pressure in Fluid | Measurement Of Pressure
Fluid pressure : -
- When a fluid is contained in a vessel, it exerts force at all points on the sides and bottom and top of the container.
- The force per unit area is called pressure.
- intensity of pressure, p = P/A
P = The force
A = Area on which the force acts
- The pressure of a fluid on a surface will always act normal to the surface.
1. Absolute pressure :-
- Absolute pressure is the pressure which is this is measured reference to absolute vacuum pressure is known as absolute pressure.
2. Gauge pressure :-
- Gauge pressure is the pressure which is measured with the help of a pressure instrument, in which the atmospheric pressure is taken as datum is known as gauge pressure.
- The atmospheric pressure on the scale is marked as zero.
3. Vacuum pressure :-
- Vacuum pressure is the pressure which is below the atmospheric pressure is known as vacuum pressure.
4. Atmospheric Pressure :-
- The atmospheric air exerts a normal pressure upon all surfaces with which it is in contact,and it is known as atmospheric pressure.
- The atmospheric pressure is also known as ‘Barometric pressure.
- The atmospheric pressure at sea level (above absolute zero) is called ‘Standard atmospheric pressure.
Mathematically ,
P ab = P atm + P gauge
(a) Absolute pressure = Atmospheric pressure + Gauge pressure
(b) Vacuum pressure = Atmospheric pressure - Absolute pressure
- S.I. unit of pressure is newton per square metre (N/m2). This is also known as Pascal.
Fig.Relationship in pressures
Measurement of pressure
Pressure of a fluid is measured by the following devices :-
A). Manometer .
B). Mechanical gauges .
(A) Manometer :-
It is a device by which the pressure at a point in a fluid is measured by balancing the column of fluid by the same or another column of the fluid .
They are classified as :-
1. Simple manometer .
2. Differential manometer .
1.Simple manometers
It is a device which consist of glass tube having one of its ends connected to a point where pressure is to be measured and other end remain open to atmosphere.
There are mainly three types of manometer :-
i). Piezometer .
ii). U-tube manometer .
iii). Single column manometer .
i). Piezometer :-
In this type of manometer one end is open to atmosphere and another is connected to the point where pressure is to be measured. It is the simplest form of manometer used for measuring gauge pressure.
ii). U-tube manometer :-
In this type of manometer one end remain open to the atmosphere while another end is connected to a point where pressure is to be measured .in this manometer glass tube bent in U shape. The tube generally contain mercury or any other liquid whose specific gravity is greater than specific gravity of the liquid whose pressure is to be measured.
iii). single column manometer :-
- In this type of manometer a reservoir having a large cross sectional area (about 100 times ) as compared to the area of the tube is connected to one of the limbs of the manometer .
- Due to large cross-sectional area of the reservoir, for any variation in pressure, the change in the liquid level in the reservoir will be very small which may be neglected and hence the pressure is given by the height of liquid in the other limb.
- The other limb may be inclined or vertical . This manometer is also a modified form of a U-tube manometer.
- Single column manometer is mainly two types :-
a. Vertical single column manometer .
b. Inclined single column manometer .
2. Differential manometer
It is the device which consist of a u tube, used for measuring the difference of pressure between two points in a pipe or in two different pipes . It containing a heavy liquid , whose two ends are connected to the points whose difference of pressure is to be measured.
Differential manometer are mainly two types :-
i).U-tube differential manometer .
ii). Inverted U-tube differential manometer.
(B) Mechanical gauges :-
- It is a device by which the pressure is measured by balancing the fluid column by the spring or dead weight.
- The commonly used mechanical pressure gauge are :-
i). Diaphragm pressure gauge .
ii).Bourdon tube pressure gauge .
iii). Bellows pressure gauge .
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