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How do you work?
• Pressure is defined as force exerted on a unit
• Mathematically, we have: P = F/A
where P=Pressure(Pa), F=Force(N) and A=Area(m2)
• Basic unit of pressure in SI units is Pascal (Pa).
– Pascal is defined as force of 1 Newton (N) per square
meter (m2). That is: 1 Pa = 1 N/m2
• Since the Pascal (Pa) is a very small unit (1
Pa = 1.45 x 10-4 PSI)
– it is more common to use units of kPa and MPa,
when we deal with the pressures in typical industrial
Absolute, Gauge and Differential
Pressures • Pressure measurements can be stated as either
– absolute or differential.
• Gauge pressure is the unit we encounter in everyday work (e.g. tire ratings are in gauge pressure).
• A gauge pressure device will indicate zero pressure when bled down to atmospheric pressure (i.e. gauge pressure is in reference to atmospheric pressure – that pressure above atmospheric pressure).
• Gauge pressure is denoted by a (g) at the end of the pressure unit (e.g. kPa(g)).
• Absolute pressure includes the effect of atmospheric pressure with the gauge pressure. It is denoted by an (a) at the end of the pressure unit (e.g. kPa(a).)
An absolute pressure indicator would indicate
atmospheric pressure when completely bled down -
it would not indicate scale zero.
The relationship between absolute pressure and
gauge pressure is:
Absolute Pressure = Gauge Pressure + Atmospheric
• The standard value of atmospheric pressure is
taken as the atmospheric pressure at sea level,
which is 101.3 kPa.
• Note a reading of less than 101.3 kPa(a) indicates
a vacuum condition.
– For example, a typical condenser pressure is 5 kPa(a) or a
vacuum of 96.3 kPa.
• Differential Pressure is the difference of two pressures.
• In order to produce a standard (4 - 20 mA) electronic
signal which represents the pressure in a process,
the pressure must be sensed and a physically
detectable motion or force in proportion to this
applied pressure must be developed.
• To sense the process, we use a pressure sensor.
Pressure Measurement Devices
• Some common sensors include:
– Bourdon Tubes
When a force acts
against a thin
it causes a deflection
of the diaphragm with
its centre deflecting
• If it is fixed at the
air inlet, it can
expand like a
• It expands in both
ways whereas in
expands in one
DIFFERENTIAL PRESSURE DEVICES
Differential Pressure Devices • Used to measure Differential Pressure (that
is, the difference between a high pressure input and a low pressure input) and hence called DP transmitters or DP cells.
• A differential pressure capsule is mounted inside a housing.
• One end of a force bar is connected to the capsule assembly so that the motion of the capsule can be transmitted out of the housing.
• A sealing mechanism is used where the force bar penetrates the housing.
• This seal also acts as the pivot point for the force bar.
A Typical DP Cell
• Provision is made in the housing for high
pressure fluid to be applied on one side of the
capsule and low pressure fluid on the other.
• Any difference in pressure will cause the capsule
to deflect and create motion for the force bar.
• The top end of the force bar is connected to an
electronic motion detector, which via an
electronic system, will produce a 4 - 20 mA
signal that is proportional to the force bar
DP Capsule Construction • The DP capsule is formed by welding two metallic
(usually stainless steel) diaphragms together.
• To provide over-pressurization protection, a solid plate
with diaphragm matching convolutions is mounted in the
center of the capsule.
• Silicone oil is used to fill the cavity between the
diaphragm for even pressure transmission. Most DP
capsules can withstand static pressure of up to 14 Mpa
(2000 psi) on both sides of the capsule without any
• However, the sensitive range for most DP capsules is quite
low. Typically they are sensitive up to only a few hundred
kPa of differential pressure.
DP Transmitter DP Transmitter • A DP transmitter is used to measure the gas
pressure (in gauge scale) inside a vessel.
• In this case, the low pressure side of the
transmitter is vented to atmosphere, and the
high pressure side is connected to the
vessel through an isolating valve.
• The isolating valve facilitates the removal of the
• The output of the DP transmitter is
proportional to the gauge pressure of the gas
in the tank, i.e., 4 mA when pressure is 20 kPa
and 20 mA when pressure is 30 kPa.
• The strain gauge is a device that can be affixed to
the surface of an object to detect the force applied
to the object.
• One form of the strain gauge is a metal wire of
very small diameter that is attached to the surface
of a device being monitored.
• For a metal, the electrical resistance will increase
as the length of the metal increases or as the
cross sectional diameter decreases.
• When force is applied as indicated in Figure, the
overall length of the wire tends to increase while
the cross-sectional area decreases.
• The amount of increase in resistance is
proportional to the force that produced the change
in length and area. The output of the strain gauge
is a change in resistance that can be measured
by the input circuit of an amplifier.
• Strain gauges can be bonded to the surface of a
pressure capsule or to a force bar positioned by
the measuring element.
Strain Gauge DP Cell Shown in figure is a
strain gauge that is
bonded to a force
beam inside the DP
The change in the
will cause a resistive
change in the strain
gauges, which is
then used to
produce a 4-20 mA
Capacitive pressure sensor
– A measuring diaphragm and a fixed plate electrode
consist a capacitor
– The measuring diaphragm moves relative to fixed
– Changes in capacitance are detected by an oscillator
or bridge circuit
– For a plate capacitor we can write:
transducer • Measuring diaphragm
moves by pressure difference
• Two fixed electrodes are mounted on the glass insulation
• Measuring diaphragm Glass insulation Protective diaphragm
• Measured pressure acts through a protective diaphragm and silicone oil on the measuring diaphragm.
Impacts on Operating Environment
• Vapor Content
• The effect of vibration is obvious in the
inconsistency of measurements, but the more
dangerous result is the stress on the sensitive
membranes, diaphragms and linkages that can
cause the sensor to fail.
• Vibration can come from many sources.
– most common are the low level constant vibration of
an unbalanced pump impeller and the larger effects
of steam hammer.
– External vibration (loose support brackets and
insecure mounting) can have the same effect.
• The temperature effects on pressure sensing will
occur in two main areas:
– The volumetric expansion of vapor is of course
– The second effect of temperature is not so apparent.
An operating temperature outside the rating of the
sensor will create significant error in the readings.
• The bourdon tube will indicate a higher reading
when exposed to higher temperatures and lower
readings when abnormally cold due to the
strength and elasticity of the metal tube.
Vapor Content • The content of the gas or fluid is usually controlled and
• Since the purity of the substance whose pressure is being monitored is of importance - whether gaseous or fluid especially, if the device is used as a differential pressure device in measuring flow of a gas or fluid.
• Higher than normal density can force a higher dynamic reading depending on where the sensors are located and how they are used.
• Also, the vapor density or ambient air density can affect the static pressure sensor readings and DP cell readings.
• Usually, lower readings are a result of the lower available pressure of the substance.
• However, a DP sensor located in a hot and very humid room will tend to read high.
Failures and Abnormalities
• Blocked sensing lines
• Leaking sensing lines
• Loss of electrical power
Over Pressure • All of the pressure sensors are designed to
operate over a rated pressure range.
• Plant operating systems rely on these pressure
sensors to maintain high accuracy over that given
• Instrument readings and control functions derived
from these devices could place plant operations in
jeopardy if the equipment is subjected to over
pressure (over range) and subsequently
• If a pressure sensor is over ranged, pressure is
applied to the point where it can no longer return
to its original shape, thus the indication would
return to some value greater than the original.
• They are also however, the most prone to
fracture on over-pressuring. Even a small
fracture will cause them to read low and be less
responsive to pressure changes.
• Also, the linkages and internal movements of the
sensors often become distorted and can leave a
permanent offset in the measurement.
• Bourdon tubes are very robust and can handle
extremely high pressures although, when
exposed to over-pressure, they become slightly
distended and will read high.
• Very high over-pressuring will of course rupture
• Diaphragms and bellows are usually the most
sensitive and fast-acting of all pressure sensors.
Faulty Sensing Lines
• Faulty sensing lines create inaccurate readings
and totally misrepresent the actual pressure.
• When the pressure lines become partially
blocked, the dynamic response of the sensor is
naturally reduced and it will have a slow
response to change in pressure.
• Depending on the severity of the blockage, the
sensor could even retain an incorrect zero or
low reading, long after the change in vessel
• A cracked or punctured sensing line has the
characteristic of consistently low readings.
Sometimes, there can be detectable down-
swings of pressure followed by slow increases.
Loss of Loop Electrical Power
• As with any instrument that relies on AC power,
the output of the D/P transmitters will drop to
zero or become irrational with a loss of power
Assignments (Last Date: April 23)
1. Find Pressure Transducers to be used in Nuclear Industry? Find one to be used with their photos and information. (Check data sheet)
2. Each of you should give a write up on (include figures and animations for higher marks) 1. Bourdon Tubes
4. Strain gauge
5. Failure reasons of Pressure sensors
6. Pressure Transmitter (with circuit)
7. Capacitive differential pressure transducer