I'm trying to keep the humidity level up in my (small 8'x10') music room using a small table top humidifier. I have a couple of guitars and a concertina that I'd like to protect from low humidity conditions during heating season. I can control the intensity of the humidifier, i.e. the rate at which extra moisture is added to the air, but it does not have the equivalent of a thermostat (hydrostat?). I'm having a hard time keeping the level close to 50%; I'm seeing swings from close to 30% up to 60% over the course of a day, depending on how much heating is going on (which in turn depends on what's going on outside). Without the humidifier, the RH would less than 30% (at least some of the time). I'm monitoring using a hygrometer that is just sitting out on a shelf.
Are there techniques to "buffer" the humidity level in a room, i.e. make it so that the humidity level is harder (slower) to change?
An idea that comes to mind is living plants, but I'm not sure if they'd have any effect. I think that some of the instrument case humidifier products might behave like this, but would it require a prohibitive amount to affect the behaviour in an entire room?