MODIBBO ADAMA UNIVERSITY YOLA
FACULTY OF ENGINEERING
DEPARTMENT OF CHEMICAL ENGINEERING
COURSE: PROCESS INSTRUMENTION (CHE 303)
PRESENTATION ON THE TOPIC;
PRESSURE MEASUREMENT
GROUP ONE
CHE/20U/1157 CHE/20U/1160 CHE/20U/1165 CHE/20U/1168
CHE/20U/1171 CHE/20U/1179 CHE/20U/1181 CHE/20U/1185
CHE/20U/1191 CHE/20U/1199 CHE/21D/1119 CHE/19U/1278
CHE/18U/1304
TABLE OF CONTENTS
➢ Introduction
➢ Pressure Measurement
➢ Instruments Used For Measuring Pressure
➢ Conclusion
➢ References
INTRODUCTION:
Pressure is defined as the force per unit area that can be exerted by a liquid, gas or
vapor on a given surface. The applied pressure can be measured as absolute, gauge or
differential pressure.
In physical sciences the symbol for pressure is “P” and the SI unit for
measuring pressure is Pascal (symbol: Pa). One Pascal is the force of one Newton per
square meter acting perpendicular on a surface. Other units commonly used to
represent pressure are; bar, torr, atm, psi (pounds per square inch). Use of pressure
units has regional application preference: psi is commonly used in the United States,
while bar the preferred unit of measure in Europe. In the industrial vacuum
community, the preferred pressure unit is torr in the United States, mbar in Europe and
pascal in Asia.
A major portion of all industrial measurements relates in some way to pressure in its
various forms.
Pressure can be measured directly by measurement of the applied force or
indirectly, e.g. by the measurement of the gas properties. Examples of indirect
measurement techniques that are using gas properties are thermal conductivity or
ionization of gas molecules. Before mechanical manometers and electronic diaphragm
pressure sensors were invented, pressure was measured by liquid manometers with
mercury or water.
In this presentation, different pressure measurement and instruments used for
measuring are explained extensively.
PRESSURE MEASUREMENT:
Pressure can be measured as Absolute Pressure, Gauge Pressure or Differential
Pressure.
Absolute Pressure:
Absolute pressure is the measure of pressure with respect to absolute
zero pressure, which is the pressure of a perfect vacuum; it is pressure relative to zero
pressure. In simpler terms, it can be defined as pressure measured above absolute zero
pressure. Absolute pressure is measured using a Barometer. Mathematically, absolute
pressure is the sum of Gauge pressure and Atmospheric Pressure.
Pabsolute = Pguage + Patmospheric
Gauge Pressure:
Gauge pressure is the pressure relative to atmospheric pressure. Gauge
pressure is positive for pressures above atmospheric pressure and negative for
pressures below atmospheric pressure. The difference between gauge pressure and
absolute pressure is clear, absolute pressure measures pressure with respect to absolute
zero pressure while gauge pressure measures pressure with respect to atmospheric
pressure.
Mathematically;
Pguage = Pabsolute - Patmospheric
The diagram below explains absolute pressure and gauge pressure.
Increasing Pressure
Gauge Pressure
Absolute Pressure
Atmospheric Pressure
Absolute Zero Pressure
Fig. 1 (absolute and gauge pressure)
Differential Pressure:
Differential pressure is the pressure difference between two pressure
media’s. Differential pressure can be measured using Pressure Transducers with a
pressure sensor diaphragm where one of the sensor diaphragms is exposed to one
pressure media and the other side of the diaphragm is exposed to the second pressure
media. The differential pressure value is the pressure difference between the two
pressure media’s
INSTRUMENTS FOR MEASURING PRESSURE:
Pressure in fluids is measured by the following devices
• Manometers;
Manometers are devices used to measure the pressure at a point in fluid by balancing
the column of fluid by thesame or another column of liquid. Manometers are classified
into;
1. Simple Manometers;
A simple manometer is one which consists of a glass tube with one end connected to
the pipe where pressure is to be measured and the other end is open to the atmosphere.
Common types of simple Manometers are listed below;
(i) Piezometer (ii) U-tube Manometer (iii) Single Column Manometer
2. Differential Manometers;
Differential Manometers are used to measure the difference in pressure between two
points in a pipe, or two different points. Most commonly used type of Manometers
are;
(i) U- tube differential Manometer (ii) Inverted U-tube differential
manometer.
• Mechanical Guages
These devices are used for measuring high pressure
i. Bordon tube pressure Guage
ii. Diaphragm Pressure Guage
iii. Bellow Pressure guage
iv. Dead weight Pressure Guage
Simple Manometers
1. Piezometer
Piezometer is the simplest form of manometer which can be used to measure moderate
pressure of liquids. It consist of a glass tube inserted in the wall of vessel or pipe,
containing liquid whose pressure is to measured. The tube extends vertically upward
to such a height that liquid can freely rise in without overflowing. The pressure at any
point in the liquid is indicated by the hieght of liquid in the tube above that point,
which can be read on the scale attached to it.
Fig.2 (piezometer tube fitted to an open vessel)
2. U-tube Manometer
A U-tube manometer consists of a glass tube bent in U shape, one end of which is
connected to a point where pressure is to be measured and the other end remains open
to the atmosphere as shown in the figure below.
Fig.3(U-tube Manometer)
Differential Manometer:
1. U-tube differential Manometer:
The U-tube differential manometer has its two ends connected to two different points
A and B containig same liquid which might be at same level or not.
Fig. 4 (U-tube differential manometer with points A and Bat same level)
Where; h = difference of mercury levels(heavy liquid)in the U-tube.
h1 = distance of the centre of A from the mercury level in the right limb
s = specfic gravity of mercury
s1 = specific gravity of liquid A
s2 =specific gravity of liquid B
the line X-----X represents the datum line
The pressure in the left and right limb above the datum line are equal.
2. Inverted Differential Manometer:
This manometer is used for measuring difference of two pressure where accuracy is
the major consideration. It consists of an inverted U-tube, with a light liquid whose
two ends are connected to the points A and B whose pressure difference is to be
measured. The inverted U-tube Manometer is shown below.
Fig. 5 (Inverted Differential Manometer)
MECHANICAL GUAGES
Manometers (as have been discussed earlier) are suitable for comparatively low
pressure. For measuring medium and high pressures,we make use of mechanical
guages. We will look at three types of mechanical guages.
1. Bourdon Tube Pressure Guage:
Bourdon tube pressure guage is used to measure both high and low temperatures. The
pressure elemen concists of metal tube of approximately elliptical cross section. This
tube is bent in the form of a segment of a circle and responds to pressure changes.
When one end of the tube wich is connected to the guage case, is connected to the
source of pressure, the internal pressure causes the tube to expand. The free end of the
tube moves and is in turn connected ny suitable levers to the rack, which engages with
a small pinion mounted on the same spindle as the pointer. Thus the pressure applied
to the tube causes the rack and spindle to move. The pressure is indicated by a pointer
on dial which can be graduated to suitable scale. The bourdon tube guage are genrally
made of bronze and nickel steel. The former is generally used for lowe and the latter
for high pressure.
A diagram of the bourdon guage is shown below:
Fig.6 (Bourdon tube Pressure guage).
2. Diaphragm Guage:
This type of guage employs a metalic instead of a bent tube. This disc or diaphragm is
used for actuating and indicating the device. When pressure is applied o the lower side
of the diaphragm, it is deflected upwards. The movement is the transmitted to a rack
and pinion. The latter is attached to the spindle of the needle movig on a graduated
dial.
Fig. 7 (Diaphragm Guage)
Conlusion
Pressure and vacuum measurement is widely used in industrial and scientific
applications. It can be measured with many different types of sensors, techniques and
measurement technologies. The selection of measurement technique and sensor type depends
on the application and requirements.
Refrences
A textbook of fluid mechanics and hyraulics by R.K Rajpuy
Coulson and Richardson volume 1
