Do I need to dump the residual water out of my diffuser before I add new oils/water?

Q: Do I need to dump the residual water out of my diffuser before I add new oils/water?

A: No you do not need to.  You can just add more water and oil to it.  If you want to clean it you can dump the water into a glass, take a q-tip with some vinegar or alcohol and wipe the inside, particularly the metal plate in the center.  Then let it air dry before running it again. But I wouldn't recommend just letting water sit in there all the time if you're having a long period of time between diffusing.