Via PM barryham said:
Dealing with the last one first. These controllers (like the ATC800, ATC800+, TC10), are really for the control of fridges/Freezers and low powered heating devices, although the ATC 800+ can control up to 3KW on the heating circuit (IIRC). They are really only digital thermostats, you set the temperature and how much you will permit it to vary . . . and they then switch the required device (cooling / heating) on as required . . . There is no PID Control, it is a simple ON OFF switch and as such is ideal for controlling fermentation temperature.
PID Controllers (PID stands for Proportional Integral Differential), are more than a simple thermostat, they contain a microprocessor which determines the difference (Differential) between the current value and the Set value . . . Integrates (Integral) this with the heating devices known heating characteristics . . . and applies an output pulse which is Proportional to the difference and the amount of heat required. . . so that the temperature reaches the set value and stays there. Generally the output is a simple low DC Voltage of a few milliamps (10V 40mA), although it can be a relay . . . but this is often only capable of handling around 3A . . . ideal for a small Aquarium heater but little else, plus the rapid switching of the PID would rapidly wear out a mechanical relay. . . hence the reason a Solid State Relay is used. SSR's are capable of switching huge currents, and are ideal for the 3KW elements we generally use in brewing (With appropriate heat sinking). These are the ones you'll see in use in HLT controllers, and HERMS/RIMS controllers, where an accurate temperature control is required.
It is important to choose the correct device for the application you have in mind . . . And also the correct temperature sensor . . . for our work the best sensor is a PT100 probe.
Hope this helps
