Calibrate your hygrometer to the application. For species that like 70-85% RH use damp salt in a bowl placed in a ziplock bag with the hygrometer. For species that like 85-100% RH use a wet towel in the bag calibrating for %99.
To adjust an analog hygrometer use a screwdriver on the adjustment screw on the back.
To adjust a digital hygrometer change the calibration setting till it reads close to the target calibration.
Place the hygrometer and/or sensors into the ziplock bag with the back of the hygrometer or digital sensor just over the salt container or wet cloth. Wait 30-60 min for the bag to stabilize then adjust the calibration setting till it reads as close to 75% or 99%. Poke a small hole with the adjustment screwdriver so you're not opening the bag to calibrate. If your digital hygrometer is reading 99.99 that means it's maxed out, lower the calibration till it reads 98-99%. Allow the hygrometer to sit for 20 min after adjustment to level out and verify calibration.
Digital RH probe and temp sensor, note the RTV on the connection where the wires go, this prevents corrosion. Do net let this sensor get wet. If it does get wet treat it like any other electronic device and place it into a bag of rice for a day. These sensors respond faster to RH when located in a moving airflow such as a circulation fan/duct.
Synthetic human hair hygrometers are fairly accurate and relatively cheap, starting around $35.
Both being calibrated inside a ziplock with a container 1/2 filled with damp wet salt. These are being calibrated to 75% for my oyster grow.
Adjusting the screw while still in the bag