HERMS PID Control

[CJV8]

I'm busy revamping my whole brewing set up and I'm taking the opportunity to add some PID control and HERMS.
I'll be running a separate small HERMS pot and I was wondering what will be the best way to control it.
Is it best to control the HERMS element with the PID, and monitor the HERMS water temp, and adjust it to suit the desired mash temp?
Or would you run the HERMS element with the PID, but monitor the mash temperature so that the PID responds to the demands of the mash temp? Or is that a recipe for a PID tying itself in knots?

There'll be temperature probes in both vessels anyway so I can swap it all around at a later date, I was just curious as to what the suggested method is.

 

[peebee]

I wrote a bit on sensor position a while ago ... ahh, managed to dig it up: Pumps and PIDs

 

[Buffers brewery]

I use a small flip-top waste bin as my HERMS tank. Holds a fair bit of water so I just control the tank temperature. I know from experience what tank temperature achieves my required mash temperature. I also have a temperature display for the wort returning from the HERMS tank to the sparge box over the mash tun. There's also a temperature probe in the bottom of the mash tun so I can see the size of the temperature gradient across the mash, top to bottom. Providing the strike water temperature is right I get pretty good temperature control over the hour.