A farmer asked me if it was possible to determine the moisture content of his grain, by knowing the relative humidity (RH) of the air being expelled from the grain bin. Here is what I told him:
OK Kelsey, I am behind my laptop and I now can give you an in depth reply. Your question: “a way to determine the moisture content of the grain inside a aeration bin? By measuring the relative humidity of the air exiting the top of a bin?” You need to know two things, the Relative Humidity RH and the Temperature T. Knowing only the relative humidity, is not enough, in fact without knowing the temperature, the relative humidity is meaningless. The amount of water that the air can hold is very much dependent on the temperature. The relative humidity only tells you the percentage of that capacity, at that particular temperature. OK, so let’s assume that you do have knowledge of both the temperature and the relative humidity of the air leaving the bin. There are EMC (equilibrium moisture content) equations in which you can plug in the Temp and RH and out pops the moisture content (MC) of that particular grain. These equations were made by trial and error. Scientist would take grain at a certain MC and Temp; put it into a sealed container and let it equalize with the air for an hour or so and then measure the RH of the air. They would do this hundreds of times for various temperatures and MC and then plot the results on a graph. They then tried to define the points with the best fitting equation. These equations have been around for a long time and are published in ASAE (American Society of Ag Engineering) journal D245.5 “Moisture Relationship of Plant Based Agricultural Products” However they are not for the faint of heart — they are ugly equations. Yes they relate Temperature, RH and MC and you kind of have to know what you are doing with conversions etc. For example we are used to MC given on a WB or wet basis. Whereas the EMC use a DB dry basis for MC. I tried to help some farmers through these calculations, and then it dawned on me that we have computers and smart phones that are really good and fast at doing calculations and conversions. So, I made a sort of app for your smart phone that does these calculations for you. You can find it at planetcalc.com/4959/ called the grain drying calculator. It is normally used to determine if you have conditions for drying; but we can use it also the other way to determine the MC if you know the Temp and Relative Humidity. The calculator asks for inputs of the Temp of Grain, the Temp of the Outside Air, the MC and it gives you the thresholld RH to which any RH below this RH thres — you will have drying conditions. I uploaded this calculator to my Iphone, so that I can use it anywhere.
Let’s make up an example to show how we can get the MC. Assume we have Canola, the RH of the air leaving the bin is 70 % and the Garin Tempi is 20, and Outside Air is 15 deg C. The first thing to do is to set the Grain Temp and Air Temp to 20. We don’t know the MC, but we will guess — put in 10% in the MC; and press calculate. Under Canola we get an RH of 74. This is too high, So guess 9 for the MC and that gives us 68.6. A bit too low. 8 gives us 61% and 11 gives us 79.9%. But we want 70% So, I try 9.2% for the MC and it gives a thres RH 69,96. This is very close to 70, so I will claim that your Canola is at a MC of 9.2% — by a little bit of trial and error. Just make sure that the outside air is set to the grain temp when you are doing this (even though it isn’t)I hope this helps.
//Ron Palmer Ph.D. P.Eng.