PRESSURE is force exerted per unit area
P = F/A
P = pressure
F = force
A = surface area exposed to the force
Pound / Square Inch = P / SI = PSI
Newton / Square Meter = N / m2
100 000 N / m2 = 1 Bar
14.5 psi = 1 Bar
Inches of mercury “inch Hg”, “mm Hg”
TYPES OF PRESSURE
The pressure at
point X can be
o 10” of mercury
o 20” of mercury
o -20 i h of
20 inches f
PRESSURE SENSING ELEMENTS
A basic instrument system consists of three elements:
1. SENSOR or INPUT DEVICE
2 SIGNAL PROCESSOR
3. RECEIVER or OUTPUT DEVICE
The block diagram of a sensor is shown below.
Pressure sensors either convert the pressure into
mechanical movement or into an electrical output.
h i l
l t i l
Complete gauges not only sense the pressure but indicate
them on a dial or scale.
1) Bourdon Tube
Bourdon tube is a flattened metal tube has an oval cross-section with one end of the
tube connected to the process pressure. The other end is sealed and connected to the
pointer or transmitter mechanism. When a pressure difference exists between the inside
and outside, the tube tends to straighten out and the end moves.
o Range as low as 0 - 15 psi up to 0-1500
o They are simple and accurate, but they are
bulky and get damaged with over-ranging.
o Range as low as 0 - 200 psi up to 0 - 6000
o Heavy-duty helical bourdons can
sometimes tolerate as high as ten times
the maximum range pressure.
o Range as low as 0 -10 psi up to 0 100 000
2) Bellows sensor
A bellows sensor is an axially
flexible cylindrical enclosure
with folded sides. When
pressure is applied through an
opening, the closed end
A bellows sensor can
accurately measure much
lower pressures than a
Bellows elements can measure
absolute pressure, gauge
pressure, vacuum, or
3. Diaphragm sensor
o Diaphragm is usually metallic and comes in two different
configurations; single and capsular.
o The single diaphragm is either flat or with concentric corrugations.
o Th capsular diaphragm consists of two diaphragms welded together at
l di h
i t ft
ld d t
pressure reference and single
o Evacuated capsules are used for absolute p
diaphragms for very sensitive measurements.
ELECTRONIC PRESSURE SENSORS
This table shows the ranges of pressures and vacuums that various
sensor types are capable of detecting
4. Strain-Gauge Pressure Sensor
o Strain-gauge pressure sensors are
used in most types of electronic
pressure transmitters. When metallic
conductors or semiconductors are
subjected to mechanical strain,
there is a change in resistance This
resistance is then electrically
converted into a 4-20 mA signal
proportional to the pressure.
o There are many different designs of
strain-gauge pressure sensors. The
most common designs use a metallic
diaphragm to i l t th process
t isolate the
fluid and exert a force on a force
o Most of the strain elements in
current use are semiconductor type.
5. Capacitive Pressure Sensor
o Capacitive pressure sensors are also used in electronic pressure
transmitters. With these devices the change in capacitance
resulting from the movement of an elastic element is
proportional to the pressure applied to the elastic element.
o The elastic element is usually a stainless steel diaphragm. As shown, a
high-frequency oscillator is controlled by the sensing element.
Changes in pressure deflect the diaphragm and the resultant change in
capacitance changes the oscillator frequency.
o The variation in oscillator frequency is converted to a 4-20 mA signal,
proportional to the pressure
C t i applications will b so severe and th
pressure sensor will not remain functional for
reasonable amount of time.
The following protection devices can be used to
protect the pressure sensor.
1) Diaphragm Seals
o Di h
Diaphragm seals are used to
isolate the pressure sensor from
the process fluid.
o This is done when the fluid is
toxic, corrosive, dirty, solidifies at
ambient temperature, or is
extremely cold and may freeze
o The diaphragm seal is a thin,
flexible disk, which separates the
pressure sensor from the process
• Siphons are generally used to isolate a hotp
process media from the p
• The siphon is a metal, tubular device shaped
in a l
loop. It can either b f ll d with a h h
h be filled
h highboiling-point liquid or process condensate
which acts as a barrier to the heat contained
in the hot gases or steam.
• The path the hot vapor takes to the pressure
sensor is relatively long and narrow with a
lot of surface area for cooling siphons.
3) Throttling Devices
o Throttling devices are commonly used to dampen high-frequency
pressure fluctuations by putting a restriction in the inlet to the
a) Throttling Screws
o Throttling screws are the simplest
means of providing a restriction.
o Throttling screws have several
They are inserted into a tapped
t di t
hole in the base (socket) of the
pressure sensor to provide a flow
b) Pressure Snubbers
o Pressure Snubbers are very common for
reducing pressure fluctuations and filtering
the media. Snubbers have a porous
element, which restricts the velocity and
filters the fluid.
o The pulsation dampener is also sometimes
called a pressure Snubbers, but does not
have a filtering element.
o The most common design has a bar-stock
fitting. As the pressure pulse comes
through the dampener, the piston is forced
up and restricts the flow from the large
chamber by closing the outlet of th
b b l i
tl t f the
c) Pressure-Limiting Valves
o Pressure-limiting valves protect the pressure
sensor from overpressure by blocking the
process fluid at a preset limit.
o There are several designs of pressurepressure
limiting valves. One has the fluid coming in
the inlet, passing through a filter, and out to
the pressure sensor.
PRESSURE MEASUREMENT DEVICES
1 MECHANICAL DEVICES
In mechanical pressure measurement devices the measuring
elements (sensors) can never separate it from the rest of the
system (signal p
processor and receiver/output device) as in many
cases they are all integral (e.g. a mechanical pressure gauge
incorporates all of these elements).
o Tubes for gauges to be
used on Acetylene must
b made of steel.
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o Associated Fittings for use
on oxygen must be kept
entirely free of oil.
o Gauges used on
Hydrogen plants must be
oA gauge with a zero error will always read high or low by a
oA gauge with a span error has an internal magnification error
where the error increases as long as the measured pressure
oA gauge with a linearity error may read correctly at 0 and
100% but will not follow a linear path between these points.
a) Pneumatic transmitter
Supply: 20 Psi OR 1.4 Bar
Output: 3 to 15 Psi OR 0.2 to 1.0 Bar
b) Electronic transmitter
Supply: 24 V DC
Output: 4 to 20 mA
1. Pressure is applied to the sensor.
2. A change in pressure is measured by a change in the sensor
3. The sensor signal is conditioned for various parameters.
4. The conditioned signal is converted to an appropriate analogue
output (i.e. 4 – 20 mA)
c) Smart Transmitter
HHC (Hand Held
o Smart Transmitter can be programmed with a HHC (Hand
o They come with addressable and communication facility.
o Transmitter can be re-ranged and zero checked without
disconnecting from the line.
o An electric pressure switch
senses pressure and opens or
closes an electrical switch
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element at a set pressure to
signal another electrical device.
o Most pressure switches trip at a
pressure above atmospheric, and
are called gauge p
g g pressure or
simply pressure switches.
o Switches can also be
manufactured to trip at pressure
referenced to a complete
vacuum and this is called
absolute pressure switches
o Pressure switches are set to trip
at a certain point with rising or
o The difference in the trip point
and the set point is called dead
band or reset or switch
Electrical Switch Contacts
The electrical switch is
o Pneumatic regulators are used
with every pneumatic
o Regulator maintains a steady
outlet pressure irrespective of
variation in the upstream
o Regulator comes with a filter to
remove wet the dust.
o A drain valve remove any
condensation of air.
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