It does this by first measuring the temperature (process variable), it then compares it to the desired value (set value). The difference between these values is known as the error (Deviation). Temperature controllers use this error to decide how much heating or cooling is required to bring the process temperature back to the desired value. Once this calculation is complete the controller will produce an output signal that affects the change required. This output signal is known as the (manipulated value) and is normally connected to a heater, control valve, fan or some other "final control element" which actually injects or removes heat from the process.

Temperature controller form one of the four parts of a temperature-controlled system. To help visualize this we will consider an oven. The four parts would be:

1. The oven
2. The heater
3. The thermometer (or thermocouple)
4. The controller

The role of the temperature controller is to measure the temperature on the thermocouple, compare it to the setpoint, and calculate the amount of time the heater should remain switched on to maintain a constant temperature. It also helps in biomedical equipment repair.

Many factors change the amount of time that the heater needs to on, to maintain the process temperature. For example the size of the heater, the size of the oven, the amount of insulation surrounding the oven and the ambient temperature will all change the rate at which the oven will heat up or cool down. At the end of the day a temperature controller replaces the function of a person whose job description would read something like - watch that thermometer, keep the temperature stable at 80Deg C and if we need more heat, switch on the heater.

The important point is that the temperature controller has one input, one output and one set point.

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