Violin cases designed for more professional players have hygrometers so that you can verify how much moisture is in the air. There are two types: dial-type mechanical hydrometers or digital electronic hygrometers. Hygrometers can help you verify if the air is too humid. This is something important for professionals who have gut strings. Gut strings, just like your gut lining, naturally absorb moisture. If the air is too humid inside your violin case, the gut strings absorb all of the excess water. Then the gut strings don’t hold their tautness, and they go out of key, so you have to retune them.