Leave the probe in air. Again, liquid changes temperature more slowly than air, but what you're directly controlling is the air temperature. If the probe is in liquid, the controller will keep driving the air temperature down until it detects the set temperature in the liquid. But now the air will be much colder, and the liquid (and your beer) will continue to drop until it's below set temperature. Then the controller will want to warm things up until set temperature is detected, and you're in the opposite situation, and you get into an endless cycle with your poor beer fluctuating all over... and stress on the equipment. The one thing you need to remember is that during active fermentation the beer will be a few degrees above ambient temperature. So just monitor the beer temperature and you'll learn to set the controller to compensate.