How are you going to control the wort flow in the herms coil. I my RIMS, I have the RTD probe in the wort flow as it RETURNS to the top of the mash tun. The temp will overshoot a little, then go under, but stabilizes after about 5 minutes. The wort moves all the time using a small march pump. No worries about having the probe sticking out into the mash tun. Having the probe positioned after the heat source prevents overheating the wort as the controller will turn off the heat source when the wort hits the setpoint and the residual heat in the kettle will keep things going. If you put the probe in the center of the mash tun, the cycle time will probably be 4-5 minutes to affect the mash temp so the wort in the herms coil and the water in the kettle, will get WAY overheated and your mash temp will never really stabilize.