There is a false sense of security when we think that dry grain is actually dry. Let’s take for example wheat, at 20 deg C, with a moisture content (MC) of 14%. Wheat is typically considered dry if it has a MC < 14.5%; but just how much water is in the grain as opposed to in the air? Let’s see! Consider a volume or space of one cubic meter. This is 27.5 bushels and at 60 lbs per bushed, this would have a weight of 1650 lbs. We said the MC was 14%, so 1650 x 0.14 = 231 lbs or 104.78 kilograms. That is, one cubic meter of wheat at an MC of 14% has 104,780 grams of water in it. Now air at 20 deg C that is saturated, 100% relative humidity (RH) contains 17.5 grams of water. But we shouldn’t be looking at saturated air in the bin, it is more likely to be around 75% RH and one cubic meter of air at 20 C, 75% RH would have 13.2 grams. However in our one cubic meter of grain, the grain occupies 60% of the space, and 40% is air. Therefore the water in the air is 0.40 x 13.2 = 5.28 grams.
So, this so called dry grain is actually caring 104,780 grams of water per cubic meter, and the air surrounding it is holding a tiny fraction of that, 5.28 grams. Even wet air at 100% RH contains thousands of times less water than the so called dry grain it surrounds. So, it is not so dry after all. And it is truly amazing that a seed can retain such a large amount of water.
What we have learned here is important to understanding grain drying. Let’s suppose that we were blowing air through the wheat and that it went from 50% RH to 75% by picking up some water from the grain. How many air exchanges would it take to lower the MC of the grain by one percent? 14% was 104.780 grams of water, so 1% would be about 7,484 grams of water to be removed. One air exchange going from 50 to 75% would take out 1.75 grams. Therefore we would need over 4,000 air exchanges to lower the MC by one percent. Now one bushel is 1.2446 cubic feet, but the air in between the wheat would only be about a half a cubic foot. At an air flow rate of 1 CFM/bu, an air exchange would occur every half a minute and we need 4000 air exchanges, 4000/ (60 x 2) ,which would take 33 hours. It would take over a day of absolutely ideal conditions to knock down the MC by one percent. If we are using the grain dryer calculator this means we would need the outside RH to be 25% less than the threshold RH for 33 hours. This example contains gross approximations and assumptions like the grain and air remaining at 20 C (it won’t); but it does suggest how an estimate of the drying time could be determined.