The experimental closed system vapour content x is a dimensionless ratio between 0 and 1. It is defined by the ratio of mass of vapour and total mass. The total mass is calculated from the sum of fluid mass and vapour mass. If the vapour content is x=0, the evaporation medium is completely liquid, x=1 means dry saturated vapour, a value in between means wet vapour with a variable liquid content. Separating and throttling calorimeters are used to determine the vapour content. In practice, devices to determine the vapour content are used in steam power plants, downstream of steam turbines or at steam boilers upstream of the super heater.

A downstream throttling calorimeter is used to determine vapour contents between x=0,95 and x=1. The wet vapour is depressurized in this process. The remaining vapour part is depressurized and then liquefied in a water-cooled condenser and also collected in a measuring cup. The two quantities can be used to determine vapour mass and total mass to calculate the vapour content.

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