pressure sensing system is based on the idea that bending of
flexible material is directly relative to the pressure being
considered. There are mainly three sensor types that are used in
this system of sensing pressure: Bourdon-tubes, diaphragms and
fluid pressure sensors are of the flexible type, where the fluid is
enclosed in a small compartment with at least one flexible wall. The
pressure reading is thereby calculated by sensing the movement of
this flexible wall, resulting in either a direct readout through
suitable connections, or a transducer electrical signal. Flexible
pressure sensors are sensitive; they are commonly delicate and
liable to throb, however. In addition, they tend to be much more
costly than manometers, and are therefore preferentially used for
transmitting detected data and sensing pressure differences. A broad
diversity of flexible elements could possibly be used for flexible
pressure sensors; the majority of devices use some form of a
pressure transducers as:
Bellows elements are
cylindrical in form and contain many folds. They deform in the axial
direction (compression or expansion) with changes in pressure. The
pressure that needs to be detected is applied to one side of the
bellows (either inside or outside) while atmospheric pressure is on
the contrary side. Absolute pressure can be detected by evacuating
either the exterior or interior space of the bellows and then
sensing the pressure at the contrary side. Bellows can only be
associated to an on/off switch or potentiometer and are used at low
pressures, <0.2 MPa with a sensitivity
of 0.0012 MPa. The need for a pressure
sensing element that was extremely sensitive to low pressures and
provided power for activating recording and indicating mechanisms
resulted in the development of the metallic bellows pressure sensing
element. The metallic bellows is most accurate when sensing
pressures from 0.5 to 75 psig. However, when used in conjunction
with a heavy range spring, some bellows can be used to measure
pressures of over 1000 psig. Figure 1 shows a basic metallic bellows
pressure sensing element.
The bellows is a
one-piece, collapsible, seamless metallic unit that has deep folds
formed from very thin-walled tubing. The diameter of the bellows
ranges from 0.5 to 12 in. and may have as many as 24 folds. System
pressure is applied to the internal volume of the bellows. As the
inlet pressure to the instrument varies, the bellows will expand or
contract. The moving end of the bellows is associated to a
mechanical linkage assembly. As the bellows and linkage assembly
moves, either an electrical signal is generated or a direct pressure
indication is provided. The flexibility of a metallic bellows is
similar in character to that of a helical, coiled compression
spring. Up to the flexible limit of the bellows, the relation
between increments of load and movement is linear. However, this
relationship exists only when the bellows is under compression. It
is necessary to construct the bellows such that all of the travel
occurs on the compression side of the point of equilibrium.
Therefore, in practice, the bellows must always be opposed by a
spring, and the movement characteristics will be the resulting force
of the spring and bellows.
1.2 Bourdon Tube Gauges2:
principle behind all Bourdon tubes is that an increase in pressure
on the inside of the tube in comparison to the outside pressure
causes the oval or flat formed cross-section of the tube to try to
achieve a circular form. This phenomenon causes the tube to either
straighten itself out in the c-type or spiral cases or to unwind
itself for the twisted and helical varieties. This change can then
be detected with an analog or digital meter associated to the tube.
Tube materials can be changed accordingly to suit the required
process conditions. Bourdon tubes can operate under a pressure range
from 0.1-700 MPa. They are also portable
and require little maintenance; however, they can only be used for
static measurements and have low accuracy.
Bourdon tubes include C-type, spiral (a more coiled C-type tube),
helical and straight tube Bourdon tubes. C-type gauges can be used
in pressures approaching 700MPa; they do have a minimum recommended
pressure range, though -- 30 kPa (i.e.,
it is not sensitive enough for pressure differences less than 30
bourdon tube pressure instrument is one of the oldest pressure
sensing instruments in use today. The bourdon tube (refer to Figure
3) consists of a thin-walled tube that is flattened diametrically on
contrary sides to produce a cross-sectional area elliptical in form,
having two long flat sides and two short round sides. The tube is
bent lengthwise into an arc of a circle of 270 to 300 degrees.
Pressure applied to the inside of the tube causes distention of the
flat sections and tends to restore its original round cross-section.
This change in cross-section causes the tube to straighten slightly.
Since the tube is permanently fastened at one end, the tip of the
tube traces a curve that is the result of the change in angular
position with respect to the center. Within limits, the movement of
the tip of the tube can then be used to position a pointer or to
develop an equivalent electrical signal (which is discussed later in
the text) to indicate the value of the applied internal pressure.
Diaphragm elements are made of circular metal discs or flexible
elements such as rubber, plastic or leather. The material from which
the diaphragm is made depends on whether it takes advantage of the
flexible nature of the material, or is opposed by another element
(such as a spring). Diaphragms made of metal discs utilize flexible
characteristics, while those made of flexible elements are opposed
by another flexible element. These diaphragm sensors are very
sensitive to rapid pressure changes. The metal type can measure a
maximum pressure of approximately 7 MPa;
while the flexible type is used for sensing extremely low pressures
(.1 kPa - 2.2 MPa)
when associated to capacitative
transducers or differential pressure sensors. Examples of diaphragms
include flat, corrugated and capsule diaphragms. As previously
noted, diaphragms are very sensitive (0.01 MPa)
. They can measure fractional pressure differences over a
very minute range (say, inches of water) (flexible type) or large
pressure differences (approaching a maximum range of 207
kPa) (metal type).
Diaphragm elements are very versatile -- they are commonly used in
very corrosive environments or with extreme over-pressure
Pressure Sensors Summary2
In a bellows-type
pressure is applied to the internal volume of a bellows and
mechanical linkage assembly.
pressure changes, the bellows and linkage assembly move to cause an
electrical signal to be produced or to cause a gauge pointer to
In a bourdon
pressure is applied to the inside of a slightly flattened arc formed
tube. As pressure increases, the tube tends to restore to its
original round cross-section. This change in cross-section causes
the tube to straighten.
the tube is permanently fastened at one end, the tip of the tube
traces a curve that is the result of the change in angular position
with respect to the center. The tip movement can then be used to
position a pointer or to develop an electrical signal.
are used to measure gauge pressures over very low ranges.
may be made up of Metallic diaphragms gauge (brass or bronze) Slack
diaphragms gauge (Rubber).
perform the following basic functions:
If a pressure sensor
spare sensor element may be used (if installed).
local mechanical pressure gauge can be used (if available).
precision pressure gauge may be installed in the system.
Merits & Demerits3
Flexible Pressure transducer
Flexible Pressure transducer cost is low.
Flexible Pressure transducer has easy structure
Flexible Pressure transducers have been time tested in application.
Flexible Pressure transducer allows high accuracy, spatially in
relation to cost.
Flexible Pressure transducer
Flexible Pressure transducer has a low spring pitch.
Flexible Pressure transducer is inclined to shock and throb.
Flexible Pressure transducer is inclined to hysteresis.
retrieved at 11/10/2010
Kirk, Franklin W. and Rimboi, Nicholas
R., Instrumentation, Third Edition, American Technical
Publishers, ISBN 0-8269-3422-6.
Wightman, E.J., Instrumentation in Process Control, CRC Press,
Rhodes, T.J. and Carroll, G.C., Industrial Instruments for
Measurement and Control, Second Edition, McGraw-Hill Book Company.
Process Measurement Fundamentals, Volume I, General Physics
Corporation, ISBN 0-87683-001-7, 1981
retrieved at 11/10/2010