What is Pressure ?

We know that whenever liquid ( such as water, oil etc.) is contained in a vessel, it exerts force ( pressure )  at all points on the sides and bottom of the container.
This force per unit is called as pressure. If “F” is the force acting on area “a” then intensity of pressure is
P =  F/a ( N/mm2)
The direction of this pressure is always at right angles to the surface, with which the fluid at rest is in contact.

In fluid mechanics, pressure refers to the force per unit area exerted by a fluid on its surroundings. It is a fundamental concept used to describe and understand the behavior of fluids, including liquids and gases.

The pressure in a fluid is caused by the random motion of its molecules or particles. As molecules collide with each other and with the walls of their container, they transfer momentum, resulting in a net force exerted on the walls. This force is distributed over the area, giving rise to pressure.

Pressure in a fluid can vary at different points within the fluid and is influenced by factors such as depth, density, and gravitational acceleration. For example, in a liquid, the pressure increases with depth due to the weight of the fluid above pushing down. This is known as hydrostatic pressure.

Additionally, pressure can also change with the velocity of the fluid flow. When a fluid flows through a pipe or a constriction, the velocity may increase or decrease, leading to changes in pressure according to Bernoulli’s principle.

Understanding pressure is crucial in various applications of fluid mechanics, such as determining forces on submerged surfaces, calculating flow rates, designing hydraulic systems, and analyzing fluid behavior in pipes, pumps, and turbines.


1) Atmospheric pressure 

A bank of air surrounding the earth surface is called as atmosphere. The height of atmosphere above the earth surface varies from 100 to 1000 km.
At the earth surface, the pressure due to the weight of air above the earth surface is called as atmospheric pressure.
It is measured by mercury barometer.

Atmospheric pressure refers to the pressure exerted by the Earth’s atmosphere on objects within it. It is the force per unit area exerted by the weight of the air above a given point.

Atmospheric pressure is caused by the weight of the air molecules in the Earth’s atmosphere pressing down on the surface of the Earth. The air molecules are constantly in motion and collide with each other and with objects at the Earth’s surface, exerting a force.

At sea level, the average atmospheric pressure is approximately 101.3 kilopascals (kPa), which is equivalent to 1 atmosphere (atm) or 14.7 pounds per square inch (psi). This standard atmospheric pressure is often used as a reference point for measuring and comparing other pressures.

However, it’s important to note that atmospheric pressure is not constant and can vary with altitude, weather conditions, and geographic location. As you move higher in the atmosphere, the air becomes less dense, and the atmospheric pressure decreases. For example, at higher elevations, such as on top of a mountain, the atmospheric pressure is lower than at sea level.

Atmospheric pressure is commonly measured using devices called barometers. A mercury barometer, for instance, consists of a glass tube filled with mercury, inverted into a dish of mercury. The weight of the mercury in the tube is balanced by the atmospheric pressure, and the height of the mercury column in the tube provides a measure of the atmospheric pressure.

2) Gauge pressure 

If the pressure is measured above or below the atmosphere pressure it is called as gauge pressure.
When the pressure is measured above the atmosphere pressure, it is called as positive gauge pressure.
If the pressure is measured below the atmosphere pressure it is called as negative gauge pressure or vacuum pressure.

Gauge pressure refers to the pressure measured relative to atmospheric pressure. It represents the difference between the absolute pressure and atmospheric pressure at a given location.

When measuring pressure, there are two common reference points: absolute pressure and atmospheric pressure. Absolute pressure is the total pressure exerted by a fluid, including the pressure due to the atmosphere. On the other hand, gauge pressure measures the pressure above or below atmospheric pressure.

Gauge pressure is often used in practical applications where the focus is on the pressure relative to the local atmospheric conditions. By subtracting the atmospheric pressure from the absolute pressure, you obtain the gauge pressure. Mathematically, it can be expressed as:

Gauge Pressure = Absolute Pressure – Atmospheric Pressure

For example, if the absolute pressure at a certain point is 200 kPa and the atmospheric pressure is 101.3 kPa, the gauge pressure would be 98.7 kPa (200 kPa – 101.3 kPa).

Gauge pressure readings are frequently used in various industries and applications, including pressure measurements in tires, hydraulic systems, industrial processes, and many other systems. It provides a convenient way to monitor pressure differentials without the need to account for the atmospheric pressure separately.

3) Absolute pressure 

If the pressure is measured above the absolute zero (or complete vacuum) then it is called as Absolute pressure.
Absolute pressure = Atmosphere pressure + or – Gauge pressure

Absolute pressure refers to the total pressure exerted by a fluid, including the pressure contributed by the atmosphere. It is the pressure measured relative to a perfect vacuum.

Unlike gauge pressure, which is measured relative to atmospheric pressure, absolute pressure provides the complete picture of the pressure being exerted at a specific location. It includes both the pressure caused by the fluid itself and the pressure due to the weight of the atmosphere.

Absolute pressure is often measured using devices such as absolute pressure sensors or transducers, which provide readings without the need for additional calculations or reference to atmospheric pressure.

In fluid mechanics, absolute pressure is typically expressed in units such as pascals (Pa), pounds per square inch (psi), or bars. For example, a pressure reading of 300 kPa absolute means that the fluid is exerting a pressure of 300,000 pascals above a perfect vacuum.

Absolute pressure is crucial in many applications, including designing and analyzing hydraulic systems, calculating forces on submerged surfaces, understanding gas behavior in enclosed spaces, and evaluating pressure differentials across various components and processes.

Absolute Vaccum 

It is the emptiness or space which contains no form of matter. Instead of complete or absolute vacuum is not used generally. So partial vacuum is used. The relationship between absolute pressure gauge pressure and atmospheric pressure is –


Absolute vacuum refers to a state of complete absence of matter or particles in a given space. It is a theoretical concept that represents the lowest possible pressure or the absence of any gas or fluid.

In an absolute vacuum, the pressure is zero, and there are no gas molecules or particles present. This state is unachievable in practice because even in highly evacuated spaces, trace amounts of gases or particles may still be present.

In practical terms, the lowest achievable pressure is often referred to as a high or ultra-high vacuum, where the pressure is extremely low but not zero. High vacuum systems are commonly used in scientific research, electronics manufacturing, and various industrial processes.

Achieving and maintaining a high vacuum requires specialized equipment, such as vacuum pumps, chambers, and seals, to remove or minimize the presence of gases and particles. These systems work by creating a pressure differential between the vacuum space and the external environment, causing gases to be pumped out or condensed.

Vacuum technology finds applications in fields such as semiconductor manufacturing, electron microscopy, space simulation, and particle accelerators. The absence of gas molecules in a vacuum can provide a controlled environment for experiments, reduce unwanted interactions, and facilitate certain manufacturing processes.

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By Aditya

Hi, I’m Aditya Sharma, a professional blogger from Gurgaon, India and I launched this blog called aadityacademy on July 2021. aadityacademy.com is a mechanical Project-oriented platform run by Aditya sharma and I got the motivation to start aadityacademy blog after seeing less technical education information available on google.

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