Hi Mark. The PID's all do the same task, ie switch the heating element on and off to control the temp to a pre set level. The difference between them is the way they are controlled, and what they control.
You have options of the way they are controlled, generally either by what are known as K type sensors, or by Pt100 sensors. The Pt100 sensors are more accurate, and seem to be the favoured choice among the brewing fraternity, although K type are perfectly acceptable. It is just a matter of choice as to haw accurately you want to control temps.
The next option is how they control. Here you have two choices, first is a relay output, this means you need a relay capable of switching the power of what you are controlling, because you are switching a element probably rated at over 2KW, and it will be switching on and off quite frequently, this is not a good choice because a relay is an electro/mechanical switch and would suffer a short life with the constant switching.
The other choice is known as an SSR output. An SSR is a 'solid state relay' the switching is done by electronic components which can switch on and off faster than you can say it. These are the ones to use.
The next thing is, are you using your HLT as a HEX or are they seperate. If they are seperate then normally you would use two PID's one to control each. You could possibly get away with one PID and use two temp sensors, and swap the sensors for different operations, but for the cost of PID's it does not really make sense. Another reason for using different PID's is that they can be set to learn the heating parameters of the operation they are controlling, which makes for more accurate control.
I hope this makes sense, as it is late and I have sampled one or two of the last brew.
