Temperature Controller

A temperature controller and the resistance-temperature-detector (RTD) sensor it often reads together complete the same basic closed loop that holds any heated or cooled surface at its setpoint — the RTD measures actual temperature through a precisely known electrical resistance change, and the controller uses that reading to continuously adjust heating or cooling power.

Heated/cooled surface needs to hold a precise, stable temperature
Requires continuous measurement feeding continuous power adjustment
Temperature control loop
RTD sensor
Measures temperature via precisely known resistance-vs-temperature relationship
Temperature controller
Reads the RTD, continuously adjusts heater/cooler power to hold setpoint

RTDs are chosen over a thermocouple specifically when a process needs higher absolute accuracy and better long-term stability, at the cost of slightly slower response. A thermocouple generates a small voltage directly from temperature difference, which responds quickly but can drift in calibration accuracy over time and across different thermocouple types; an RTD instead measures temperature through a metal element's electrical resistance, which follows a very precisely known and stable relationship to temperature — giving RTDs an edge in long-term accuracy and repeatability for applications where that precision matters more than the fastest possible response.

RTD and Temperature Controller — the Closed Loop RTD trades a bit of response speed for long-term accuracy and stability RTD — resistance vs. temperature precisely known, stable relationship Controller — closed-loop regulation RTD read power adj. continuous feedback loop this closed-loop pattern underlies every thermal control surface already covered — pedestals, chucks, bake plates

The controller's continuous feedback loop is what distinguishes true temperature control from simply applying a fixed amount of heating or cooling power. Open-loop heating — applying a set power level and hoping it produces the right temperature — drifts easily as ambient conditions, load, or component aging change; a temperature controller continuously compares the RTD's actual reading against the target setpoint and adjusts power up or down in real time, holding temperature steady despite whatever disturbances the surrounding process throws at it.

RTDs and temperature controllers together implement the basic closed-loop control pattern that underlies essentially every precisely-held temperature surface this project has covered, and the choice of RTD specifically over a thermocouple reflects a deliberate tradeoff favoring long-term measurement accuracy and stability.

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