It is helpful to have a humidifier in the room where you practice. However, when you take the instrument away from the room for an extended time, you need other means of keeping the instrument safe from fluctuations in humidity. Good humidifiers installed in instrument cases are helpful, but only if the case remains closed for several days. On short trips, small humidifiers (Dampits) can be inserted through the instrument's soundholes. Dampits work well because they can go wherever the instrument goes, but they don't hold much water and need to be checked often. Also, if you over-moisten a Dampit, it can drip and damage the instrument. To prevent dripping, pull the Dampit through a towel after soaking it under a faucet.
Hygrometers indicate changes in humidity. However, most of them don't remain accurate, so do not rely on them. A dramatic drop in the reading should always be taken as a warning sign, even if the percentage of humidity indicated on the hygrometer is still OK. The level of humidity considered OK depends somewhat on the climate where you live. In an area with an average of 90 percent humidity, you may experience problems when the humidity level drops to 50 percent. However, if you live in an area where the humidity is normally around 50 percent, and the instrument is accustomed to this low humidity, the instrument should be safe. A problem can occur if you move or ship an instrument from a humid location to a dry one. If you and your instrument are ever caught in a dry spell without access to a humidifier, you can take the instrument and its case into a bathroom. With the windows and doors closed, the humidity in the bathroom is higher than in the rest of the house. Running the shower for a few moments before entering the bathroom will help raise the humidity even further. Opening the case for a short time will trap moist air inside when it is closed again.
These suggestions are no guarantee against damage caused by changes in humidity, but they will help reduce the risk.