While it is probably possible to use a PID with a set temperature, the problem becomes control the strength of the boil. First Ill explain it in theory what should be happening, then the real world.
In theory, water boils at 212 (on a standard day, altitude, pressure and temp change this but thats more complex), so at 211 the wort doesnt boil and at 212 it does. So logic would then state put the temp to 212. The problem arises in that once the water reaches 212 it no longer rises in temperature. So adding more heat doesnt make it hotter it just makes it boil more vigorously. With a PID, if you set it to 212 there is no way to control the level of boil other than setting it to 212 gets you some level (depending on PID settings and element) or setting it to any temperature above would give you the max output of the element. This is how it would work in theory
In reality, some parts of the water hit boiling before 212 so you get a simmer before a boil. The problem still arises that there is no way to control the level of boil based on temp, because the PID cannot determine the level of boil. while you might be able to set it at a temp just below boiling and get a strong simmer that is close to what you want there is no way to control that. The reasoning behind wanting a manual setting is to be able to lower the power of the element to get the boil to an equilibrium with the element power to get the right amount of boil to control boil off.
Now you might be able to get away with using the temp setting and the difference depending on your system and location, but that would change day to day and would not be very controllable. I have not tried using a PID in auto mode to boil so if someone has experience with this and would like to add to this please do. But for the price I would search for a PID that has a manual mode, you will be happier in the long run.
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