Pressure is defined as a force per unit area.
Pressures are exerted by gases, vapours and liquids. The instruments that we
generally use, record pressure as the difference between two pressures.
Devices indicate pressure either above or below that of the atmosphere.
Atmospheric pressure: The atmospheric air exerts a normal pressure upon all
surfaces with which it is in contact, and it is known as atmospheric pressure. The
atmospheric pressure is also known as barometric pressure.
The atmospheric pressure at sea level (above absolute zero) is called standard
Gauge pressure: It is the pressure, measured with the help of pressure measuring
instrument, in which atmospheric pressure is taken as datum.
The atmospheric pressure on the scale is marked as zero.
Absolute pressure: Any pressure measured above absolute zero of pressure is
termed as an absolute pressure.
Absolute pressure = atmospheric pressure + gauge pressure
Vacuum pressure = atmospheric pressure – absolute pressure
A simple manometer is one which consists of a glass tube whose one end is
connected to a point where pressure is to be measured and the other end
remains open to atmosphere.
Common types of simple manometers are enumerated and discussed below:
2. U-tube manometer
3. Single column manometer (micro-manometers).
A Piezometer is the simplest form of
manometer which can be used for measuring
moderate pressures of liquids.
pressure at point A :
p = w * h
p = pressure,
w = specific weight of the fluid
h = height of fluid in piezometer tube
A U-tube manometer consist of a
glass tube bent in U-shape, one
end of which is connected to a
point at which pressure is to be
measured and other end remains
open to the atmosphere.
Single column manometer is
modified form of a U-tube
For any variation in pressure,
the change in the liquid level in
the reservoir will be so small
that it may be neglected and
the pressure is indicated by the
height of liquid in the other
Elastic Pressure Elements
Elastic pressure elements or mechanical type of transducers are used for
measurement of very high pressures up to about 700 MN/𝑚2
There are three main types of pressure elements:
1. Bourdon tube
Errors in bourdon tube pressure gauge
Following three types of errors are commonly found in bourdon tube gauges:
1. Zero error.
2. Multiplication error.
3. Angularity error.
Electrical Pressure Transducers
A transducers is a device for converting a signal of one sort into other type. In
other words, a pressure gauge is a transducer for converting the distortion of a
bourdon tube into an indication through use of a calibrated dial and so on.
In case of electrical transducers for measuring pressure, various transformations
which are needed to convert pressure to some sort of usable electrical signal.
Resistance Type Pressure Transducers
Resistance type devices used in
pressure transducers include:
1. Strain gauge
2. Moving contact