If you plan on performing relative humidity calibrations (if the sensors are provided with a means to do so), or just plan on checking the accuracy of your hygrometers, use a sling psychrometer as the reference standard. Here is an example of what these instruments look like:
https://www.globaltestsupply.com/pro...oaArpiEALw_wcB
A sling psychrometer (wet bulb thermometer/dry bulb thermometer) is the most accurate method available to measure RH. It is rather inconvenient to use, though. That's why the small digital electronic units are so popular. Unfortunately the accuracy and precision, of the thin-film capacitive sensor, is somewhat sloppy.