Hey guy's need a little help with keeping my humidity at a steady 50-55%.

I have a 3ft beech vivarium by vivexotic which has 4 small rectangle vents, and lately im having to spray more than twice a day! It seems to be dropping fast, and always drops to 40% never any lower.

My gauge used to measure is Exo Terra Analogue Hygrometer?

Whats the best method i can use which keeps it at that level????

Auto misters??