I've read some other posts, but I think my situation is slightly different.
Recently built my keezer. Older 20 cuft Admiral chest freezer. Added a 2x6 collar and a Ranco digital temp controller.
Originally I had the sensor going through a hole in the back of the 2x6 placing the sensor end in the center of the 2x6 board in the center of the freezer.
I have the temp controller set for 36* and to allow for 5* of variation as I also planned to use the keezer for storage of extra soda and water. To verify that the temp controller was working properly I also had a small analog thermometer sitting in one of the hanging baskets in the freezer which was reading low/mid 30's.
Fast forward to last night.....I come home late after brewing at a buddies house to find the Temp Controller sitting at 34*, but the inside thermometer sitting at 20*......and two 12 packs of Coke had exploded and covered all sides of my new keezer.
I temporarily adjusted the temperature controller sensor and allowed it to hang down about 2/3 down along the side of the freezer and it promptly dropped to 23*. I wiped down the door and the wood, but I have frozen Coke slushy everywhere inside the lower parts of the freezer.
I've read about people putting the sensor into some distilled water or the gel from freezer packs, but I'm hesitant as my sensor isn't encased in copper like the analog ones, but just has some plastic tubing over it.
Where and how should I put the sensor?