Problem :

  • We had need of Humidity sensor for sensing moisture of drying chamber in Drying Apparatus. As per requirement we get Temperature & Humidity sensor (DHT21 series) from market. But these sensor was getting damage after 4-5 days. Temperature sensing was good but humidity sensing was too poor.
  • we dependent on dry bulb and wet bulb temperature apparatus using these temperature data we can  calculate humidity.
  • So we are looking low cost & better performance sensor, but we didn’t found.


  • we calculate humidity using principle of dry-bulb & wet-bulb thermometer.
  • we thought can we make this model/equipment using electronics components (digital temperature sensor & controller).

Mathematical model

  • Saturation Vapor Pressure @ wet bulb temp:
    = 0.61078*EXP((17.08085*T(wet))/(237.175+T(wet)))
  • Actual Vapor Pressure:
    = Sat. V.P.@wet bulb – [ (psy. constant) (Atm.Pressure in kPa) (T(dry)-T(wet)) ]
  • Relative Humidity:
    = (Actual V.P./ Saturation V.P.)*100

Arduino code