Comprehensive measurement of accuracy of cheap humidity sensors
A guy has put in a lot of time in order to check if some inexpensive humidity sensors actually are accurate (spoiler: They are surprisingly good).
Instead of varying concentrations of a particular salt I use saturated solutions of several different salts. Depending on the solubility of a particular salt, a different relative humidity will be generated in equilibrium with the solution.
Read more: Link - Test and Calibrate DHT22 / AM2302 / RHT03 hygrometers
Compare DHT22, DHT11 and Sensirion SHT71
One can buy the DHT22 here and there are some libraries too: