Water is the enemy of these things. I did swamp cooler chilling with two of them, and the wet t shirt covered the fermometer. Soon, the temp reading was simultaneously showing 68, 70, 72, 74... on up to as high as the numbers went. I now attach the fermometer to the carboy with 2" wide clear packing tape, in hope of sealing the edges, and try to keep them away from constant water contact. With this method, the temps are pretty close to spot on.