Not too sure how much this is needed in beer making as I am new to it, but I use this alot in winemaking and figured it may have a use here as well. I dont heat my house very warm in the winter "65-67*f " same in the summer. So I had bit of trouble keeping my must at a better "and stable" temp for fementing some wines "70-72". So I made this pid controlled heater. I used a two part epoxy putty on the side of the primary and while it was still soft I stuck the threaded probe into it. Just before it hardened I unscrewed it out of the putty. After the putty hardened I can easily thread the probe on or off the primary. Then I simply plugged a heating pad into the pid controller and strapped it to the primary with old lid gaskets. I just set the temp and it keeps it within one degree. There was difference in the temp of the must and the temp of the outside of the primary in the putty. Using a digital thermometer I found the must was three degrees warmer than the temp the pid was displaying. Easy fix, just programed the pid to display three degrees higher. I triple checked this at different temps and it is right on.
Here is a pic. The pid is mounted in a New Kids on the Block lunchbox..lol.