First we must agree on what the objective of the controller would be? I assume that we only want to run the fan when conditions are right for drying; and secondly that we want the grain to be as safe (least spoilage or deterioration) as possible. And finally, that we want to do this in the most economical way.
What do we need? We need to know the temperature (T) and relative humidity (RH) of the outside air near the intake of the fan. We also need at least one OPI moisture cable with T and RH sensors every 4 ft. This string could be hung pretty much down the middle, with the highest sensor bud, being just under the center ring, but above the grain. This sensor would be used to detect the T and RH of the air being discharged from the bin. Also a controller than can turn the fan on and off based on pyschrometric equations. (I did this in a previous blog- How much water is in the air?)
We mentioned before the importance of getting the fan turned on ASAP. The fan would be turned on while the bin is being filled with the freshly harvested grain, whether it is dry or not. Then every hour the amount of water in the air would be calculated for the air entering and leaving the bin, using the T and RH sensors for the outside air, and the OPI T and RH sensor, at the top of the bin sampling the discharge air. These psychrometric equations can calculate the water in a specific volume of air, by only knowing the T and RH of the air. If there is more water in the discharge air as compared to the input air, the fans would remain on. When an hourly decision time has a calculation that shows that there is more water going into the bin than out, the fan would be shut off. Then we wait for an hour and then will decide whether or not the fan will be turned on? But we can’t use our water in/out balance technique that we used to turn the fan off because now that the fan is off, the air around the top sensor is not necessarily representative of the discharge air. We need to make the decision to turn the fan on based on something else.
To make the decision to turn the fan on, we will base it on the principle of the grain drying calculator . We can easily calculate the amount of water in the outside air using the T and RH plugged into the psychrometric equations. But the outside air becomes the grain temperature (or very close to) when it hits it, and since we know how much water is in that air, we can calculate its RH for its new temperature, again using the psychrometric equations at a temperature of the grain as given by the OPI sensor. If this newly calculated RH is less than the RH of the OPI sensor at that sensor bud, then we have a drying condition, for at least that bud. But the sensor buds are every 4 feet along the OPI string, so a calculation must be done for each bud that is in the grain. When a majority of the buds have a RH higher than the calculated RH, then we have a majority of the buds indicating that a drying condition does exist, and therefore the fans should be turned on. Then we wait an hour and use the water balance technique to turn the fan off.
The OPI also can give us the moisture content of the grain by utilizing EMC equations. However the fans do not have to be operated until all the grain is dry, but only until the average is dry. When the grain is eventually unloaded, the over dry and under dry grain can be blended to give an overall grain that is just dry. The above technique also cools the grain, and even if there is some slightly tough grain (probably at the top), it will not spoil because it will be cool or cold.
And that’s it, the ultimate controller. The really neat thing is that it does not depend on the type of grain, the accuracy of EMC equations, or the moisture content of the grain (other than when we are deciding to terminate the operation when the average of the grain is dry). It will keep the grain cool and safe, with the minimum amount of fan time. It is the ultimate.