Attach probe so its against where the beer is, not head space. Insulate the probe so its only reading the fermenter wall not the air inside the freezer. I set my hot and cold side to the same temp with one degree differencial. So if mine is set at 50, the freezer comes on at 51 and the heater comes on at 49. At first it will work hard but once all that beer mass is at 50 it barely kicks on at all. Maybe once every hour or two. Within a couple hours of putting my beer in the chest its at the set temp and always stays right there. The display on your controller becomes your new thermometer. Measuring the air or a glass of water is a sheer act of futility. The air im my chest is usually the same as the beer temp or close, but when tge freezer kicks on the air will be freezing and when the heater kicks on the air will be hot. I already know that, because I have a freezer and a heater... I only care about the beer temp.