Congratulations on your purchase of a glass carboy. Carboys are both easier to use and to sanitize than plastic buckets. A well-treated carboy should last a lifetime.
Before using your carboy for the first time, we recommend filling the carboy with water, one gallon at a time, and marking-off each one-gallon increment with permanent black marker. Now you will always precisely know how much liquid you have in the carboy.
For accurate temperature measurement when using carboys, we recommend using a plastic stick-on thermometer on each carboy. The thermometer's adhesive is waterproof and permanent. Once it is in position, it will not come off. For the most accurate temperature measurement, place the thermometer at about the 2.5 gallon mark on the carboy. If your carboy has ribs, stick the thermometer on the un-ribbed "window" area.