You already have a post in regard to this. You asked I answered. For growing weed, avoid ultrasonic and go with the evaporative one I linked for you in the other post. Or something similar, I love my vornado.
from Midwest in the other post…Many are Chinese made and pretty much all the same. I suggest getting a wicking, evaporative humidifier versus one that works with ultrasound. The ultrasound humidifers will aerosolize the minerals in your tap water and leave white mineral dust all over your room. The only way to avoid it is to use RO or distilled. Evaporative humidifiers to not cause this problem.