Different Types of Pressure in Fluids [PDF]

What is Pressure?

Pressure (p)  is defined as force per unit area applied in a direction perpendicular to the surface of an object. Pressure is an effect which occurs when a force is applied on a surface.

Pressure : gauge pressure and absolute pressure

The measurement of fluid pressure can be categorized into two systems: one measuring pressure above absolute zero, and the other measuring pressure below it. This distinction gives rise to various terminologies in fluid mechanics to describe different pressure ranges.

The pressure is the amount of force acting on a unit area. This is compressive stress on the fluid and the mathematical expression is,


= Force / Area = F / A


  • p is the pressure and the SI unit is Pascal.
  • F is the normal force and the SI unit is Newton (N).
  • A is the area and the SI unit is m2.

Pressure unit conversions

  • 1 Pa = 1 N/m2
  • 1 kPa = 1000 N/m2
  • 1 bar = 100 kPa = 105 N/m2

Different types of Pressure

Following are the types of pressure:

  • Absolute Pressure
  • Gauge Pressure
  • Vacuum Pressure

Absolute Pressure

Absolute pressure is the measurement of pressure relative to the absolute zero point on the pressure scale. It is denoted by ‘p’. It is important to note that atmospheric pressure represents the pressure in the atmosphere, while absolute zero pressure or vacuum level refers to an absence of pressure.


Absolute Pressure = Atmospheric Pressure + Gauge Pressure

Gauge Pressure

Gauge pressure is measured using a pressure gauge, with atmospheric pressure set as the reference point marked as zero on the scale. When the pressure matches atmospheric pressure, the gauge pressure reads zero. It is important to note that gauge pressure can have negative values, indicating a pressure lower than atmospheric pressure.


Vacuum Pressure  = Atmospheric Pressure – Gauge Pressure

Vacuum Pressure

Vacuum pressure refers to pressure levels below atmospheric pressure. It is employed when gauge pressure registers as negative. In this context, positive vacuum pressure indicates a negative gauge pressure.


Pvacuum = Pa – P

Relationship between Pressures

Relationship between Pressures
Relationship between Pressures pdf
pressure pdf
absolute pressure pdf
pressure diagram
pressure chart
vacuum pressure
atmospheric pressure
gauge pressure


In conclusion, understanding the different types of pressure is essential in various fields, including fluid mechanics. Absolute pressure provides measurements relative to the absolute zero level, while gauge pressure is referenced to atmospheric pressure. Vacuum pressure is utilized when gauge pressure becomes negative. By comprehending these distinctions, we can accurately measure and interpret pressure values in different contexts, contributing to advancements in science, engineering, and other industries reliant on pressure measurements.

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