Just for what it’s worth, incidentally, I’m not personally a great fan of managing the temperature of 25 litres of wort by applying heat to a square foot or so of the outside of the FV. To me that’s just not enough surface area, and leaves a big question mark over the temperature differences within the body of fluid itself. So I have fitted a coil of (sterilised!) plastic pipe inside my FV, through which I continuously pump temperature controlled water. I then measure the temperature of the water coming back from this heat exchanger coil as a good proxy for the average temp of the beer - a lot more accurate IMHO than just taking a reading at a single point.