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Proportional Integral Derivative = PID.
There are many types of PID, but for our requirements of temperature-control we are interested in the type capable of receiving a thermocouple or RTD signal.
Sounds complicated, but this device can be thought of as nothing more than a glorified thermostat.
The Advantages over a standard thermostat are, the PID type device 'learns' the heating cycle during operation, and can be more efficient and a finer control of the temperature attained. As they can be very accurate, they are often used by the home beer brewing community where fine temperature of the brew is essential.
For biodiesel making they perform much better than a standard thermostat, as often two temperatures are needed for different stages of the process, and need fine control of temperature.
There are many makes and models to choose from, but most work in roughly the same way.
Often they have two displays, one marked SV and the other PV. SV means the Set Value or the temperature at which the user requires the process to run, while PV is the Process Value meaning the actual temperature presently.
They are often available new and direct from manufacturers via internet auction sites.