Abstract: An energy conservation enthalpy control system utilizes a sensing unit for sensing the wet bulb temperature of sampled air, both outside air and return air, under conditions which approximate adiabatic saturation. The unit includes a housing adapted to pass filtered air through the unit. Water circulated throughout from a sump in the housing is intimately contacted and mixed with the air by a spray or an evaporative pad. A temperature sensing element located in the sump or in the airstream provides a signal indicating the wet bulb temperature of the air under adiabatic saturation conditions. The unit provides the inputs to a control system which operates to coordinate the operations of return air dampers, outside air dampers, and air conditioning equipment.