I placed my probe half way to the middle of MLT (and half way vertically), it is 4" RTD probe mounted in welded bung, but I started to hate it because it's pretty tough to stir the mash equally, especially bellow the probe..
After some thinking I decided to shift it, make thermo-T couple (like Lonie Mac did it on Brutus ten) and use it to measure input temperature. I measured this temperature before with K-type thermometer placed in drilled hole in MLT input tubing, but now I be able to use RTD probe for other measurements too, like output of HEX when cooling wort (I use CFC and HLT filled with ice for chilling).
I would like to keep measuring my mash temperature too, so I will leave welded bung (1/4") on MLT so I can pull K-type probe trough it to measure mash temperature when i want to.
Problem is to secure wort from leaching outside of MLT and I came to an idea: mount 1/4" ball valve on bung (usually used for gas) and put wide rubber ring on outside of ball valve. Ring has small hole in center, but large enough to pull probe trough it.
I imagined it like this: inserting the probe- when top of probe is behind the rubber ring I'll open valve and push it to the end. Ring should prevent leaks when probe is in.
To take out the probe- drag it to pass the valve, shut off the valve and pull it off.
This way I should be able stir mash without taking care of the probe, measure temperature in middle of mash when I want without taking lid of, and also continue to monitor input temperature. Besides that, I think T couple w/RTD probe is nice solution to take temperature readings in several places.
This is just an idea, but I hope I will realize it until next batch.