I have a 500 count humi my wife bought me for x'mas that has a large open area at the bottom and two trays on top. I purchased an e-humidier (Hydra-sm) for it.

I also purchased two extra fans (including one fan kit) to help circulate air (and thus humidity) evenly around the humi.

Originally I bought one XiKar (Madeleine) digital hygrometer:

http://www.amazon.com/Madelaine-Adju.../dp/B0021XIDO4

But the first one was broken so I then bought 3 Caliber III digital hygrometrs:

http://www.amazon.ca/Western-Digital.../dp/B0007W1EA6

One for the 500 count, one for my 100 count and one for the travel humi. I bought those while waiting for the replacement XiKar.

The Caliber III's do not require any calibrating. In fact, the documentation that comes with them specifically says, don't do it! The XiKar did need to be calibrated. I've noticed over time the XiKar loses its calibration slowly. Right now, it shows about 4% RH less than the Caliber's when at first, it showed the exact same. The 3 Calibers have always been exactly the same both temp and RH.

I never did calibrate the e-humidifier. I didn't bother because I had the Caliber's. I simply adjusted the RH setting to keep my humi at about 65/66% RH which seems to suit my location and cigars the best.

I don't know that it matters, but I suggest you ignore the e-humi's hygrometer if your digital hygrometer's are accurate.