The modelling of psychrometric processes in ApacheSim, MacroFlo and ApacheHVAC requires a set of calculation procedures for converting between such quantities as:
· Dry-bulb temperature
· Wet-bulb temperature
· Dew-point temperature
· Relative humidity
· Percentage saturation
· Humidity ratio
· Water vapour pressure
· Enthalpy
· Air density
· Atmospheric pressure
For these conversions ApacheSim, MacroFlo and ApacheHVAC use published procedures set out in Reference 9.
Air Moisture Content Dynamics
The storage of water vapour in the room air mass is represented by
( 40) 
is the air density (kg/m 3 ) 
is the net water vapour gain to the air (kg/s) 
is the room air humidity ratio (kg/kg)