Temperature Probe Lenght 20 Gal Kettle

[RickCask]

What is a good temperature probe length in a 20 gallon brew boil kettle ?
I will most likely use a rtd probe with a 1/2 inch male npt tied to a Auber PID
Temp Controller.

 

[Gameface]

For me I'm going with the 2" probe. I don't think it needs to be particularly long as the wort will be moving as I come up to a boil and during chilling so as long as it's touching liquid I think it'll be fine.