Pressure is defined as force per unit area. It has various units like pascals and pounds per square inch. High pressure can result from a small contact area like spike heels. Atmospheric pressure is caused by air weight and varies with altitude. Vapor pressure depends on temperature and indicates evaporation rate. Differential pressure is the difference between two pressures. Overpressure refers to absolute pressure above atmosphere. Manometers like U-tubes measure pressure differences using fluid columns. Laminar flow has constant velocity while turbulent flow is irregular. Pressure gauges measure vessel interior pressure.

1. What is Pressure?
Pressure (P) is defined as the amount of force (F) acting per unit area (A). The mathematical
equation for pressure can be written as:
P= F/A = mg/A
where P is pressure F is the normal force (g is acceleration) and A is the area of the surface.
Although the normal force is a vector quantity, pressure is a scalar quantity (vector page).
The SI unit for pressure is the pascal (Pa), equal to one newton per square metre (N/m2 or
1kg/(m-s2).
Other units of pressure, such as pounds per square inch and bar, are also in common use. The
CGS unit of pressure is the barye (ba), equal to 1 dyn·cm2 or 0.1 Pa.
A good example of how a force on small area can result in a very high pressure is seen in
women's shoes with high spiked heels. Did you ever get stepped on by a woman wearing high
heel shoes? It would be less painful if she wore a flat shoe because the soles are larger and the
pressure is less.
As an example --an average shoe distributes the weight of the person over about 20 square
inches. Thus, a 150-pound person applies 150/20 = 7.5 pounds per square inch on the floor.
Since a spike-heel is only 0.25 square inches, the 150-pound person would be applying 150/0.25
= 600 pounds per square inch on the floor at the heel.
Fluid Pressure
All matter is made up of tiny particles called atoms. The forces that exist in fluids are caused by
the mass and velocity of these atoms making up a fluid.
The pressure exerted by a static fluid depends only upon the depth of the fluid, the density of the
fluid, and the acceleration of gravity. (see hydrostatic pressure page for more details)
Atmospheric Pressure
Atmospheric pressure is the force per unit area exerted on a surface by the weight of air above
that surface in the atmosphere. In most circumstances atmospheric pressure is closely
approximated by the hydrostatic pressure caused by the weight of air above the measurement
point. The standard atmosphere (symbol: atm) is a unit of pressure equal to 101325 Pa.
Equivalent to 760 mmHg (torr), 29.92 inHg, 14.696 psi. The atmospheric pressure at the sea
level is equal to 760 mm of height of the column of mercury.

2. Vapor Pressure
Vapor pressure (or equilibrium vapor pressure) is the pressure (at a given temperature) that is
exerted by a gas in equilibrium with either a solid or liquid that is in a closed container. The
equilibrium vapor pressure is an indication of a liquid's evaporation rate. Vapor pressures
increase with temperature. The vapor pressure is an indication of a liquid's evaporation rate.
The units for vapor pressure :There are several units used for vapor pressure: Pascals (Pa), tor
(mm Hg), atmospheres (atm) and bar (bar).
The torr (symbol: Torr) is a unit of pressure, now defined as exactly 1⁄760 of a standard
atmosphere. Thus one torr is exactly 101325⁄760 pascals (~133.3 Pa). Historically, one torr was
intended to be the same as one "millimetre of mercury".
The torr (symbol: Torr) is a non-SI unit of pressure with the ratio of 760 to 1 standard
atmosphere, chosen to be roughly equal to the fluid pressure exerted by a millimeter of mercury,
i.e. a pressure of 1 Torr is approximately equal to 1 mmHg.
1 Pa = N/m2 ---1 bar = 10^5 Pa---- 1 atm = 1.01325 bar--760 Torr = 1 atm ---1 torr
Differential pressure
The difference between two pressures,p1 and p2, is known as the pressure differential, Δp = p1 - p2. In
cases where the difference between two pressures itself represents the measured variable, one refers to the
differential pressure,p1,2.
Overpressure (gauge pressure)
The most frequently measured pressure in the technological field is the atmospheric pressure differential,
Pe (e = excedens = exceeding). It is the difference between an absolute pressure,pabs, and the relevant
(absolute) atmospheric pressure (pe = pabs - pamb) and is known, in short, as the overpressure or gauge
pressure.
A positive overpressure is referred to when the absolute pressure is greater than the atmospheric pressure.
In the opposite case,one speaks of negative overpressure.
The indices of the formula symbols “abs”, “amb” and “e” clearly define the reference point of the
respective pressure. They are only attached to the formula letter p, and not to the unit symbols.

3. A manometer is a device used for measure the pressure of a fluid by balancing it with against a
column of a liquid. Five different types of manometers are shown below with images.
U-Tube Manometer:
It consist a U – shaped bend whose one end is attached to the gauge point ‘A’ and other end is open to
the atmosphere. It can measure both positive and negative (suction) pressures. It contains liquid of
specific gravity greater than that of a liquid of which the pressure is to be measured.
where ‘γ’ is Specific weight, ‘P’ is Pressure at A.
Pressure at A isP = γ2h2 – γ1h1
Differential U-Tube Manometer:
A U-Tube manometric liquid heavier than the liquid for which the pressure difference is to be measured
and is not immiscible with it.

4. Pressure difference between A and B is given by equation
PA – PB = γ2h2 + γ3h3 – γ1h1
Inverted U-Tube Manometer:
Inverted U-Tube manometer consists of an inverted U – Tube containing a light liquid. This is used to
measure the differences of low pressures between two points where where better accuracy is required.
It generally consists of an air cock at top of manometric fluid type.

5. Pressure difference can be calculated from equation
P1 – ρ1*g*H1 – ρm*g(H2– H1) = P2 – ρ2*gH2
Micro Manometer:
Micro Manometer is is the modified form of a simple manometer whose one limb is made of larger
cross sectional area. It measures very small pressure differences with high precision.
Let ‘a’ = area of the tube,
A = area of the reservoir,
h3 = Falling liquid level reservoir,
h2 = Rise of the liquid in the tube,
By conversation of mass we get A*h3 = a*h2
Equating pressure heads at datum we get
P1 = (ρm – ρ1)*gh3 + ρm*gh2 – ρ1*gh1

6. Inclined Manometer:
Inclined manometer is used for the measurement of small pressures and is to measure more
accurately than the vertical tube type manometer. Due to inclination the distance moved by the fluid
in manometer is more.
Pressure difference between A and B is give by equation.
Incompressible and Compressible Flow
Incompressible flow refers to the fluid flow in which the fluid's density is
constant. Fora density to remain constant, the controlvolume has to remain
constant. Even though the pressure changes, the density will be constant for an
incompressible flow. Incompressible flow means flow with variation of density
due to pressure changes is negligible or infinitesimal. All the liquids at constant
temperature are incompressible.
Compressible flow means a flow that undergoes a notable variation in density
with trending pressure. Densi ty r (x, y, z) is considered as a field variable for the
flow dynamics. When the value of Mach number crosses above0.3, density begins
to vary and the amplitude of variation spikes when Mach number reaches and
exceeds unity.
The behavior of control volume (CV) for incompressible and compressible flow is
depicted in the image below.

7. It can be seen that the CV remains constant for a flow that is incompressible and
CV is squeezed for compressible flow.
Bernoulli's equation is applicable only when flow is assumed to be incompressible.
In case of compressible flow, Bernoulli's equation becomes invalid since the very
basic assumption for Bernoulli's equation is density r is constant
For compressible flow, .
Laminar Flow:
the flow of a fluid when each particle of the fluid follows a smooth path, paths which never
interfere with one another. One result of laminar flow is that the velocity of the fluid is constant
at any point in the fluid. Turbulent Flow: irregular flow that is characterized by tiny whirlpool
regions.
A pressure gauge is a mechanical instrument designed to measure the internal
pressure and/or vacuum of a vessel or system. Trerice Pressure Gauges are offered

8. in a variety of styles, sizes, and wetted part materials to meet the demands of
standard and special applications.