Pressure volume diagram
（重定向自Indicator diagram）
![](/Images/godic/202501/20/P-v_diagram_of_a_simple_cycle.svg0149.png")
![Watt's indicator diagram](/Images/godic/202501/20/Indicator_diagram0149.png")
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![](/Images/godic/202501/20/Indicator_diagram_steam_admission.svg0149.png")
A pressure volume diagram (or PV diagram, or volume-pressure loop) is used to describe corresponding changes in volume and pressure in a system. They are commonly used in thermodynamics, cardiovascular physiology, and respiratory physiology.
PV diagrams, originally called indicator diagrams, were developed in the 18th century as tools for understanding the efficiency of steam engines.