Conversion of saturated vapor to standard conditions (calculation of valve Cv values)

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The Cv value requires the volume Qn under standard conditions (0°C, 101.325 kPa). There is a problem with Qn; let’s illustrate it with an example. It is known that for 18 kg of water at saturated conditions, the pressure is 1.0 MpaA, the temperature is 180°C, and the volume is 0.194348885018 m^3/kg. First calculation method: 22.4 * mass/molecular weight = 22.4 Nm3, which gives the volume under standard conditions as 22.4 Nm3 ； The second calculation method uses the formula PV = mRgT; given the temperature and volume at high temperatures, the volume under standard conditions can be determined as follows: 18*0.194348885018*273/(273+180)*1.0/0.1 = 21.08 Nm3 ； The difference between the two can be seen. The actual value is saturated R245fa at 121°C, which is a 50% difference – quite alarming… Using Aspen to input the mass gives a result close to that of the second method. I would appreciate your guidance as well. Thank you!