the product coefficients for flax, but they should be similar to canola as they are both oil-seeds).  Now when this air hits the cold bin roof, it will cool to the point at which it can no longer hold water. This can all be calculated using the pyschrometric saturation graph and in equation form:

Ws =  0.000289 *T3 + 0.010873 * T2 + 0.311043 * T + 4.617135

where T is in deg C, and Ws is the most water that air can hold at that temperature (saturated) and is in units of gr/m^3

The relative humidity tells us what percentage of this saturated amount is in the air.  I have attached.   In our case with air at 27C and 65% relative humidity, the air is holding 17.3 gr/m^3.  But air at 19.5 C can only hold , at most 17.3 gr/m^3  –if the air gets any colder it will start dropping it as liquid water.  So if the bin roof is any colder than 20 C, you will find that water is condensing and running down the sides.  Even though the flax is dry!

We certainly don’t want water raining down on the flax.  Normally your advice of running the fan from 9 PM to 9AM is good advice, but not so much here, because at this time of year, you can pretty much guarantee that the roof will be colder than 20 C and we will get more condensation.

I think the best thing to do is wait for a sunny day (sun heats roof) above or at least close to 20 C, and start the fan and cool the flax down, and as the flax cools, the air surrounding it will contain less and less water.  For example once we get the flax down to 20 C (still at 8% MC) the temperature of the roof can be lower before condensation occurs; that is 12 C.  And once you get the flax to 15 C, the roof will have to be at or below 7 C before condensation will occur.

How do I know all this?  I have made a grain drying calculator, that I now run in Excel, but I hope to get an app made for an iphone.  It does all the math, the only thing you input is the moisture content of the grain,  the temperature of the grain, and the outside air temperature.  It gives you what I am calling the threshold relative humidity.  If the outside relative humidity is less than this threshold humidity, your grain will dry; and if the outside relative humidity is greater than this threshold relative humidity your grain will get hydrated.   And you can go through exercises like we just did above to know when your bin is going to start raining inside.  It is really neat.  I am not releasing it yet, we will be validating it on our past years of drying data.  And I don’t have the coefficients for all the grains yet.  I don’t have them for flax, rye, peas, and oats.  I would like to release the calculator with all the common grains.  Stay tuned!  And I would be more than willing to answer any other questions.
Just another thought, if the farmer’s flax is rising in temperature (heating) then you may not be able to wait for the roof to heat up — get the fan turned on, you have to get that flax cooled right now, even if there is some condensation.

Ron Palmer