im trying to buy a PID and some temp probes from amazon.com, they are very cheap there(should i waste my time with cheap PIDs??). i want to set it up so that i can use it for a precision yogurt maker, or i can easily pull out the PID computer itself and plug it into my homemade espresso machine.

i guess i just need to know what i should be looking for/buying together etc. so that im not getting the wrong things, in other words, wrong temp probe for the PID or wrong relay etc.
it would also be great if someone could point me to a COMPREHENSIVE instructional web page! iv'e typed all sorts of stuff into google and had very little luck finding info for beginners that does not involve some lengthy mathematical dribble...

what im looking at right now is two different PIDs on amazon, one says ssr, the other says snr..what does that even mean?? i also could not find that on google lol.

can i mess up buying relays? do i need a heat sink, and under what conditions?

Just a guess here, but you were looking at MYPIN brand SSRs?

They have a model that has "SNR" in the model number which I guess could be confusing --

http://www.mypinchina.com/uploadfiles/2 ... 651265.pdf

gives a decoder chart for that brand's model numbers:

SNR would mean:
SSR for OUT1
NONE output for OUT2/ALARM2
RELAY on ALARM1 output

This PID controller has a built-in SSR on output 1; it may not be beefy enough for what you want to drive.
A better choice would be a 0-10V output (I option - so it might be VNR instead of SNR) if you have a SSR that can take a 10V input, for example.

Most PID controllers can be programmed for different thermocouple or RTD types; check out the manual for the controller you are considering.
The output options vary; different manufacturers have different model naming conventions which often specify the types of inputs, outputs, ranges, programmability, etc. Your best bet is to download the manual from the manufacturer and read through their guidance.

Here's my thread on PID'ing my La Pavoni. (Another) La Pavoni PID - I detail all the parts I purchased. I wouldn't recommend super cheap components. http://www.auberins.com/ is a fairly reputable source for this type of stuff.

I wouldn't recommend swapping the PID controller (computer) between systems. First off it will be a hassle because you have like 7 different wires that hook into the back of the controller. Unless you could use some type of coupled adapter or 7 male female connectors it will be annoying. Another problem is with how each system differs. You said you want to use it for a yogurt machine and an espresso machine. This will pose a problem because one system is cooling, one is heating, and overall the target temperatures will be quite different. Thus the PID parameters and other settings on the PID controller would have to be changed each time you switch it for optimal performance. Trust me it's not a fun process to change a bunch of settings on these tiny things. You could probably find a PID controller with multiple profiles but I haven't seen one and it would probably be pricey. Also if you went this route you'd better stick with the same type of sensor to avoid changing the input setting for that as well.

In terms of the sensor make sure it's an RTD (best if pt100 RTD) if you want good accuracy. These can be had for around $30. You will also need an SSR (solid state relay) which will control the power to the boiler or cooling system in the yogurt machine. As allon stated there's an internal relay in the PID controller but it will probably be incapable of handling adequate current, so get an SSR with the proper rating to handle your machine's heating element. You will also NEED a heatsink for the espresso machine. SSR's are meant to run with a heatsink even at room temperature. For example an advertised 40A SSR with heatsink at 30 C can handle around 40A. Without a heatsink, it can handle less than 10A.