Page 112 - CITS - WCS - Mechanical

WORKSHOP SCIENCE - CITS

Types of Pressure
• Absolute pressure.
• Gauge pressure.
• Differential pressure.
• Sealed pressure or vacuum pressure.
Gauge pressure is the pressure relative to atmospheric pressure. Gauge pressure is the pressure relative to
atmospheric pressure. Gauge pressure is positive for pressures above atmospheric pressure, and negative for
pressures below it. In fact, atmospheric pressure does add to the pressure in any fluid not enclosed in a rigid
container. This happens because of Pascal’s principle.
Absolute pressure is the sum of gauge pressure and atmospheric pressure. The definition of absolute pressure is
the pressure of having no matter inside a space, or a perfect vacuum. Measurements taken in absolute pressure
use this absolute zero as their reference point. The best example of an absolute referenced pressure is the
measurement of barometric pressure.
Vacuum pressure (Figure 2 labeled C) is a measure of the pressure of a gas in a space where the pressure is
lower than atmospheric pressure. It is expressed as a negative gauge pressure, often in units of millimeters of
mercury (mmHg) or Pascals (Pa).

Differential pressure is essentially the difference in pressure between two given points. It is a type of pressure
measured within different industries using differential pressure sensors. There are 3 common types of pressure;
· Differential Pressure.

Example
A liquid gives force of 100 N over an area of 2m . What is the pressure?
2
Force = 100 N
Area = 2 m 2

Pressure = ?

P = =
= 50 N/m 2

Unit of pressure N/m , 1 N/m = 1 pascal.
2
2
This unit is too small (Pressure of a fly on a area of 1 cm ). Hence ‘bar’ is introduced as the unit of pressure.
2
1 bar = 10 pascal.
5
10 P = = 1 bar
5
a
1 bar = 1000 mbar. [SI unit of Pressure is Pascal (Pa) and Metric unit of Pressure is bar]
Pressure gauges
They are instruments or devices used to measure the pressure of liquid steam or gas contained in a vessel. There
are also known as mark meters.

Types of manometers
• Open tube
• Closed tube
• Differential type

• Inverted type
Mechanical pressure gauges
• Bourden’s pressure gauges
• Diaphragm pressure gauges
• Dead weight pressure gauges

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CITS : WCS - Mechanical - Exercise 10 CITS : WCS - Mechanical - Exercise 10