But I'm frustrated with A) the price of PID kits online (~$150 US), compared to kits on amazon (~$40 US) and B) how you ultimately lose steam functionality because the PID kit locks the temperature to just one set point, rather than two. C) how often my Gaggia Classic Pro cycles the heating element on and off. It can't hold temperature stable.

So here's my idea. My Classic Pro has two thermostats that cut off power to the heating elements at 145c, and 107c -- DM1288 steam thermostat, and DM1168 coffee thermostat. Both are M4 thread that screws into the boiler. One gets bypassed (the coffee thermostat) when you flip a switch.

There are thermocouples/RTD probes that have M4 thread connections. Most of the kits only have *one* RTD, and stick it on the coffee thermostat position. Why not put two RTD probes, one on the coffee thermostat position, and one on the steam position?

If the RTD this output curve for a 1000 ohm RTD here is correct, and is as linear as it appears https://www.bapihvac.com/rtd-overview/ then we can put two of the exact same RTD probes on the boiler, one connected at the coffee thermostat position, and then one at the steam position. The trick would be to make the PID controller think the temperature *at the steam position* is

*lower*than it actually is in reality.. so we need to decrease the resistance. How do you decrease the resistance of a resistor? You jumper across it with another resistor. This gives two paths for current to flow across increasing current capacity and thus lowering resistance. With a little math; we can figure out how to

*intentionally*offset the temperature read by the PID controller.

So lets say for the sake of simplicity the coffee temp is 100c, and the steam temp is 150c. Lets say the boiler currently read through the coffee RTD reads 100c, or 1400 ohms. So to trick a the PID into pushing the temperature up to 150c, the RTD needs to read ~200 ohms less. So when the RTD is at 1600 ohms, it needs to be 1400 ohms. With this calculator app, we can calculate the "missing resistor" parallel value paired with 1600 ohms -- 11,000 ohms: https://www.omnicalculator.com/physics/ ... l-resistor

Lucky us, 11,000 ohm is a standard value for a 5% or 1% resistor! https://eepower.com/resistor-guide/resi ... or-values/ and they're like $0.68 USD off digikey https://www.digikey.com/en/products/det ... EB/1553654

So a $0.68 resistor, a $40 PID controller, $?? for two RTD pt100 temperature probes, and $11 for a 40A solid state relay? Somebody check my work to make sure I'm not crazy?