I have some stuff called Soil Moist that I got at the local garden center. It's shredded up polymer that absorbs water and is probably the same thing as the climax beads but sold for a different purpose (mix in the soil to retain moisture).
I took a handfull of and moistened it in a similar way as the salt test used to calibrate your hydrometer. I put them in a sealed ziplock along with a calibrated hydrometer and left it over night. In the morning the hydormeter read 70%.
The Soil Moist cost about $10 for a pint sized jar. Only a small amount is needed to fill a large humidifier. As with any humidifier, don't over charge it with water.
