There are three basic types of temperature controllers available: ON-OFF, Proportional and PID.
ON-OFF controllers are the simplest form of temperature control. If the temperature is below the setpoint, the controller is in the ON state. If the temperature is above the setpoint, the controller is in the OFF state. These types of controllers are usually low cost, which can be a plus, however, because of the way in which they operate, they do not closely control temperature, sometimes cycling on and off very quickly, or overrunning or underrunning the temperature setpoint.
To control the on-off cycling, controller designers add an ON-OFF differential, or hysteresis to the operation. This differential requires that the temperature will exceed the setpoint (in either direction) by a certain value before the controller will cycle states to ON or OFF. This differential also prevents the output contactor (usually mechanical) from chattering, or quickly switching ON and OFF.
Proportional controllers are designed to eliminate the cycling issues inherent in ON-OFF controllers. A proportional controller decreases the cycle time as the temperature approaches the controller setpoint. This has the effect of slowing down the temperature increase so that the heater will not overshoot the controller setpoint. This is called “time proportioning” because it varies the ON-OFF cycle time in direct proportion to the temperature differential between actual and setpoint temperatures. The proportioning action occurs within a certain band of temperatures, called the “proportioning band”. Outside this band, the controller acts as a simple ON-OFF controller, so that if the temperature is below the band, the controller is in the ON state, and if the temperature is above the band, the controller is in the OFF state.
Inside the band, the ON-OFF cycle is a ratio of the measurement difference from the setpoint to the actual temperature. At the setpoint (the midpoint of the proportional band), the ratio is 1:1, or, the ON state is equal to the OFF state in time measurement.
The proportional band is usually measured in percentage or degrees.
PID controllers provide proportional along with Integral and Derivative control, or PID. These additional two control features allow the controller to automatically compensate for changes in the temperature of the unit being monitored. These additional two controls are time-based units, and are usually referred to by the reciprocals, RESET and RATE.
The Integral or RESET value allows for the adjustment of the load (trimming). The Derivative or RATE value allows the controller to compensate for rapid changes in the load. An example would be opening the door of a kiln. This changes the internal temperature of the kiln dramatically, and the Derivative action allows reduces the undershoot and overshoot of temperature.
The proportional, integral and derivative values must be individually adjusted or tuned to a particular unit usually by trial and error, but more advanced controllers have built in tuning mechanisms, either in a one-shot or continual method.
PID controllers provide the most accurate and stable control of the three types, and are suitable for small units which can react quickly to heat changes.
One last temperature “controller” that is not mentioned above is the so-called “infinite” control. This type of “controller” isn’t a controller as defined above, but rather a bi-metal temperature sensing mechanism that turns on and off the power to your kiln based on a dial setting. These types of (I can’t even really call them a thermostat!) devices are found generally in low-cost kilns and should be replaced by a digital controller as soon as you can afford one.