The key difference between vacuum pressure and vapour pressure is that the vacuum pressure relates to a vacuum whereas vapour pressure relates to solids and liquids.
A vacuum is a condition where no air or gas exists. We can create a vacuum by removing all the gases in a closed system. Usually, the vacuum pressure is the negative pressure exerted under space. Vapour pressure, on the other hand, is the pressure that vapour can exert on its condensed form, and this is usually positive.
What is Vacuum Pressure?
Vacuum pressure is the pressure inside a vacuum. In other words, if we create a vacuum inside a closed vessel, the vacuum pressure of that vessel is the difference between the absolute pressure inside the vessel and outside the vessel, when the pressure is greater at outside than that of the inside. Therefore, vacuum pressure usually is negative.
We measure this pressure relative to ambient atmospheric pressure. The unit of measurement is pounds per square inch (vacuum) or PSIV. There are several types of instruments that we can use to measure the pressure of a vacuum; hydrostatic gauges, mechanical or elastic gauges, thermal conductivity gauges and ionization gauges.
What is Vapour Pressure?
Vapour pressure is the pressure that a vapour exerts on its condensed form when the condensed form and the vapour are in equilibrium with each other. The condensed form can be a liquid or a solid. However, we can measure this pressure only if the equilibrium of the system exists inside a closed system with a constant temperature. The vapour pressure is a result of the conversion of the condensed form into vapour form.
The substances having a high vapour pressure at a low temperature are volatile substances. The process of formation of this vapour is vaporization. This vaporization may occur from either a solid surface or a liquid surface. Depending on the changes of the temperature of the equilibrium system, the vapour pressure also changes. For example, if we increase the temperature of the system, then more liquid or solid molecules will escape to the vapour phase. This increases the vapour pressure. This occurs because of the increase in the kinetic energy of the system. Moreover, the boiling point of a liquid or the sublimation point of a solid is the point where vapour pressure equals the external pressure of the system.
What is the Difference Between Vacuum Pressure and Vapour Pressure?
Vacuum pressure is the pressure inside a vacuum whereas vapour pressure is the pressure that a vapour exerts on its condensed form when the condensed form and the vapour are in equilibrium with each other. This is the fundamental difference between vacuum pressure and vapour pressure. Moreover, a notable difference between vacuum pressure and vapour pressure is that the vacuum pressure is a negative pressure while the vapour pressure is always a positive value. In addition to that, vapour pressure changes with the temperature changes, but the vacuum pressure does not change. Moreover, the vacuum pressure relates to a vacuum while vapour pressure relates to solids and liquids that are in equilibrium with their vapour phase. We can say this as the key difference between vacuum pressure and vapour pressure.
The below infographic tabulates the difference between vacuum pressure and vapour pressure in more detail.
Summary – Vacuum Pressure vs Vapour Pressure
Pressure is the force that exerts on a unit area. Vacuum pressure and vapour pressure are two types of pressure. The key difference between vacuum pressure and vapour pressure is that vacuum pressure relates to a vacuum whereas vapour pressure relates to solids and liquids.
1. Lish, Tom. “Vacuum Pressure: What Is It & How Do You Measure It?” Setra Systems – Pressure Transducers, Transmitters & Industrial Sensors. Available here
2. “Vapor Pressure.” Wikipedia, Wikimedia Foundation, 11 Sept. 2018. Available here