When do we get condensation, dripping and crusting in the bin?

The grain drying calculator gives the precise conditions under which condensation will occur; and that is whenever RHthres is greater than 100. (See the blog on dripping).  In playing around with the calculator today, I can give you a rule of thumb for this without using the calculator.

For oilseeds like canola and flax, that is just dry 10% MC; if the grain is more than 5 deg C warmer than the outside air (roof), there will be condensation.  I tried the calculator with GrainTemp – AirTemp,  30 – 25,  20 -15, 10 -5.  So a difference of 5 C gave me an RHthres close to 100%.

For cereals like wheat and barley, that is just dry at 14.5% MC; if the grain is more than 7 deg C warmer than the outside air (roof  & walls) , you have conditions for condensation.  Tried 27 – 20,  17 – 10, 7  – 0 and they gave RHthres close to 100.

This is interesting, because we see that oilseeds are more sensitive to condensation, and we certainly don’t need much of a difference in temperature before condensation starts to form on the top layer of grain and the inside of the roof —  only 5 deg C  difference!!  For example, if you have wheat that is 28 C and the outside temp is 20 C, you could turn on the fan and you are in for a bit of condensation on the inside of your roof, and on the top layer of wheat.

For years and years we have had spoilage from condensation because we did not turn the fans on immediately to cool the grain with a temperature that was close to the actual temperature of the grain?

The Natural Air Grain Drying Project — a WGRF project

This project was started in 2007 at the Indian Head Experimental Farm as an IHARF (Indian Head Agricultural Research Foundation) project under the supervision of Guy Lafond.  Data was collected every year from two instrumented bins.  I was given the data in 2010 with a vague mandate to find out what was happening in the bins as the fans ran continuously.  Did the fans have to run continuously?  Were there times during the day in which more drying was taking place?

The experiment was set up brilliantly such that readings were taken every hour and most importantly that  RH and T sensors were placed to measure the discharge or exhaust air at the top of the bin as well as RH and T sensors measured the air entering the bin.  This lead to the ability to measure the water going into the bin and the water leaving the bin and thus we could precisely track the drying, hour by hour.  And the Diurnal Drying Cycle was determined.

In 2012 we got a grant from WGRF (Western Grains Research Foundation) and 4 more bins were instrumented to collect more data.  Another three year grant from WGRF was obtained in 2015 to discover more.  It was called:

New Insights into Natural Air Grain Drying  (2015 – 2018)

Objective: To develop a fan control strategy using natural air that results in the safe storage of grain, that is efficient and results in less fan running time, and that results in more uniform drying of grain.

Benefit: Reducing the risk of grain spoilage and preventing revenue loss with on farm storage.

When Should I Turn the Fan On or Off? But Only have Temp Sensors

In the last blog, I talked about the ultimate controller that used moisture cables with temperature and relative humidity sensors.   But what if I don’t have this T/RH sensor cable — I only have temperature sensors?  Well, we can still do a pretty good job of turning the fan on only when the conditions are right for drying.  How, by using the grain drying calculator .  This calculator determines if conditions are right for drying, but it is subject to the accuracy of the EMC equations, whereas the ultimate controller does not require EMC equations other than to determine the MC of the grain.  Also the ultimate controller uses water balance to turn the fan off, whereas again this method requires the calculator to both turn the fan on and off.

Here is what you do; again it is important to get the fan going immediately upon filling the bin.  Then every hour one makes a calculation by entering the moisture content  (MC) of the grain, the temperature of the grain, and the temperature of the outside air.  The calculator returns a threshold relative humidity ( RHthres ) for several grains.  If the outside RH is less than this threshold, then we have drying conditions.  The greater this difference, the more drying will take place.  So we would turn the fan on.   If however the RHthres is less than the outside air RH, then the fan would be turned off.  Because conditions can change rather quickly, this calculation and  a decision should be made every hour or so.  This process would continue until  the average MC (as measured manually with a  sampling probe).  The bottom will as always dry first, but it is not necessary to continue the process until the top is dry, but only until the average is dry.  When the grain is pulled out, it will blend to give an overall dry.  The top, even if it is a bit tough, will not spoil because it will be cooled with this overall process and therefore be safe from spoilage.

After the process is terminated, the temperature of the grain should be monitored, and if the grain temperature begins to rise substantially, then the process should be restarted.  One might not have to do this all winter, but in the spring and summer the process may be used to keep the grain cool.

If one is not sure about the MC of the grain, then the dry level should be entered into the calculator.  For example, wheat may have been put into the bin, some being 14.2%  some at 15.1%  and another unknown quantity at 14.9%.  The dry level for wheat is 14.5%, so that is what should be put into the calculator, and it will do the calculations for drying the grain to this level.

This calculator is not quite as good as the ultimate control strategy, but it is pretty close.  It relies on the accuracy of the EMC equations, and it also requires manual measurements of the MC to determine the average MC and thus when the process can be terminated.

Planning Future Blogs

I hope to have future blogs — in no particular order and I am putting this out there to perhaps get an idea of what I should do first:

  • Aeration — a useful tool to get ahead of the game, and lower risk.  Combine the grain tough, you can better manage the grain in the bin as opposed to in the field.
  • Tips on storing your grain, including the above, start the fan right away, keep the fan on for the first day, keep the grain as cold as possible, take out a load to get an inverted cone, put the toughest grain at the bottom by starting each new day, when the grain is tough with a new bin, and of course fan control strategies.
  • Using the new calculator http://planetcalc.com/4959/
  • Comparing the new calculator results with those of actual bin drying.
  • The first day, why it is so important  ie get the grain cold to be safe.
  • The ultimate control,  use the OPI moisture sensor, when the fan is off, calculate the moisture in the air around the grain, and when it is greater than the moisture in the outside air, then turn the fan on.  And to turn the fan off, check the moisture in the discharge air, as opposed to the outside air.
  • A list of control strategies depending on what you have:  no sensors,  temp sensor, temp and relative humidity sensor.
  • Diurnal Cycle
  • How fast does grain warm?
  • How fast does grain cool?
  • Cooling is drying?
  • Safety, is a factor of dry grain and cool grain, and the formula that provides  a measure of safety quantitatively.
  • Why the old way of using EMC of the air does not work, along with examples of actual data to show why.
  • Why using mean daily temperatures is misleading.
  • Why does grain dry in the bottom of the bin first.
  • Correlation of actual drying, with fan on only when grain temp > air temp.
  • Correlation of actual drying with calculator threshold RH results
  • How much should grain cool, when energy is used to evaporate the water in it.
  • Condensation on the roof, why does it occur, when does it occur, how to prevent it. Using the calculator to detect conditions for condensation. http://planetcalc.com/4959/
  • This year’s runs on all six bins.
  • Organizing all these topics and items into a more cohesive organized document.

So, I have a lot to do, and as the colder weather sets in, I will have more time to get at this.  Of course your comments and questions are a good source of inspiration and motivation.



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 planetcalc.com/4959/

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