You'll find a wonderful explanation of the following topics:- Sensing Elements,Bellows ,Bourdon tube,Impact of Operating Environment ,Diaphragm,Differen,capacitance,tial Pressure Transmitters,strain gauge

1. PRESSURE
MEASUREMENT
PRESENTED BY:
MOHANAD
FAEK NOORI

2. PRESSURE MEASUREMENT
Pressure is the action of one force against another over, a surface.
The pressure P of a force F distributed over an area A is defined as: [ P = F/A ]
Units of pressure:
standard: N/m2
(Pa-Pascal),
Kpa (Kilo Pascal)
1Bar=100kpa

3. Pressure Measurement
Divided into four categories :-
1.Absolute pressure – pressure at a point in a fluid relative to a vacuum (absolute
zero of pressure)
2.Gauge Pressure – pressure relative to local atmospheric pressure.
3.Differential Pressure – difference between two unknown pressures, either of which
is atmospheric pressure.
4.Atmospheric pressure : Sea level standard atmospheric pressure is 101.325Kpa but
it changes with altitude of earth.1 atmosphere [technical] is equal to 98.0665Kpa
Relationship between absolute and gauge pressures
Pabs=Pgauge+Patm

4. SENSING ELEMENTS
The main types of sensing elements are
•Bourdon tubes
•diaphragms
•bellows
The basic pressure sensing element can be configured as a C-shaped Bourdon tube
(A); a helical Bourdon tube (B); flat diaphragm
(C); a convoluted diaphragm (D); a capsule (E); or a set of bellows (F).

5. BELLOWS
• bellows sensor is an axially flexible, cylindrical enclosure with folded sides. When
pressure is applied through an opening, the closed end extends axially.
• Bellows elements can measure absolute pressure, gauge pressure, vacuum, or
differential pressure.

6. BOURDON TUBE
• A Bourdon gauge uses a coiled tube, which, as it expands due to pressure
increase causes a rotation of an arm connected to the tube.
• bourdon are often used in harsh environments and high pressures, but can also be used
for very low pressures; the response time however, is slower than the bellows or
diaphragm.
C-type bourdon
psi Range as low as 0 - 15 psi up to 0-1500
Helical bourdon
Range as low as 0 - 200 psi up to 0 – 6000 psi
Spiral bourdon
Range as low as 0-10 psi up to 0-100,000 psi.

7. Diaphragm
• A diaphragm is a circular-shaped convoluted membrane
that is attached to the pressure fixture around the
circumference . The pressure medium is on one side
and the indication medium is on the other.
• Diaphragms provide fast acting and accurate pressure
indication. However, the movement or stroke is not as
large as the bellows .

8. Diaphragm
• There are two different devices that are referred to as
capsule.
• The first is shown in figure . The pressure is
applied to the inside of the capsule and if it is fixed only
at the air inlet it can expand like a balloon. This
arrangement is not much different from the diaphragm
except that it expands both ways.
• The capsule consists of two circular shaped, convoluted membranes (usually
stainless steel) sealed tight around the circumference. The pressure acts on the
inside of the capsule and the generated stroke movement is shown by the
direction of the arrow.
• The second type of capsule is like the one shown in the differential pressure
transmitter (DP transmitter). The capsule in the bottom is constructed with two
diaphragms forming an outer case and the interspace is filled with viscous oil.
Pressure is applied to both side of the diaphragm and it will deflect towards the
lower pressure.

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10. Differential Pressure Transmitters
• Most pressure transmitters are built around the pressure capsule concept.
• They are usually capable of measuring differential pressure (that is, The
difference between a high pressure input and a low pressure input) and
therefore, are usually called DP transmitters or DP cells.

11. strain gauge
• 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.

12. strain gauge
• When force is applied to any metallic wire its length increases due to the strain.
The more is the applied force, more is the strain and more is the increase in
length of the wire. If L1 is the initial length of the wire and L2 is the final length
after application of the force, the strain is given as: ε =(L2-L1)/L1
• Further, as the length increases, diameter decreases.

13. capacitance
• In a capacitance-type pressure sensor, a high-frequency, high-voltage oscillator
is used to charge the sensing electrode elements. In a two-plate capacitor
sensor design, the movement of the diaphragm between the plates is detected
as an indication of the changes in process pressure.
• Further, as the length increases, diameter decreases.

14. Impact of Operating Environment
All of the sensors described in this module are widely used in control and
instrumentation systems throughout the power station
Vibration
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.
Temperature
The temperature effects on pressure sensing will occur in two main areas:
The volumetric expansion of vapour is of course temperature dependent.
Depending on the system, the increased pressure exerted is usually already
factored in.
Vapour Content
The content of the gas or fluid is usually controlled and known. However,
it is mentioned at this point because 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.

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