Pv Diagrams in Thermodynamics

Thermodynamics is a very important area in engineering. We are using the tools of thermodynamics to assess engineering systems. One of these tools is the Pv diagrams. Like Tv diagrams, we are using Pv diagrams to assess the phase changes over a thermodynamical cycle. So, you can check below to understand how to read Pv diagrams of pure substances.

What is Pv Diagram in Thermodynamics?

Pv diagrams are the same as the Tv diagrams in thermodynamics. In addition, we just replace the temperature axis with the pressure. You can see the example of the Pv diagram below.

Pv diagram of a pure substance.
Image Source: Yunus A. Çengel, Michael A. Boles – Thermodynamics, An Engineering Approach-McGraw-Hill Science, 8th Ed., Pg: 120.

In this Pv diagram, we have two axes; temperature and specific volume. And then, the specific volume is the physical property where the volume of the substance per kilogram. It is the inverse of the density. The unit of specific volume is m3/kg or ft3/lbs in English units. 

Critical Point

You know that pure substances have phases like saturated liquid and saturated vapor. Saturated liquıid is the liquid where the vaporization starts at a temperature at a specific pressure. Also, saturated vapor is the temperature where all the liquid is converted to a vapor.

The critical point of a substance is where the saturated liquid and saturated vapor phases are at the same time. For example, the pressure and temperature values of water are 374°C (705°F) and 218 atm.

Also in the Pv diagram above, the green line shows all the saturated liquid and saturated vapor, and critical points of a pure substance.

Saturated Liquid Line

Image Source: Unipamplone.edu.co.

At the curve above, the saturated liquid line is at the left side of the critical point. So at this line, the vaporization of water starts.

You can see the transverse lines on this graph. These lines are the temperature lines. So you can obtain the saturated liquid and saturated vapor pressures. If you strut down from these points, you will find the specific volumes of these substances in these temperatures and pressures.

Saturated Vapor Line

Also at this line, the vaporization process of pure substance completes. All the substance is vapor. Between the saturated vapor and saturated liquid lines, substance liquid and vapor phases at the same time.

So, you can make different interpretations with Pv diagrams of pure substances.


Finally, these are the general explanations for this issue. Also, it is very easy to read these diagrams to assess the thermodynamical systems.

So, do not forget to leave your comments and questions below about the Pv diagrams of pure substances.

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