Let me explain the premise briefly without going into tons of details or the specifics of PID functionality and why some people like this type of control. Maybe this will help clear the air with a mile high overview.
1) You have control over raising boiler and vapor temps. You can not make things boil at a higher temp (nor do you want to) but you can monitor the present temp and CUT or LOWER heat to hold a temp or allow more heat so that the boil temp will rise on it's own. Thus you can control the boiling temp as it will never rise higher then the target. If you set a target temp of 173F it will supply electricity to the heating element to produce heat until it reaches that temp in a proportionate way. As it nears the target temp it will lower the heat being applied to hold that temp. It will not go higher than the target temp until the PID or controller changes set points or target temp.
You can not make the boil higher then physics allow it to be based on the alcohol content in the boiler but you can set a target and allow it to rise to that set point and can hold it until a new target is selected.
2) As the temperature being monitored approaches the set point the distillate will run slower and slower and eventually stop which is what we want to happen when temps levels off at a set point.
3) When you set a new set temp that is higher then the current boil temp, the PID or other similar controller will be adding more heat which will cause the still to produce again. The rate of flow is adjustable by ID functions but it's beyond this post.
4A) People using a rheostat type control find fault in #2 and #3 above as the still produces differently then they are used to and they can't hold a steady stream or drip rate which they expect. Thus to them PID control is "broken" or has problems.
4B) People using temperature monitoring and control (aka PID like device) want exactly #2 and #3 and DO NOT need a constant steady stream.
#4 is the source of most arguments or misunderstanding beside "can't control boiler temp" comments.
Why?
The person using the physics of the distillation process to their advantage knows that certain concentrations come out at different temperature ranges and makes the most use of this to their advantage. For example foreshots come off the still first and have the lowest boiling temp. Heads come off next with a bit higher temperature. Then Hearts come off with yet another temperature range, followed by tails which have the highest range of temperatures.
When you "force" a steady stream you are in fact smearing your alcohols (with reflux you can knock much of this back down). Now as an example with a PID you can set alarms and targets. So for example an audible alarm could sound at 170F followed by a target of 173F and an alarm. So when you hear the alarm you know foreshots are coming and can be ready to dump X amount as usual. Since the boiling temp is less than 173F heat is being applied and Heads will flow. As the temp nears 173F the stream or drips will subside until nothing is coming out as you don't allow the boil temp to rise. The maximum amount of heads have now been collected best that can be done without refluxing. Now a new target is set for Hearts and distillate will start to flow again into the new jar you just switched. When the boiler reaches a new target temp of say 204F or whatever you set it to, your Hearts will slow to a drip and then stop. You are finished the concentration of Hearts, can change jars and collect your tails with a new target of 212F.
This works like clockwork but can be fine tuned and adjusted to your style of running or spirit being distilled. For example a brandy might want late heads for flavor so 172F is used vs 173F for whiskey as the stop point of heads/hearts. In this case you want the late heads in your hearts (or keeper jars)! This is what is meant by "fine tuning". With the ability to set alarms at certain temp points you can set an alarm to notify you of transitional ranges. So you can collect in big containers, get to an alarm/transitional point, collect in small jars for later checking and then collect in big jars again until your next transition. Basically the transitions are heads to hearts and of course hearts to tails.
This is a DIFFERENT way of running a pot still, but in many ways is similar or will make some sense to those who reflux. The person who does refluxing will have no issue with stops in distillation to build up concentrations/compacting before starting again. This is a "poor mans" version of that on a pot still to separate as much as possible (within reason since it's a pot still) and stop as much smearing as possible between your transition jars. It of course is not doing any refluxing but the control of boiler temps helps to only allow the main concentration of alcohols out that said temperature will allow as dictated by physics. What it does is give you the cleanest cuts or transitions you can get on a pot still and allows you to make runs exactly the same time and time again assuming what's in the boiler is the same.
This works really well on SPIRIT runs when you strip to specific ABVs.
many people want to use a PID to control their still. There’s pretty much one one way to do that a device that turns the heating element on to get to a desired temperature/result then off when it reaches the target temperature/result. PID and other devices do just that in a different way. It’s basically on/off one is mechanical and one digital. So with no explanations about theory of either device. Where would you put a thermometer to register the necessary heat/result required for deciding what temperature/result you want at any given time. Pot or reflux. Paint by the numbers please. Remember not many have the skill set you have and are only interested in getting results. Hope that makes things a bit clearer.
. The cost factor is what I would consider high, about $80.00 for the PID. The up side once you get it set up, experience, it’s as close as you can get to automation.