Fluid Mechanics - Pressure and its Measurement in terms of fluid flow

Consider a small area dA in large mass of fluid. If the fluid is stationary, than the force exerted by surrounding fluid on the area dA will always be perpendicular to the surface dA. If the F force acting on an area dA than the pressure is defined as the force (F) per unit area is known as the intensity of pressure.

Pascal's Law :- It state that the pressure or intensity of pressure at a point in a static fluid is equal in all directions.

- Absolute Pressure :- Absolute pressure is defined as the pressure which is measured with the reference to absolute vacuum pressure.

- Gauge Pressure :- Gauge Pressure is defined as the pressure which is measured with the help of a pressure measuring instrument , in which the atmospheric pressure is taken as a datum. The atmospheric pressure on the scale is marked as a zero.

- Vacuum Pressure :- Vacuum Pressure is defined as the pressure below the atmospheric pressure.
The relationship between the absolute pressure, gauge Pressure and vacuum pressure are :-

1) Absolute Pressure = Atmospheric Pressure + Gauge Pressure

2) Vacuum Pressure = Atmospheric Pressure - Absolute Pressure

- Measurement Of Pressure :- The pressure of a fluid is measured by the following devices :-

1. Manometers :- Manometers are defined as the devices used for measuring the pressure at a point in a fluid by balancing the column of fluid by the same or another column of the fluid. They are further classified as :-

(A) Simple Manometers :- A simple manometer consist of a glass tube having a one of its end connected to a point where pressure is to be measured and other end remains open to the atmosphere. Common types of simple manometers are :

* Piezometer
* U-tube Manometer
* Single Column Manometer

(B) Differential Manometers :- Differential manometer are the devices used for measuring the difference of pressure between two points in a pipe or in two different pipe. A differential manometer consist of a U-tube , containing a heavy liquid , whose two ends are Connected to a points , whose difference of pressure is to be measured. Most commonly types of differential Manometers are :

* U-tube differential manometer
* Inverted U-tube differential manometer

2. Mechanical Gauges :- Mechanical gauges are defined as the device used for measuring the pressure by balancing the fluid column by the spring or dead weight. The commonly used mechanical pressure gauges are :

(A) Diaphragm Pressure Gauge.
(B) Bourdon tube pressure gauge.
(C) Dead - Weight pressure Gauge.
(D) Bellows pressure Gauge.