I've been brewing on a 35l 3.1.1 for about a year now, and have always wondered what the temperature in the malt pipe was in relation to the set temperature on the controller. I have previously checked the calibration on my controller using plain water in the unit and two other thermometers I have and found it to be accurate.
I recently came across a meat thermometer with waterproof probes, and, as it was inexpensive, bought it. I enlarged one of the holes in my top screen to allow for the probe to be slid through it. In this mornings mash (temp set to 147 degrees), the temp in what should be close to the center of the grain bed started out at 144 at the beginning of the mash. It slowly climbed to 149 at about 40 minutes in, then retreated to 148 where it remained until done. This gives me a little confidence that the unit is maintaining my set temperature pretty well as I figure a 5 degree variance around the set temp is not bad.
I have also been dialing in my new Maltzilla mill, this was my third brew doing my own milling rather that buying milled grain from Morebeer. Last time, I had the crush too fine and my mash stuck big time. This time, I opened the gap a little and was able to maintain recirc for the whole mash, albeit VERY slow. When I drained it prior to boil, I saw the issue (attached photo). Two thirds of my top screen was covered with an eight inch of dough, not allowing flow. Next time, I'll open the gap another "click", and hopefully, it will allow for better flow through the top screen. I am wondering, however, how improved flow through the top screen will alter the temperature, if at all.