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A temperature controller is a device that controls the outputs of a heating or cooling system. This control is achieved by linking temperature sensors installed in the heated area or system to the controller. The data captured by the sensors is compared to a user preset reference; if the temperature in the system should deviate from it, the controller either activates or deactivates the heat or cooling source accordingly. There are three main categories of temperature controller operation: on/off control, proportional control, and PID control. Each fulfills the same basic function of controlling the temperature but offer differing levels of sophistication and functionality.
Heating and cooling systems typically consist of three basic components: a thermal source, a control mechanism, and one or more sensors. Depending on the complexity of the system and the level of precision required in the control of temperatures, the control and sensor mechanisms may be no more complex than a bimetal switch. Extended, intricate production systems may, on the other hand, feature sophisticated computerized controls which drive several heat sources based on the data received from numerous sensors. To achieve this range of control intervention, thermal dependent systems use one of three basic types of temperature controller.
The first temperature controller category is the on/off unit. This is the simplest of all controllers and offers a single level of control. With this type of controller, the heat source runs at full power and is either switched off or on depending on the system temperature. A feature known as on-off differential is built into this type of controller which prevents relays, contactors, or valves from being damaged by “chattering” if the temperature changes occur too rapidly. This feature allows the system temperature to exceed the set point by a certain margin before the controller switches prevent rapid on/off cycling.
The second temperature controller type is one which is capable of proportional temperature adjustment. This controller reduces the power at which the heater runs as it nears the reference set point. This avoids system temperatures from exceeding the set point as is the case with the on/off type of controller. If the temperature does stray too far from the set point due to environmental issues, the controller then functions as an on/off unit either keeping the heater power full on or switched off.
The third type is the proportional with integral and derivative (PID) controller. This kind of controller functions in the same way as standard proportional units but includes two extra, time-dependent adjustments. These adjustments are typically marked as "reset" and "rate" on the device and allow extremely stable temperature control in applications where the system load requirements change frequently. This variant, however, requires fine tuning of all adjustment parameters to function correctly.