Surprised Tony hasn't chimed it, he's probably over it, I'm bloody close. He used to monitor wort out of the Mash Tun, wort out of the HE and Wort returning to the Mash Tun.
Exactly Andy, fcuk it, if it's returning at the right (mash) temp then that is all that matters. The most acurate place to control it is as close to the HE out as possible, naturally as you don't want the wort temp to rise above your mash temp, if monitoring at the mash tun return there will be system losses between the HE and the mash return so the wort would naturally be higher at the HE out than than at the mash tun return.
Wort leaving the Mash Tun will always be at a higher temp than entering the HE due to system losses. Wort exiting the HE will always be the highest temp in the system.
Forget HERMS, think about wort in the mash tun. Whatever heat source you use to heat the wort to the next step will need to apply a higher temp than the required mash temp to achieve the mash temp within a reasonable time. Is this desireable? Do you want parts of your wort at say 88C during the time it takes to raise the mash temp, how would this effect enzymic reactions in the wort. You would have used a mash temp of whatever, say 66C but some of the wort would have been at a much higher temp for a period of time to achieve this, do you think your wort would behave in the same way as wort that had never been above 66C. Wort fermentbility is pretty much the result of averages of temp over time, even single infusion mashes. The average of the water/wort temp over time is what determines the fermentability of the wort. With HERMS the wort temp should never rise above the set step temp monitored at the HE out, wort will recirculate through the HE during the rest period and again an average mash temp will result. The upside is that the wort has never been way above mash temp as in infusion or direct heating of the mash tun. This results in better control of fermentability.
My last post on this subject, there is an encylopaedia of info available and I'm sick of rehashing it, seems there's those that get it and.....well......you know.
Use your HERMS in whatever way makes good beer on your system. And most importantly, if you don't use a HERMS feel free to advise all and sundry on their use and on the pros and cons of their inclusion in the brewing process.
Screwy
h34r: 
