The measurement chamber is a hollow cylinder whose lower end acts as a measurement orifice that is placed into contact with the test surface. Its upper end is closed with an aluminium condenser that is maintained below the freezing temperature of water by means of an electronic Peltier cooler.

When in contact with the test surface, the chamber is closed and the air within it is protected from disturbance from ambient air movements. The condenser controls the humidity in the chamber independently of ambient conditions. It acts as a vapour sink by forming ice on its surface, thus creating a zone of low humidity in its immediate vicinity. By contrast, the test surface acts as a vapour source, creating a zone of higher humidity in its immediate vicinity. This humidity difference causes water vapour to migrate from source to sink by passive diffusion, leading to a linear distribution of humidity parallel to the axis of the chamber under steady conditions. The water vapour flux is calculated from measurements of this humidity gradient and Fick's first law of diffusion.

In the AquaFlux, the humidity immediately adjacent to the condenser is calculated as the thermodynamic equilibrium value for ice at the temperature of the condenser. The humidity gradient is determined from this value, together with the readings of RH & temperature from the sensor combination approximately half-way between the condenser and the measurement orifice.

The main bebefits of this method are:-

• Measurements are protected from ambient disturbance by the closed measurement chamber. Feel free to talk and breathe during the measurements.
  
  
• Measurements are reproducible because the microclimate humidity within the chamber is controlled independently of ambient humidity.
  
  
• Continuous flux vs time measurements can be recorded for many hours, because the water vapour entering the measurement chamber is continuously removed by the condenser.
  
  
• Measurements can be performed at any site, any angle, anywhere.