Correction of vapour pressure data for pad gas

This section presents a method for correcting pressure data measured in closed system tests to remove the partial pressure of pad gas. The method is required for vapour pressure systems. Vapour pressure data obtained during tempering in an open test do not require to be corrected for pad gas. 

At the initial conditions, the test cell temperature is T, (K) and total pressure is P (N/m2 absolute). The initial pad gas pressure, P, can be obtained by subtracting the estimated vapour pressure at Ti( Pvi, from the total pressure  

The pad gas pressure at any temperature can then be estimated assuming an ideal gas. If the gas space volume is assumed constant, then  

If it is calculated that the volume of the gas space in the test cell will decrease significantly during the test, due to liquid thermal expansion, then account needs to be taken of this. Either the test can be performed at a lower fill level (at the cost of increased thermal inertia) or some estimate of the relationship between gas space volume and temperature should be made and the following equation used to find the pad gas pressure at any temperature  

The vapour pressure at any temperature can then be calculated from  

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