Y Function

The Y-function determination is a traditional part of oil PVT studies, providing an equation to match the twophase region of the flash study.
Proper use of the Y-function can lead to bubble point measurements of the highest precision, essential for reliable reservoir property correlation work and of great use in the quality control of samples, but the mathematical procedures must be tempered by a fundamental understanding of the measurements themselves.
In petroleum engineering the term is defined by the following empirical equation:

Y=(Pb-P)/[(Pabs)×(Vr-1)]

where:
Pb represents the bubble point pressure,
P represents the pressure for an equilibrium measurement in the two-phase region,
Pabs is the same pressure in absolute units,
Vr is the relative volume for the same equilibrium measurement, namely the actual volume divided by the volume corresponding to the bubble point.

Since (Pb-P) is a difference, both pressures can be in either gauge or absolute units, providing they are consistent. The term (Vr-1) may also be written (V/VPb-1). It is immediately clear that the Y-function formula becomes very sensitive for equilibrium measurements close to the bubble point where the numerator and the denominator approach zero.

https://onepetro.org/SPEATCE/proceedings/11ATCE/All-11ATCE/SPE-146394-MS/148460