I use PID to control a Lee 4-20. The unit was made by member (Pigslayer) here. The circuitry, plans and building tips are archived in Pat's posts on this forum. The PID plugs into the wall. the pot plugs into the PID and the sensor from the PID is clipped to the pot with the probe held in the melt. The pot is set to full heat and the PID cycles the pot heating element to reach and stabilize at the temp as you set the PID. There is some cycling plus and minus in degrees that gets smaller and smaller in temp swing before the temp stabilizes as the PID learns your metal response to heat and your routine. The chip LEARNS.
I cast a full pot nonstop without fluxing after initial fluxing and don't add metal or flux while casting as this wildly changes temperature and screws up stability of temperature.
The 4-20 is a bottom pour spout pot and junk on the top of the melt is of no consequence to bullet quality unless you run the pot down to under about 1.5 pounds. When I run it down that low, then I refill and melt a full pot, flux and cast continuing the cycle a pot at a time without changing the temperature setting.
Fluxing and adding metal while working defeats the stability benefit of the PID, Don't add metal and flux or you will be sitting and waiting a VERY LOT for temperature stabilization..
Every time you change the degree setting of the PID it has to re-educate itself again. Be sure of the setting you want and don't change it all even if you are going to run many potfulls in succession. Any time you change the temp setting you will be twitteling thumbs a long time.
PID units are rated in Watts, one Lee 4-20 is 500 watts. The PID chip only provides for one sensor, so if you want to run 2 pots, you need 2 separate PID units. you can't stack multiple pots on one PID.
An extremely useful feature of using a PID is that you can exactly determine ideal pot temperature for casting any bullet alloy. Set the pot high and get a fully fluid melt. Then turn the PID down to room temp and note the very first solidification sign of your metal surface and note temperature. The Ideal pot temp for casting that alloy is 100 degrees hotter than first solidification.