I see the Gaggia Classic Pro has some issue with the temperature as it uses a hysteresis to do bang-bang control. This is not necessarily something bad but from what people say the bands are quite big.
The common solution to the problem is to buy a PID kit with display, possibility to change the temperature setpoint and a shitload of crap.
However, to fix the problem you could just change the hysteresis limits so the bang-bang controller is faster to react and there’s less temperature difference. I have not looked into how to change these but I would imagine it would be a matter of some small soldering and a few cents of components vs 100 usd. Perhaps you can get away without soldering? Has anyone looked at this option? Of course you will use more the relay and may need to change sooner but these are also dirty cheap.
So here’s a simple diagram of the Gaggia: https://imgur.com/a/445X7xq It seems it would be the element 4 - coffee thermostat - that does everything. A quick google search shows that you can find some for like 5 usd: https://www.bluestarcoffee.eu/en/Gaggia-Coffee-Thermostat-107C/m-435.aspx. Note that they sell them with different setpoints therefore the whole thing must the the control and there should be some available with narower hysteresis bands maybe.
I’m not sure whether people have give you the answers you’re looking for, but a slightly different response: there are cases where people have installed very simple PID algorithms which dispense entirely with the ID part and only do the “proportional” part. These work well!
So you don’t need something complicated. However I still think these algorithms will instill a proportional response (so time of input will be proportional to the error) and this is crucial.
While the PID is primarily installed to fix the temperature stability issue, it also allows you to adjust your brew temp if you want to try a 90-degree shot and then crank it back up to 98 for a different roast.
People talk a lot about how the Gaggia doesn’t hold temp while brewing, but the boiler can also get too hot, hurting your shot.
Exactly. Mechanical thermostats can also be unreliable over time as they continue to heat cycle which will eventually incur plastic deformation and lead to leading and lagging temperature engagements. Prefer not to have to swap out a thermostat every time I want to change my temperature let alone after they start to drift or fail. Installing the PID is not that cumbersome even for those that are not that handy.