Are you trying to determine the liquid level in the tank by the pressure in the tank?
If so, if you know the total mass of ethylene and ensure there is sucifecint space above the liquid phase to hold the evaporating gas. If there is insufficient volume for the evaporating ethylene tank must be vented or it will rupture from over pressurization. Picture this, if you start with a full tank of liquid, any phase change to gas will need to be vented to atmosphere and you will not know the total mass of ethylene. In contrast, if your tank is partially full and sealed off at some known pressure and temperature then you could calculate the increase in the vessel pressure as the liquid changes phase.
One way to keep track of content is with load cells on the tank legs. Then you subtract the empty tank weight from the loaded tank to determine quantity.
Provided you have sucifecint volume in the tank for the evaporated gas without requiring venting then you would have a number of equations to solve simultaneously. Mass balance, Volume balance, liquid phase mass, vapor phase mass, liquid phase vol, and gas phase volume. If you know the physical properties at the range of temp and press you need then write the equations, combine the equations if possible, and then iterate to a solution. This will only work provided difference in physical properties for different temp and pressures is great enough to make the errors in your measurements of temp and pressure minor.
Not sure what you are trying to do; however, I suspect that you will not have accurate enough temp, pressure, total mass and physical properties to calculate a meaningful solution.
