What a PID actually changes
A PID holds brew temperature to a fraction of a degree. That matters for a light roast and a repeatable shot, and much less for a dark roast in milk.
Temperature
Brew temperature by roast level, and how much stability really matters.
Articles
Single boiler, heat exchanger or dual boiler
The three ways a machine makes brew water and steam at once, what each one costs you in waiting, and which one suits how you actually drink.
Water temperature, and what it actually changes
The 90-96 °C window, how roast level moves the right number, and why stability matters more than the setting. Plus how to test it by taste rather than by spec.