Specific volume is the volume occupied by 1 kg of a material. This part is simple to understand. Say I have some fluid at a temperature and pressure. I can find the specific volume for it by looking at the tables and find its quality. Simple enough. Now lets move to stage two where one or all properties (temperature, pressure and volume) change. Now I must find the quality again. At this point why do I keep the same specific volume as in the previous stage (vf & vg change of course). I mean if you decrease the pressure you would expect each kg of gas to occupy more space which would change the specific volume found initially. A question to help you help me understand the concept better: Propane is contained in a rigid container at 8 bar and 80°C. Propane is in the vapor region under these conditions . If pressure drops to 5 bar find the quality. At 5 bar propane is in the liquid-vapor region. My question again is that why do we use the specific volume found at 8 bar and 80°c to find the quality at 5 bar and some new temperature.