MECHANICS OF FLUIDS
BSCE 3A/3B
3. PRINCIPLES OF HYDROSTATICS
LEARNING OUTCOMES:
• Define pressure
• Give examples of pressure from everyday experience
• Define gauge pressure and absolute pressure
• Calculate pressure using the basic formula of pressure
DISCUSSION:
Unit Pressure
Pressure, is the force per unit area exerted by a liquid or gas on a body or surface, with the force acting
at right angles to the surface uniformly in all directions.
The unit of pressure is the pascal (Pa), but it is also expressed in bars or meters of water column. 1 atm
is standard 1 atmospheric pressure in meteorology and is called the standard atmospheric pressure.
1 atm = 760mmHg = 101 325Pa
Conversion Units of Pressure
PASCAL’S LAW
Pascal’s law, developed by French mathematician Blaise Pascal, states that the pressure on a fluid is
equal in all directions and in all parts of the container.
Absolute pressure and gauge pressure
There are two methods used to express the pressure: one is based on the perfect vacuum and the other
on the atmospheric pressure. The former is called the absolute pressure and the latter is called the
gauge pressure. Then,
, In gauge pressure, a pressure under 1 atmospheric pressure is expressed as a negative pressure. Most
gauges are constructed to indicate the gauge pressure.
Gage Pressure: Gage Pressure are pressures above or below the atmosphere and can be measured by
pressure gauges or manometers.
Atmospheric Pressure: Atmospheric pressure is the pressure at any point on the Earth’s surface from the
weight of the air above it.
Vacuum: A vacuum is a space that has all matter removed from it. A vacuum can also be described as a
region of space where the pressure is less than the normal atmospheric pressure of 760 mm of Mercury.
Under normal conditions at sea level:
Patm = 2166 lb/ft2
= 14.7 psi
= 29.9 inches of mercury (Hg)
= 70mm Hg
Absolute Pressure: Absolute pressure is the pressure above absolute zero(vacuum).
MEASUREMENT OF PRESSURE
Manometer
A device which measures the fluid pressure by the height of a liquid column is called a manometer. For
example, in the case of measuring the pressure of liquid flowing inside a pipe, the pressure p can be
obtained by measuring the height of liquid H coming upwards into a manometer made to stand
upright as shown in Figure.
BSCE 3A/3B
3. PRINCIPLES OF HYDROSTATICS
LEARNING OUTCOMES:
• Define pressure
• Give examples of pressure from everyday experience
• Define gauge pressure and absolute pressure
• Calculate pressure using the basic formula of pressure
DISCUSSION:
Unit Pressure
Pressure, is the force per unit area exerted by a liquid or gas on a body or surface, with the force acting
at right angles to the surface uniformly in all directions.
The unit of pressure is the pascal (Pa), but it is also expressed in bars or meters of water column. 1 atm
is standard 1 atmospheric pressure in meteorology and is called the standard atmospheric pressure.
1 atm = 760mmHg = 101 325Pa
Conversion Units of Pressure
PASCAL’S LAW
Pascal’s law, developed by French mathematician Blaise Pascal, states that the pressure on a fluid is
equal in all directions and in all parts of the container.
Absolute pressure and gauge pressure
There are two methods used to express the pressure: one is based on the perfect vacuum and the other
on the atmospheric pressure. The former is called the absolute pressure and the latter is called the
gauge pressure. Then,
, In gauge pressure, a pressure under 1 atmospheric pressure is expressed as a negative pressure. Most
gauges are constructed to indicate the gauge pressure.
Gage Pressure: Gage Pressure are pressures above or below the atmosphere and can be measured by
pressure gauges or manometers.
Atmospheric Pressure: Atmospheric pressure is the pressure at any point on the Earth’s surface from the
weight of the air above it.
Vacuum: A vacuum is a space that has all matter removed from it. A vacuum can also be described as a
region of space where the pressure is less than the normal atmospheric pressure of 760 mm of Mercury.
Under normal conditions at sea level:
Patm = 2166 lb/ft2
= 14.7 psi
= 29.9 inches of mercury (Hg)
= 70mm Hg
Absolute Pressure: Absolute pressure is the pressure above absolute zero(vacuum).
MEASUREMENT OF PRESSURE
Manometer
A device which measures the fluid pressure by the height of a liquid column is called a manometer. For
example, in the case of measuring the pressure of liquid flowing inside a pipe, the pressure p can be
obtained by measuring the height of liquid H coming upwards into a manometer made to stand
upright as shown in Figure.
