Product review (5 stars): Works and not expensive
Used this unit to monitor and control a humidifier in a refrigerator sized cigar humidor. This unit appears to work ok, but I don't have a reference so I can't tell how accurate it is. I assumed it was more accurate than the cheapo humidor hygrometer, so I set the hygrometer to this sensor output. It's straight differential control, so on when humidity too low, off when humidity too high. You get set point and a hysteresis parameters. Bare bones control, but it tracks and seems to work.

I ran into an issue with the sensor when it first arrived. I wired up the unit to power and thought I'l test it. I had the sensor in open air ( in my garage ) and it read 60%... I set the setpoint to 70 and I wanted to see the relay kick off, so I breathed on the sensor. The first time, I barely registered anything, so I was a bit more energetic on the second breath. After the second breath, the display read 70-81-92-OL and remained displaying OL for an hour or more. Then it slowly, and I mean one percent per 10 minutes slowly, returned to 60% over the course of the next several hours. It didn't appear to suffer any permanent damage, but It certainly didn't like my humid breath.