PRESSURE MEASUREMENT CLASS NOTES

 

PRESSURE MEASUREMENT

CLASS NOTES

The measurement of pressure is one of the most

important measurements in almost all industries.

Applications needing measurement of pressure are:

1. Measurement of steam pressure in a boiler for

safe operation of the boiler.

2. Pressure measurement is necessary in

manufacturing and chemical industries.

3. Pressure measurement determines the liquid

level in tanks.

4. Pressure measurement determines the density

of a liquid.

5.  Pressure measurement indications the flow

direction.

6.  There is a requirement of Pressure measurement

in daily life. It is of maintaining optimal pressure in

vehicle tires.

7. Manometer is an instrument which

measures the pressure of a liquid

or of a gas. There are

many types of manometers. Each

has its particular application.

 Pressure definition

Pressure (P) is amount of force (F) acting per unit area (A). Mathematically
equation for pressure is:
P= F/A = mg/A
Where P is pressure
F is the normal force (g is acceleration)
A is the area of the surface.
Although a force is vector, pressure is scalar. The SI unit for
pressure is the Pascal (Pa), equal to one newton per square meter (N/m2).

 Different types of pressure

Atmospheric pressure

Atmospheric pressure is the pressure atmospheric air.

Absolute pressure

The pressure measured from a state of prefect vacuum is an absolute pressure.
Firstly Absolute Pressure = 0 absolute when perfect vacuum
Secondly absolute Pressure =1 at gage pressure zero
absolute Pressure = gage pressure + 1 atmospheric pressure

FOR EXAMPLE
Gage pressure = 3 bars
Absolute pressure = 3 + 1 =4 bars
Gage pressure applies if water is converts into vapor above 1000C.

Gauge Pressure

When a pressure (P) is greater than the atmospheric pressure (Pa). The pressure measured is the gauge pressure.
Pressure above atmospheric pressure is Positive Gage pressure.
Absolute pressure= Gage pressure + atmospheric pressure
 Static Pressure
Due to the height of the fluid column in the container
P = ρ g h
Dynamic pressure
Due to the velocity of the fluid
P =ρ V 2/2
Datum pressure
It is reference where pressure is zero.  Normally it is zero at the ground level where Z is 0.
Pressure at any height z
p = ρgZ
Therefore p = 0 at height Z=0

PRESSURE OF A FLUID

  NORMAL force of fluid per unit area.
Its units are Pascal and bars.
Pascal = 1 N/m 2
bar = 740 mm of Hg = 10 m of water
1 atmospheric pressure  = 101.3 k Pa=760 mm of Hg = 10.3 m of water=14.7 psi = 1 ata

Vacuum pressure

Negative pressure below atmospheric pressure is vacuum.
How much below atmospheric pressure is vacuum.
Vacuum pressure = atmospheric pressure – Actual  gage pressure
Atmospheric pressure = 760 mm Hg
Say Gage pressure is 700 mm Hg
∴ Vacuum = 760 –700=40 mm Hg
PRACTICAL APPLICATIONS OF VACUUM
Water converts into vapor below 100C under vacuum..
Water vapor in air is under vacuum pressure. Pressure of water vapor is much below atmospheric pressure. Total pressure of air and water vapors in atmosphere is atmospheric pressure. Evaporation takes place under vacuum at temperature lower than normal boiling point.
MEASUREMENT OF PRESSURE BY MECHANICAL AND ELECTRICAL TRANSDUCERS
Measure moderate pressures by simple pressure gauges like Piezometer and U-Tube manometers. Measure very low or very high pressures using advanced gauges.

Two Types Of Pressure Measuring Devices

1.Mechanical Devices ( Pressure Transducers)

(i) Manometer

A column of liquid balances same or different column of liquid.
(ii) Bourdon tube

A column of liquid is balances  a spring or dead weight

(iii) Spiral and Helix Bourdon Tubes
(iv) Spring and bellows
(v) Diaphragm
(vi) Single and double inverted bell

2. Electrical Devices (Pressure Transducers)

(i) Strain gauge
(ii) Vibrating wire
(iii) Piezoelectric
(iv) Capacitance
(v) Linear variable differential transformer
(vi) Optical

MANOMETERS

Fig. Manometers for Gauge & Vacuum pressures

Fig. U-Tube Differential Manometer

Types of Manometers

(i) Simple manometer
(ii) Differential manometer

Simple Manometers

(i) Piezometer
(ii) U-tube manometer
(iii) Single column manometer (Micro-manometer)

Piezometer

Piezometer tube is perhaps the simplest among the various pressure measuring devices and consists of a vertical tube. This  measures moderate pressures of a liquid in a pipe or in a vessel.

Advantages of a Piezometer

1. Simple in construction
2. Cheap
3. No moving parts
4. No wear and tear
5. No maintenance.

Disadvantages of a Piezometer

1. A Piezometer can measure pressure above atmospheric only. It cannot measure vacuum.

2. Piezometers cannot measure very high pressures.

3. Piezometers cannot measure the pressure of a gas. A gas does not form a visible free surface.

U-tube Manometers

 U-shaped glass tube partially filled with liquid. It contains water or mercury in a U-shaped tube. Connect one end of the U tube  to the unknown pressure. Connect the other end to a reference pressure usually atmospheric pressure. It usually measures the pressure of a gas.

Advantages of U-Tube Manometers

(a) Simple in construction
(b) Low cost
(c ) Very accurate
(d) Very sensitive
(e) No moving parts
(f) No wear and tear
(g) No maintenance
(h) No calibration require.

Disadvantages of U-tube Manometer

1. Fragile
2. Highly sensitive to temperature change
3. Error in the measurement of difference of height ‘h’

Characteristics of liquid used in U-tube Manometer

1. Low viscosity
2. Low surface tension
3. The boiling point of the liquid be high. There is no vaporization.
4. The liquid should stick to the walls of the tube

Single Column Manometer (Micro-manometer)

U-tube manometers require reading of two liquid levels.  Overcome this difficulty in a single column manometer. Connect one end of single column to a shallow vessel. Manometer cross section is one hundredth of a vessel. Any expansion/contraction does not affect the level in the vessel and only the height in the manometer gives the pressure.

Inverted U-tube manometer

Inverted U-tube manometer measures pressure differences in liquids. Fill the space above the liquid with air in the manometer. Bring in or bring out air with  a tap provided at the top. It adjusts the liquid level in the manometer.

Differential manometers

Attach Differential manometer to a pipe through which the fluid is flowing. A differential manometer measures the pressure difference between two points along the pipe.

 DIFFERENTIAL MANOMETERS

(a) Tube type
(b) Ring type
(c) Float type
(d) Bell type