I have been asked this question again and again and the first thing I say is that the temperature is more important than the Relative Humidity (RH). So first, only turn the fan on if: OutSide Air Temp < Grain Temp.
However, doesn’t the RH of the outside air and the Moisture Content (MC) of the grain also come into play? Absolutely. And the way we can answer this question is to use the calculator planetcalc.com/4959/ Make the grain temp and air temp the same and try at 10 C 20 C 30 C and calculate corresponding RHt
Flax 9% MC 61.6% 66.2 70.4
Flax 10% MC 68.1 72.1 75.6
Flax 11% 73.2 76.7 80
Flax 12% 77.4 80.4 83
Wheat 16 73.5 75.6
Wheat 15 68.3 70.6
Wheat 14 62.4 65
Yellow Peas 16 73.5 75.6
Yellow Peas 15 69 71.3
Yellow Peas 14 63.8 66.4
Conclusion: If one is to build a controller that is controlled by temp, that is:
Fan On IF: Outside Air Temp < Grain Temp on must also consider RH,
AND Outside Air RH < 80% to start when grain temp and MC high
< 70% when grain cooled and MC close to dry
We almost need to have the RH on a variable knob, so that on the first day when we first start the fan after filling the bin, the grain is warm and maybe a little on the tough side, we put the RH knob to 80%. After a day or so, the grain will have been cooled down, and therefore drier, so we might put the knob at 75%. And then, maybe after a week or so we turn the knob down to 70%. Following this practice should keep the grain cold and only have the fan on when we have drying conditions. This would make for a simple reliable controller that would keep the grain safe and dry.