A stopper thermowell is used 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, 6 or 6.5 gallon glass carboys or any other similarly sized opening hole. The unit comes with an 15" stainless steel thermowell and a #6.5 stopper. It can be used in any container with a stopper opening of this size where you need to accurately monitor and control liquid temperature. Inner Diameter of Thermowell: 1/4".