This is the basic rule in hydraulic engineering you must know.
There is different pressure you had come across in engineering, out of that here you will find most important three.
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So, here is the three different types of Pressure measured in Engineering

Atmospheric Pressure
It is established that the Air possesses some Weight.
And due to its Weight, Air must exert some force on the Surface of the Earth.
The Density of Air is different at different heights due to its property of Compressibility.
The Density of Air has also varied from time to time due to change in its Temperature and Humidity.
Due to these factors, the Atmospheric Pr. of Air cannot be calculated. But you can calculate in the case of Liquid.
The Atmospheric Pr. at Sea Level is 1.03 kg/cm².
It can also be expressed as 10.3 meters of water, in terms of equivalent Water Column or 76 centimetres of Mercury in terms of equivalent Mercury Column.

Gauge Pressure
It is the Pressure measured by Pressure Measuring Instruments.
The Atmospheric Pr. on the Gauge Scale is marked as Zero.
Actually Gauge Pr. is always above the Atmospheric.

Absolute Pressure
It is defined as the sum of Atmospheric and Gauge Pr. In the case of Vacuums, the Gauge Pr. is a minus, and then the Absolute Pr. will be Atmospheric minus Gauge.
If the Intensity at a point is more than the Local Atmospheric, the difference in these two is called “Positive Gauge Pr.”.
However, if the intensity at a point is less than the Local Atmospheric Pressure, the difference in these two is called “Negative Gauge Pr. or Vacuum “.
Besides this information, you are suggested to read something more from below engineering books
So, Here you find Best Engineering Resources for further details
To get the more details about the topic, I further recommended reading
 A Textbook of Hydraulic Machines
 A Textbook of Hydraulics, Fluid Mechanics, and Hydraulic Mechanics
 Textbook of Fluid Mechanics and Hydraulic Machines
 A Textbook of Fluid Mechanics and Hydraulic Machines
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