My thermocouple I'll be using is only about 1/4" long into the keg. It's the same one Pol used on his first electric build and it seemed to work fine. On the other hand, I don't think I'd be worried about a thermometer probe causing any flow issues if you went the with 4"
Tap Room Hobo
I should have stuck to four fingers in Vegas. :o - marubozo's mom