The biggest enemy of a RIMS is stuck mash, and/or uneven flow-rate from the pump. This will eventually setoff the PID-control. Hence, a dedicated chapter till Stuck Mash is needed even short.
Stuck Mash is the situation when the flow of water/wort is no longer happing in the desired range (whatever that might be), and/or stopping all together. For a RIMS to work a given flow is needed.
Stuck Mash happens when the gain is milled to much/fine and space around the gain is to narrow. Hence, if you see stuck mash when dealing with Pale or Pilsner malt, you should mill it with a larger gap in the miller. For oats, flaked wheat and especially flour these will fast give issues if used in to large quantities.
The mill sieze I use is 1.3-1.4 mm!
To overcome the stuck mash situation you will need to build in a over-flow system, there leads the water/wort towards the heater/bottom of pot as else the system gets hick-ups if the flow is not sustained. I guess it is also important the temperature probe is somewhere near the area where the over-flow system is located, so the probe is measuring both the wetted malt temperature, but also the liquid temperature in a stuck mash situation.
To futher comebat the issue of stuck mash, you should always make use of Rise Hulls (2-5% of malt bill) as soon as you put oats, wheat either flaked or base into you mash. This should be the first stuff to buy when going RIMS-way!
Just 2 more picture to show the overflow system. Fromt he bottoom to silicon tubes are mounted, one for the pump intake, the other for the overspil system. The silicon tube can tangle/twist itself and hence adjust to any volume of gain/water!