# How to understand Nelson-Obert charts?

The compressibility factor is given by: $$z=\frac{P\overline{V}}{RT}$$ The Nelson-Obert chart is:

This chart brings the following information:

1. At very low pressure (Pr << 1), gases behave as an ideal gas regardless of temperature.
2. At high temperatures (Tr > 2), ideal gas behavior can be assumed with good accuracy.
3. The deviation of a gas from ideal gas behavior is greatest in the area of the critical point.

Where:

$$T_R=\frac{T}{T_{(At\ critical\ point)}}\\ P_R=\frac{P}{P_{(At\ critical\ point)}}$$

What I don't understand is:

• Does this chart describe the ideal behavior of a supercritical fluid or a superheated gas or plasma?

Having in mind any phase diagram: