You want to measure the temperature of the beer in the FV, not the air in the fridge (which will vary much more than the thermal mass that is the beer). Ideally, you would dangle the probe I to the beer but that opens up a whole load of issues around sanitation, probe life, etc. Soost people, me included, tape or otherwise attach it to the FV with some foam or something on the outside - so the probe is sandwiched between the outside of the FV and the foam. This means the probe is as near as damnit measuring FV temp. Hope that helps - I can take a piccy later once I get back home of you like.