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.

Fan Control Strategies from the Worst to the Ultimate

For years farmers have wondered what to do — when should they turn the fan on, when should they turn the fan off.  Intuitively they knew that there were ambient conditions that were better for drying and there were times when there were terrible conditions for drying; like when it was raining.  The experts for years have not really known either, so the recommendation for years has been to just leave the fan on continuously until you thought the grain was dry.  For years this has been working, but it does beg the question:  Is there a better way.   I have now been looking at this for years now, and I have been analyzing the data that we have been collecting for eight years now, and the following is what I think should be done to control your fan.  It goes from worst practice to what I think is the ultimate or optimum controller.  I will have more details on each of these in subsequent blogs.

  • The worst thing to do is nothing.  Let that hot grain coming in off the field sit for a while and sweat.  This is not good, your crop is starting the process of spoiling. Even if the grain is coming in dry, you still need to cool it down to stop the spoilage process.  There are two factors in keeping your grain safe from spoilage, and that is cooling and drying.  You can get the grain cool quickly, so get it cooled down quickly.  Turn the fan on as soon as the floor or screen is covered — don’t wait.
  • Turn the fan on only on hot days.
  • Turn the fan on, only during the day.
  • Turn the fan on continuously.
  • Turn the fan on only at nights.
  • Turn the fan on, on cold nights only.
  • Turn the fan on if the grain temperature is greater than the outside air temp, and the relative humidity is less than 80%.
  • Turn the fan on when the outside relative humidity is less than the threshold relative humidity given by the grain dryer calculator  and turn it off when the outside RH > threshold RH from the grain drying calculator. I have done correlations with this strategy comparing it to when the drying actually occurs and it has R correlation values > .75.
  • The ultimate would be to have the fan turned on when the conditions are right, and headed for good drying with the grain drying calculator, and then shutting the fan off when there is no drying.  This is calculated by watching how much water is going into the bin, and how much is coming out.  As long as there is a net amount of water coming out of the bin, we will keep the fan on, but as soon as we see more water going in than coming out.  This requires temperature and RH sensors on both the air going in and being discharged.  The temperature and relative humidity are plugged into the pyschrometric equation for saturated air and pro rated with the relative humidity to get the absolute humidity.  We need a few more sensors with this, but it is the ultimate, the Cadilac, of controllers.

Suck or Blow?

I have often been asked whether to blow air into the bin, or suck it out.  The short answer is to blow it in.  There are several reasons to blow:  First we will utilize the heat given off by the motor to aid in the drying process.  If we suck, that heat is just wasted.    Also when we blow, there is compression of the air that occurs, which manifests itself in the form of heat.  We have measured this compression to be 5 or 6 inches of water (supports a column of water in a manometer) and this results in an increase temperature of a couple of degrees.  This also helps in the drying process.  And the third reason is somewhat more practical.  Blowing will keep all the little holes in the screen open, whereas sucking will plug the holes with fines.

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

How Much Water is in the Air?

Obviously, the amount of water that is in the air affects its drying ability. Less water results in a lower vapour pressure and a better chance that the grain vapour will overcome it. But how does one determine the amount of water in the air. It turns out that it isn’t that onerous. When air is holding the absolute most it can hold, it is said to be in saturation. The saturated amount of water that air can hold is determined almost exclusively by its temperature and it can be obtained from something called a psychrometric chart. The formula for this is:

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

Where Ws is the saturated amount of water (grams) per cubic meter at a given temperature, T (degrees Celsius, ⁰C).

If air is cold, it cannot hold much water.  At 0 ⁰C the most it can hold is 4.6 gr/m3   whereas when the air is hot at 35 ⁰C it can hold almost ten times that at 41.2 gr/m3.

Vapour Wars

Grain drying is a battle between two forces of vapour pressure, that of the air and that of the grain. The water in the air has a force or pressure trying to push its water into everything it comes in contact with. Likewise grain has a vapour pressure in which water is being pushed out of the seed. When air vapour pressure is greater than grain vapour pressure, water is pushed into the grain, and the grain is wetted down. However when the vapour pressure of the grain exceeds that of the air; water is transferred from the grain to the air and we have drying. The vapour pressure (Vp) of the air is determined by its temperature and by the amount of water it is carrying. At any given temperature the greatest vapour pressure occurs when the air is holding its maximum capacity and this is called the saturated vapour pressure (Vps). Compared to atmospheric pressure (14.7 psia), vapour pressure is typically much smaller (1psia).

Grain vapour pressure is determined by the temperature of the grain, the moisture content (MC) and the type of grain. It is measured indirectly by leaving it equalize in vapour pressure with its surrounding air in a sealed environment. This has resulted in the creation of Equilibrium Moisture Content equations which will be discussed later.

Learning the Process of Grain Aeration

Throughout the upcoming blogs, I intend to go through the basic science of how grain aeration dries or wets grain.  I will take you on the journey of how this project started, what we learned along the way, and in the end I will give you the specifics that you need to know to control your fan.  The conditions that will result in drying — turn the fan on; and conditions in which there will be wetting — turn your fan off.  There are very few people who know about this, so you will be on top of very recent research.  I am excited, not only in telling you about the new control strategy but also in educating you as to why it is the ultimate way to control your aeration fan.   Comments of course will be welcomed.