OK, so you want to: “measure the moisture of grain using only a sensor in the plenum and headspace.”
To determine the moisture content of the grain, you would use EMC equations that would require the parameters of the air surrounding the grain in question. These parameters would be temperature and relative humidity. So you need sensors, that would sense the temp and RH of the air surrounding the grain. Now we need to make some reasonable assumptions here. I will assume that you have just been running your fan and as such the top layer of grain will probably be wetter and warmer than the bottom layer. But it is the top layer that we are interested in, why? Because it is the last layer that the air goes through before exiting. So the temp of the exhaust air will be the same as the temp of the air in the top layer. The same can be said about the RH. So whether you have the sensors in the top layer, or in the stream of air exiting the bin, it will be the same. The second assumption is that the top layer of grain in the bin is consistent in moisture content, and as such the air surrounding it will be consistent in temp and RH and that the air exiting the bin would be the same in both temp and RH. And assuming this is the case, then it would not matter if the bin has exhaust vents, because the air exiting the bin, whether by vent or main portal, the exhaust air would be the same in both temp and RH. Now, to get to the question of where to place the sensors. In the experiments we did, we had the temp and RH sensors mounted under the roof, just inside and down a couple feet from the main portal. In your case, I will assume that you will be using a moisture cable with temp/RH sensors every few feet. I would hang this string right from the collar of the main portal. It should be a bit to the side so as not to interfere with the auger’s loading chute. In our experiments, we used as many as 9 moisture cables per bin; but, I think you only need one — one in the center. The layers going across the bin were fairly consistent; but there was a difference in the top to the bottom layers. So, there you have it, my recommendation would be to hang one moisture cable from the collar of the main portal. That all being said, I am not a real fan of the moisture cables because:1. They are expensive2.The RH sensor is not reliable and not that accurate.3. One must use EMC equations by inputting the T and RH to get moisture content, and they are ugly equations, with questionable accuracy.4. EMC equations give moisture content on a dry basis, and this must be converted to wet basis. (This conversion is often over looked)
I like the idea of using only a temperature cable. To determine the actual moisture content, one would sample and measuring it directly. The top layer’s moisture content does not change quickly and is the last to dry.