Product review (4 stars): ThermoPro TP50 Digital Hygrometer/Thermometer: Did fairly well considering its low price!
For a very inexpensive hygrometer, it's reasonably accurate. I plan on using it in the room where I store my acoustic guitar. I tested it using a Boveda 32% test kit. The pic was taken after the hygrometer was in the bag for 3 days. It read 34%, & since there's no calibration method for this model, I'll just subtract 2%. Not bad for its low price! I also tested the Caliber IV thermometer/hygrometers. The advantage is that they have a correction capability, so I recommend them over this model for acoustic musical instruments/guitars.