I've read quite a few posts regarding fermentation temperatures and how brewers around the country measure and attempt to control them, in hopes of getting the best results from the yeast strains they use. After doing some research on this topic I found the following bit of information on the Brew Your Own website that I wanted to share with you.
"However, in a small fermenter such as a 5-gallon (19-L) carboy the difference between the air temperature and the beer temperature is usually within about 5 ºF (3 °C). So if you have a yeast strain that produces the best beer when fermentation is held at 70 ºF (21 °C) the surrounding air temperature should be around 65 ºF (18 °C). You can periodically monitor this by inserting a thermometer into the fermenting beer."
The reason I found this post so interesting is that it talks about the exact same temperature control setup I have been using at home to ferment and carbonate my beers, all 27 of them. In my upstairs office/brewery where I have been fermenting and carbonating my Mr. Beer recipes I run a small 110 volt window air conditioner set to 70F.