If you put the probe in a bottle of water, your fermentation temp can be 5 to 10 degrees higher than ambient. This might not be too bad with an ale but not good for a lager. Taped to a glass carboy with insulation over it is fine. I shoot a few degrees lower when doing it that way.
if the tape on thermometers are to be believed taping to the outside provides a pretty good read (at least with the way I do it). At most the temp probe might read 1F lower than the stick on thermometer. That's what I've been seeing on my current ferment. I have the controller set to 63F and the highest I've seen on the stick on thermometer is 64F.
I don't think I've ever seen a 10F differential in my pre-ferm-control days. +5F was pretty much the limit. maybe some yeasts could go further - just not the ones I use I guess.
also - just some more data:
temp controller setting: 63F
typical stick on thermometer reading: 63-64F
typical air temp reading when freezer isn't running: 59-60F
Edited by Evil_Morty, 29 September 2014 - 07:33 AM.