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We were discussing the basic definition and significance of Pascal’s Law along with its derivation and also the concept of vapour pressure and cavitation in our previous posts. 

Now we will understand here the concept of Absolute pressure, Gauge pressure, Atmospheric pressure and Vacuum pressure with the help of this post. 

Pressure on a fluid will be measured with reference to some datum. Pressure on a fluid will be measured in two different systems. In first system, pressure will be determined by taking the absolute zero pressure or complete vacuum as datum line and it will be termed as absolute pressure. 

While in second system, pressure will be measured by taking atmospheric pressure as datum line. 

Absolute pressure

Absolute pressure is basically defined as the pressure which will be measured by taking absolute zero pressure or complete vacuum as datum line. Absolute pressure will be always positive. 

Absolute zero pressure exists in complete vacuum where there will be no fluid molecules at all.

Gauge pressure

Gauge pressure is basically defined as the pressure which will be measured above or below the atmospheric pressure i.e. atmospheric pressure here will be taken as datum line. 

Gauge pressure will be measured with the help of pressure measuring instrument. Atmospheric pressure will be marked as zero on the scale of pressure measuring instrument. 

Pressure measuring instrument will measure the pressure difference between the liquid to which they are attached and surrounding sir. 

If pressure is measured above atmospheric pressure then it will be termed as positive gauge pressure and if pressure is measured below atmospheric pressure then it will be termed as negative gauge pressure.

Vacuum pressure

Vacuum pressure is basically defined as the pressure which will be measured below than the atmospheric pressure and therefore that measured pressure will be with negative sign and we will say it vacuum pressure or negative gauge pressure.

Atmospheric pressure

Atmospheric pressure is basically defined as the pressure exerted against a surface by weight of the air column over that surface.

Relationship between Absolute pressure, vacuum pressure and gauge pressure

Absolute Pressure = Atmospheric pressure + Gauge pressure 
Vacuum Pressure = Atmospheric pressure -  Absolute pressure 

Do you have any suggestions? Please write in comment box.
We will now discuss the measurement of pressure, in the category of fluid mechanics, in our next post.


Fluid mechanics by R. K. Bansal
Image Courtesy: Google


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