PRESSURE AND ITS MEASUREMENT

Fluid Statics: - Study of fluid at rest or study of fluid in rigid body motion.

There is no shear stress acting on the fluid, when fluid is at rest.

When fluid at rest, there is only normal stress acting on it.

This normal stress is equal to the pressure acting on it.

Pressure: - Normal component of force per unit area.

Unit- N/Mm2 or Pascal

1 ATM pressure = 760mm of Hg =760 Torr = 101.3KPa = 1.013 bar

1bar =105 Pa or 1torr =133.28 Pa

Pascal’s law: It states that pressure at any point in a fluid at rest is equal in all directions.

Px= Py = Pz = P

Pressure at point 'P'
Fig.1 | Pressure at point 'P'

Types of Pressure: -

1.   Absolute pressure:It is the pressure which is measured with respected to absolute zero pressure.

 

2.   Gauge Pressure:It is the pressure which is measured with the help of pressure measuring instrument, in which atmospheric pressure is taken as datum.

Pabs= Patm + Pgauge

Relation between gauge pressure and absolute pressure
Fig.2 | Relation between gauge pressure and absolute pressure

3.   Vacuum Pressure or negative gauge pressure:It is define as the pressure below the atmospheric pressure.

Pressure at a particular point in a fluid –

Absolute pressure at point ‘A’ (PA) = Patm + γh

= Patm + ρgh

Gauge pressure at point ‘A’ (PA)gauge = ρgh

 

MEASURING INSTRUMENT OF PRESSURE:

I.           MANOMETER –

1.   Simple Manometer-

a.   Piezometer: -

Piezometer
Fig.3 | Piezometer

  • It is a simplest type of manometer.
  • It can’t measure vacuum pressure.
  • It can’t measure high pressure.
  • It can’t measure gas’s pressure.

 

b.   U-tube manometer: -

U-tube manometer
Fig.4 | U-tube manometer

  • It can measure High Pressure.
  • It can measure both positive and negative pressure.
  • The density of manometric fluid should be higher than the fluid, which density is to be measured.


c.    Single column micro manometer

Fig. 5 | Single column micro manometer

It is more sensitive than U-tube manometer.


d.   Inclined U-tube manometer

 

2.   Differential Manometer-

a.   Differential U-tube manometer

Differential u-tube manometer
Fig. 6 | Differential u-tube manometer

b.   Inverted U-tube manometer

c.    Inclined U-tube differential manometer

 

II.         MECHANICAL GAUGE –

1.   Bourdon tube

2.   Pressure transducer

3.   Blow pressure gauge

4.   Diaphragm gauge