It's that time of year again. This post got me curious: Edit-- The instructions linked below are reproduced here in post #4 of this thread. Here's a link to a nice set of instructions for doing the "salt test." You can do the test with just about any hygrometer and note the error. If you want a hygrometer that you can calibrate, here's a nice little one. Some of us obsess about maintaining proper humidity. The hygrometers most of us end up with can be pretty far off.