I don't believe you can control humidity w/ the herpstat; only temps and I believe lighting cycles.
The higher models (Herpstat 1 and up I believe) allow you to use a misting system with the Herpstat, if you purchase a humidity probe. I don't know how it works, as I don't have a humidity probe and I don't need a misting system, but the option exists.
0.1 Red Axanthic P. regius | Mazikeen
0.1E. climacophora | Lan Fan