1. Pressure and Pressure Scales

2. Pressure
“A pressure is the ratio of a force to the area on which the force acts.”
Pound force, Kilogram force
Newton, dyne
Force
Area
Pressure =
Square Inches, Square feet
Square Centimeters, Square Meters
gallons per minute, pressure can be measured in different units:
• In the S.I. system of units, pressure is measured in pascals (Pa). One pascal
(1 Pa) is defined as one newton (1 N) of force applied to an area of one square
meter (1 m2). Therefore, 1 Pa is equal to 1 N/m2. Since the pascal is a very small
unit, the kilopascal (kPa) is used more often. One kilopascal (1 kPa) is equal to
1000 Pa.
• In the U.S. system of units, pressure is measured in pounds-force per square
inch (psi). One pound-force per square inch (1 psi) is defined as a force of one
pound (1 lbf) applied to an area of one square inch (1 in2).
• In the metric system of units, pressure is measured in bars (bar). One
bar (1 bar) is equal to 100 kPa or 14.5 psi.
Head
A pressure can also be expressed as a vertical height of a column of liquid
Common units include:
Pounds per Square Inch (psi)
Kilo-Pascals (kPa)

3. Pressure
An important operating parameter that is relevant in many applications
Pressure applied over a given area can be used for useful work.e.g
Steam reforming, Water pressure(energy)
Pressures can be measured to monitor the condition of other process parameters.
Flow,

4. Pressure units SI system: Pascal N/m2 (Pa), AES: psi
One Newton (1 N) of force applied to an area of one square meter (1 m2).
1 Pa is equal to 1 N/m2
small unit, often use kilopascal 1 kPa=1000 Pa.
Force of one pound (1lbf) applied to an area of one square inch (1 in2).
Metric System : Bar
One bar (1 bar) is equal to 100 kPa or 14.5 psi.

5. More Pressure Scales Inches* of water /mm Hg
PSI and kPa are the most common pressure scales but there a few more:
Inches* of water /mm Hg
Inches* of mercury/ ft of H2O
Bar 100kPa=1bar
Atmosphere (atm)
Torr (vacuum)
dynes/cm2 ,
* or millimeters when using metric

6. Applied process pressure is
Pressure Scales
“H20
“Hg
Bar
Atmos
Applied process pressure is
1 psi or
6.89 kPa
The choice of scales will depend on
the amount of pressure being measured (high pressure = psi/kPa, low pressure = inches H20)
The type of application ( flow = inches H20, blood pressure = inches of Hg.)

7. Pressure Conversion Chart
Pressure Units
psi
kPa
inches of Hg
inches of H20
atmospheres
bar 1
6.894
2.036
27.681
.0681
.06895
0.1450
.2953
4.0147
.01
0.4912
3.3864
13.595
.03342
.03386
inches of H2O
.2491
.07355
14.696
101.33
29.92
406.8
1.0133
14.504
100
29.53
401.86
.9869
1 psi = 6.89 kPa
1 inch Hg = 0.49 psi
100 inch H20 = 3.61 psi
1 Bar = 14.5 psi = 100 kPa

8. Conversion Factors
1 cubic foot of water that weighs 62.4 lbs acting over an area of 144 in2 produces a pressure of pound per square inch (psi)
The same volume of water weighs 28.3 Kilograms over an area of 929 cm2, therefore the pressure is 0.03 kilograms per square centimeter.

9. Fluid Pressure and Pressure Head
Pressure is exerted on the top of the cylinder of the water by the atmosphere and on the bottom of the cylinder itself by the water.
The Pressure at bottom of the static (nonmoving )column of the water exerted on the sealing plate is
P = pgh + Po
P=Pressure at bottom of the column
Po=Pressure at the top of the column

10. In AES (psi) Area= 1cm2 Height 50cm
Sp gr at 20C and density of Hg is 13.55g/cm3.
Then force exerting by Hg on the bottom plate with 1cm2 area is
6.64N
In AES (psi)

11. Pressures caused by a fluid:
Fluid pressure
If a fluid is flowing through a horizontal pipe and a leak develops, a force must be applied over the area of the hole that causes the leak.
The fluid pressure may be defined as the ratio F/ A, where F is the minimum force that would have to be exerted on a frictionless plug in the hole to keep the fluid from emerging.
Fluid Pressure on the base of a tank

12. Pressures caused by a fluid:
Hydrostatic Pressure (HP)
The pressure at the base of a vertical column of fluid with density and height is called the hydrostatic pressure.
The mass of the fluid will exert a force on the base of the container i.e. HP.
F thus equals the force on the top surface plus weight of fluid.
P=Po + pgh
Caused by the mass of a fluid . Area not matter, so applicable to every system
Pressure at base of fluid column (HP)

13. Pressure Head
A pressure can also be expressed as a vertical height of a column of liquid
The relationship between the pressure at the base of a column of fluid of height h and the pressure at the top is 'particularly simple if these pressures are expressed as heads of the given fluid: if the column is mercury, for example, then
The height of a hypothetical column would exert the given pressure at its base if the pressure at the top were zero.
The equivalence between a pressure P (force/area) and the corresponding head Ph (height of a fluid) is given

14. Absolute and Relative Pressure Scales
Expressed as Absolute or relative.
Depends upon nature of measuring device to make measurements
Open End would measure Relative pressure
Ref is Atm P
Close End
No Pressure = Vacuum
Absolute pressure
Precise value, unchange AT
Abs 0= Perfect Vaccum
Relative 0=Atmospheric P

15. Types of Pressure Atmospheric Pressure
Is the pressure caused by the weight of the earth’s atmosphere. Often called Barometric Pressure.
Absolute Pressure is the Total Pressure. An absolute pressure of zero is a perfect vaccum. Absolute Pressure must be used in all calculations unless a pressure difference is used.
Gauge Pressure, is the Pressure Relative to atmospheric pressure.
Vaccum Pressure, is a guage pressure i.e pressure below atmospheric pressure.

16. Suffix ‘a’ and ‘g’ Psi and atm often carry
Indicate whether Pressure is absolute or guage
psig: Guage Pressure in psi
psia: Absolute pressure in Psi
atma: Absolute pressure in atm
atmg: atmospheric pressure in guage

17. Standard Atmosphere
Pressure equivalent to 760mmHg at sea level and at 0C.
Unit is atm
Pressure equivalent to standard atmosphere are

18. Figure