Measuring humidity accurately with long term repeatability has always been a significant challenge for sensor companies. At the core of that challenge is a good reference measurement from which to calibrate all sensors. Humidity chambers mix dry air and water vapor to create atmosphere and often this is further improved with an instrument like a chilled mirror dew point sensor.
These reference instruments and chambers are extremely expensive, cost tens of thousands of dollars and have an accuracy of ± 1%. The typical hand held meter used in restoration is a hygrometer with and accuracy ±2%, and this is achieved with meters with a cost of $600 or less.