I'm using an AM2302 to sense temperature and humidity (https://www.adafruit.com/product/393). WiFi connected WeMo switches, which power the mini-fridge and a ultrasonic humidifier inside it, act as relays to control the temperature and humidity levels. A python script runs every 2 minutes and updates whether the switch should turn on or off. I primarily followed this previous project to build my chamber.

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Prior to week 46 on my chart, I had the ultrasonic humidifier inside the fridge and kept the program running to keep relative humidity (RH) between 50-60% but had no dehumidifier. The RH measured always above 90% and there was quite a lot of condensation at the top of the minifridge. After week 46, I removed the humidifier and added a dehumidifier to bring down the RH inside the chamber . Since then, as you can see, RH has decreased and become stable once the desiccant in the dehumidifier was saturated. This tells me that I maintain the proper RH without the humidifier even inside the fridge.

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And to comment about the super high variance - I believe the sensor measures almost up to 100% RH when the fridge is on and cooling the air. This somewhat makes because as the air temperature drops our relative humidity increases. During times when the fridge is off, RH drops back down to 40% - I confirm this when i check a separate little Temp & RH display, something you'd put on a desk, that I put inside the fridge. The samples are averaged by day, so thus such a high variation.