Always timely, always worth a read.
The discovery I made this year was that my humidification controller was really spotty and off by at least 5 points. This is the one that had the full rebuild about four years ago. It's getting replaced, today, with the "new and improved" version from Bob Staebell.
Point being, you need at least one independent RH monitor and you need to salt check them at least annually when you replace the battery. More often is better. It's called "relative" humidity for a reason and you really need to keep an eye on it to ensure things are proper. Coolers with bags of beads in them are almost maintenance free, but checking them is a very good thing. I'm really digging the SensorPush modules (I have two) but there are less expensive ways to monitor the RH. I've also found monitoring the ambient RH is valuable, just to know what the system is trying to keep up with.
Bottom line: use more than one hygrometer, calibrate it at least annually, and expect things to change when your HVAC system changes from cooling to heating or vice versa. And don't freak out over a point or two.....