The sensor’s RH readings are lower than the reference RH meter’s? What can cause this?
Most inexpensive RH meters are generally not accurate. Please check the meter’s specifications.
this may be cumbersome, but will work as a meter check. Take your meter to where NOAA ( http://forecast.weather.gov ) states the readings are made, typically "podunk airport", and check against the NOAA value against your meter. for a quick check guess where you are has the same RH as the nearby reporting site
Measuiring RH is very difficult. One degree C of heating reduces RH by 5%. There can large local flucutaitons in RH just due to small temperature differences or due to pockets of humidity that have not dispersed. RH can also be affected by materials nearby that absorb or emit humidity. Unfortunately, the only reliable method for RH accuracy check is to use a calibrated meter rated for sufficinet accuracy. Be cautious of low cost meters as many can be inaccurate or have accruacy rated as "typical". A chilled mirror hygrometer or calibrated chamber is the best method for accurate RH measurement.
Take your meter to where NOAA ( http://forecast.weather.gov ) states the readings are made,
I stated this for the above reasons.
WereI to do this, I'd go to the airport, ask where the weather station was, bring up a NOAA screen and wait for the "time of reading" to change, then compare