This test checks if water vapour mass is conserved. Two air streams with different mass flow rate are humidified by a mixing volume with two different vapor mass flow rates. These flows are then mixed. Afterwards the added water is removed again. The final water concentration, mass flow rate and enthalpy flow rate should then be equal to the initial values.
Note, however, that there is some approximation error because in its default configuration, the conservation balance models simplify the treatment of the water that is added to the fluid. See Buildings.Fluid.Interfaces.StaticTwoPortConservationEquation and Buildings.Fluid.Interfaces.StaticTwoPortConservationEquation for a discussion.