Russian Tortoise I'm not sure of his exact age. I have had my guy for 2 months. Keeping/maintaining the humidity at a high level is really my question. His indoor terrarium is 50 gallon by size. with a wire grated top. He loves the outdoors and is usually indoors just in the evening.
A Russian does not need as high a humdity as some others. 40-60 is a good range. Try covering 1/2 of the enclosure and placing some damp coconut coir, soil, or moss on that end with a heat source above. If that doesn't get it high enough, try covering move of the enclosure, or placing more damp substrate under a top layer of dry, throughout the enclosure.
How big is your guy? Younger tortoises are thought to need higher humidity.
I live in a fairly humid area, the Tidewater part of VA, and my indoor humidity ranges from about 45% to about 75%. When I see it getting less than 60% I'll hang a wet towel in the room to bring the humidity up a little and add water to the coco coir/topsoil mix substrate.
As Russians almost always dig down into the substrate I think they would like to have the humidity 60% or higher.
I don't have any hatchling Russians, so my experience is limited to adult and near adult Russians.