Hygrostat? Never heard of that......
Not an uncommon device; they work just like a thermostat, only to regulate rh. Just like a thermostat, they come in two types; the ones to lower humidity will be all you will find in the South without a special order. The ones to raise humidity will be stocked in the arid West. Be sure you know which type you want before you buy!
I used to sell cigars in south Florida. Man, y'all should have seen the looks I got when I told these crusty ol' guys I was trying to RAISE the humidity, to protect moisture levels in the cigars in inventory! Why did we need to raise rh levels in south Florida? Because the AC dried the air inside the warehouse too much! Sounds familiar, doesn't it?
This, folks- this is why I bought a chiller. AC kills your rh, chillers don't. If you think buying 2 tons of cooling for $3000 is a lot of money, add up your AC, humidifier, dehuey for nighttime, and additional chilling for your hydro system. Then factor in the fact that my chiller is likely to be roughly twice as efficient as any single component of that assemblage (including the little chiller, bigger is much better), AND with my chiller none of those components work against one another, like dehuey and AC do, and you can see why I'm such a passionate proponent of water chilling as the way to go.