Please, if you happened to read it, disregard a previous post by me that I've deleted. There is a mistake in it.

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Update:

You assume that you have 3 componentes (isobutane, n-butane and propane)

Two phases

So for intensive properties you have:

F = C-P+2 = 3 degrees of freedom

Extensive variables add one more per phase that is, a total of 5 degrees of freedom.

You know:

- Pressure

- Temperature

- Total volume of Liquid

- Total volume of Gas.

You are missing one variable to be able to solve the system. You may assume a constant ratio of butanes (iso/normal), probably without making a big error (at least better than assuming a constant density).

But... you have the following difficulties:

- You assume that temperature is uniform. If the tank is not insulated and exposed to sun, this might not be true.

- The the tank is perfectly mixed and in equilibrium.

- There are no other components apart from propane, isobutane and normal butane. Small amoung of volatiles/non condensables will distort your pressure (ethane, methane, nitrogen)

If you really need a measurement of density, the best way is to purchase a ASTM D1657 hydrometer. The price is accesible even to a small operation if having an accurate inventory control is an issue.

**Edited by Saml, 25 June 2018 - 06:57 PM.**