The "manual" way to measure relative humidity is by means of a psychrometer, which is nothing more than a double thermometer with one wetted bulb. Swirling the device results in evaporation of moisture causing the temperature to drop. The temperature difference in combination with the "dry" temperature determines the actual rel.humidity. So, two temp. sensors, some water and a ventilator should do the job. Easy and very acurate! See http://www.cis-online.co.za/tables/rh_tables.htm for table.