3G_ wrote:(snippage)
If i mount the temp probe at the sparge arm i would get the temp correct there but i am worried about denaturing the enzimes in the HE.
You will not denature enzymes in the wort from passing it through the HE coil. The heat gained by the pass through the coil is in turn passed in to the mash in a matter of a couple of seconds.
In general, for a TYPICAL HERMS and mash, the best placement of the sensor is either in the mash, near the FB/manifold, or at the MLT wort OUTLET. For larger mash tuns, like 125+ quart coolers, you can add a second sensor to the Ranco ETC (see wiring in operating instructions) to get an average mash temp. Placing it at the wort return will lead to very long ramp times. Note that I specified TYPICAL HERMS setups, with a fixed HLT setpoint, in which the controller swithes the pump or valves. There are designs in which the HLT temp is the controlled variable, in which case the wort return may make sense.
The keys to efficient heat exchange and being able to ramp up in a reasonable amount of time are delta T and flow rate. Assuming equal flow rates, the control scheme using a higher delta T (i.e. higher HL temp) will have shorter ramp times. I run my HL at 190 F until halfway through the ramp up to mash out. Then I lower the setpoint to sparge temp (170), and top off HLT with a small amount cold water if needed to get down to it.