From the product description, check product page for current description, price and availability:

This carboy thermowell is used in conjunction with the Digital Controller, FE601A, FE610A, or FE602, 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 either 3, 5 or 6 gallon PLASTIC carboys or any other similarly sized opening hole. The unit comes with a 15″ stainless steel thermowell and a #10 silicone stopper that will accommodate our sanitary filter, FIL90, as well. It can be used in any container with a stopper opening of this size where you need to accurately monitor and control liquid temperature.

Will not work with Johnson Temperature Controllers (FE600 or FE611) because the probes are too large.