Multiple fermenter temp??

[baer19d]

I have a large temperature controlled fermentation chamber that I can fit several batches in at the same time. If I have say, three batches, all of which are ales, that are at different stages in their fermentation which one should I put the temperature probe into or next to in order to control the temperature in the chamber?

 

[pelipen]

My experience has been that measuring the surrounding air beside a carboy, with a fan running, and a reasonably wide hysteresis, holds the beer temp the most constant.
Anymore I just dangle it close, nothing fancy.