What are the Common Units of Pressure?

Pressure is something that we experience in daily life but fail to recognize it as our body is so accustomed to it. For example, the atmospheric air applies pressure on our body, but we don’t feel it as we are exposed to it since birth and our body is habitual to it. When we write using a pen, we need to apply some pressure over the pen to make it start writing. But these are our day-to-day activity which we fail to recognize

Pressure is defined as force applied on a surface per unit area, given by the expression P = F/A, where P is pressure applied, F is force and A is the surface area. Force is given as a product of mass and acceleration of the body. Force is always a cause of change in state of an object.

Even when we don’t see mass, acceleration in the pressure definition, they play a key role in it. The SI unit of pressure is Pascal Pa =N/m².It is used to quantify internal pressure. Pressure is not the same as that of stress but is a physical quantity. Where

Pa= Pascal

N= Newton

m= meter

Common Mistake

The pressure unit in Kilogram force per square centimeter should be always written as kgf/cm², but we often observe that it is represented as kg/cm² without the letter “f”. Similarly, pound force per square inch (pfsi) is normally written as pounds per square inch (psi)

Various Types of Units’ System followed for Pressure are

International system of units commonly known as SI system or metric system

SI system of unit is derived from the metric system of units. It was introduced in 1960. It uses meter, kilogram, second system rather than centimeter, gram and second system. The units in the SI system are further divided into base units and derived units. It has seven base units and others are derived from it.

Derived units are formed by multiplication and division of the base unit. For example, the unit of speed is derived using a meter per seconds, m/s.

Pressure, the effect of force applied to a surface, is a derived unit. The unit of pressure in the SI unit is Pascal denoted (Pa), which is defined as the force of one newton per square meter.

1 Pa = 1 N / m².

$\text{Pa= }\frac{\text{N}}{{\text{m}}^{\text{2}}}\text{=}\frac{\text{Kg}}{{\text{m × s}}^{\text{2}}}$$\text{ Pressure of unit = }\frac{\text{1bf}}{f{t}^2}$

Pascal is a smaller unit of pressure. For example, atmospheric pressure is around 101325 Pascal on absolute scale.

We also see the unit of pressure in terms of bar, it is based on the metric system, but it does not fall under the category of SI unit systems. Bar is 100000 times that of pascal. 

Usage of Prefix

When the value of pressure comes out to be very large or very small, prefixes such as kilo (1000), mili (1/1000), centi (1/100) are used. These prefixes are independent of which metric system one uses. For example, a unit of atmospheric pressure can be said as 101.325 KPa.

Imperial Units of Pressure

Different countries use units which are local to their use only. For example, the USA uses different units of mass and area other than the SI unit system. Hence it creates a different set of units of pressure. Mass is commonly known as pound or ounce and the distance is measured in inches or feet.

When we derive unit of pressure we get as following

Some others are psi, ozf/in², in terms of height of water column.

CGS Unit of Pressure

Centimeter-gram-second (CGS) unit system uses, centimeter for distance and a gram for measuring mass. It is a very old system of measurement. In this pressure is measured in barye, which is defined as below

$\text{1barye=}\frac{\text{(1dyne)}}{{\text{Cm}}^{\text{2}}}$, here dyne is the force which is equal to gram/ cm-s-2. 1 barye is equal to 0.1 Pascal.

Atmospheric Units

When one wants to calculate atmospheric pressure, unit standard atmospheric pressure is used, written as atm. This atm is equal to 101325 Pascal.

Similarly, one more popularly used unit of atmospheric pressure is torr. It is used to measure atmospheric readings which are smaller in quantity. It is an absolute pressure scale. It is equal to 1/760 of standard atmosphere. Torr is not part of the SI system.

Common Mistakes

People confuse standard atmosphere (atm) with technical atmosphere which is represented as (at). The technical atmosphere is defined as 1 kilogram force per square centimeter. So, 1 technical atmosphere is equivalent to approximately 0.968 atm

Pressure in Terms of Height of Liquid Column

In order to measure pressure in pipes in industries, earlier U-tube manometer were used. They are nothing but a transparent tube with U shape and markings on them, with a high-density liquid filled in it, the difference in height of the liquid column is used to measure the pressure in the tube.

The pressure is represented in terms of liquid column. In this u-tube, if one side is open to atmosphere, then the other gives reading of pressure which will be a gauge pressure. In some pressure scale is marked as well, so the difference in liquid levels gives the pressure readings.

The very common liquid used is water, but when we need to measure higher pressure, we use liquid of high density to measure the pressure. The advantage of higher density liquid is the length of tube will be lesser.

We know pressure, $\text{P = ρgh}$,

For given constant pressure, if density is higher, the height will be lesser. For example, to measure the blood pressure if we use water instead of mercury, we have to use a very long tube instead, which is not feasible practically.

Some common application of this is to determine the level of water in the tank. With the help of pressure, we can determine the level of water in it.

Various factors like gravity, temperature could affect the pressure readings. The length of the tube can be marked in different units like meter, centimeters, inch, and feet.

Different densities serve different purposes.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for   

• Bachelor of Engineering 
• Bachelor of Technology
• Master of Engineering 
• Master of Technology