The Humidity Sensor will not be damaged by incidental condensation but should not be immersed in liquid. If condensation has formed, the Humidity Sensor will not be able to give a reliable measurement until all the droplets have evaporated. Evaporation may be speeded up by movement of the Sensor through the air or by using a fan. 


The electronic circuit inside the Sensor itself cannot be completely sealed from the atmosphere. If the Sensor is to be used in an environment where there is a risk of condensation, precautions must be taken to ensure that liquid cannot enter this part of the Sensor. 


The response time of this sensor will vary considerably with air circulation i.e. it will respond more quickly in a moving current of air. 


If used in a dirty environment, protect the Sensor from dirt using a piece of nylon stocking mesh.


Do not expose to chemical vapours such as acetone, organic solvents, or chlorine, which are harmful to the Sensor. 


Theory 

The warmer the air is, the more water vapour it can “hold”. Humidity is an expression used to describe the amount of water vapour in the air. 

 

Relative Humidity (RH) is the term used most often in weather information meant for the public. 

 

Relative Humidity is the amount of water vapour actually in the air compared with the amount of vapour needed to make the air saturated at the air’s current temperature. It is expressed as a percentage and can be computed in a variety of ways.  

 

e.g. %RH = 100 x Pw / Ps(t) 

 

Where Pw = partial water pressure 

Ps(t)  = water vapour’s saturation pressure at ambient temperature, t (0C). 

 

Example

  • The air temperature is 30 degrees and the air has 9 grams of water vapour per cubic meter of air. 
  • Divide 9 by 30 and multiply by 100 to get a relative humidity of 30% - the air has 30% of the water vapour it could hold at its current temperature. 
  • The air temperature drops to 20 degrees. Divide 9, the vapour actually in the air, by 17, the vapour it could hold at its new temperature, and multiply by 100 to get a relative humidity of 53%. 
  • The air cools down to 10 degrees, divide 9 by 9 and multiply by 100 to get a relative humidity of 100% - the air now has all the vapour it can hold at its new temperature. 


Useful Definitions 

  • Absolute humidity: The mass of water vapour in a given volume of air. 
  • Actual vapour pressure: The partial pressure exerted by the water vapour present. 
  • Dewpoint: The temperature air would have to be cooled to in order for saturation to occur. 
  • Mixing Ratio: The mass of water vapour divide by the mass of the dry air. 
  • Saturation of air: The condition under which the amount of water vapour in the air is the maximum possible at the existing temperature and pressure. 
  • Saturation vapour pressure: The maximum partial pressure that water vapour molecules would exert if the air were saturated with vapour at a given temperature. 
  • Specific humidity: The mass of water vapour divided by the total mass of the air.