From MoreBeer: This carboy thermowell is used in conjunction with a Digital Controller, FE601, FE601A, FE602, or FE611, to more accurately regulate internal temperature during fermentation. It is exceptionally useful during the start of fermentation when the lag between ambient temperature in the refrigerator and actual temperature in the carboy can vary greatly. Now you can keep your beer or wine within one degree of the set point on your digital controller. Designed to fit 3, 5,6, and 6.5 gallon glass smooth neck carboys, as well as 5 and 6 gallon plastic carboys, the unit comes with a 15″ stainless steel thermowell and a carboy hood that will accommodate a standard airlock or 1/2 inch blow off tubing.