week 5-6 Pressure measurement devices lecture

EE218-INSTRUMENTATION &
MEASUREMENT
DR. MONA IBRAHIM

LECTURE 5-PRESSURE
MEASUREMENTS

PRESSURE MEASUREMENT AND DISPLAY
In order to
display
pressure
values, two
approaches
are used:
Elastic
Sensing
Elements
Balancing
Techniques
Types:
❑Bourdon
Tube
❑
Diaphragms
❑ Bellows Types: Manometers:
U-type
manometer
Well-type
manometer
Inclined-type
IN ORDER TO DISPLAY PRESSURE VALUES, TWO
APPROACHES ARE USED:

Elastic Sensing Elements
Diaphrag
m
Bourdon
Tube
Spiral
Helical
Capsules
C-Type
Bellows
 Most pressure sensors used in process control, converts the
pressure into a mechanical displacement
 Displacement is converted to a pointer movement via a link,
quadrant and a gear arrangement
 The elastic element can be Bourdon tube, diaphragm, bellows or
capsules.

Pressure Measurement: 2.Elastic Sensing Elements

Advantages
1. They give reasonably accurate results.
2. They are simple in construction and inexpensive to
produce.
3. They can be safely used for the accurate measurement of
high pressures.
4. They can be modified to generate electrical outputs.
5. Bourdon tube gauges have high repeatability, i.e., they
can accurately record the same pressures multiple times.
Disadvantage
s
1. A Bourdon tube responds slowly to changes in pressure.
2. An amplification mechanism is needed to generate
readings, as the movement of the free end of the Bourdon
tube is quite low.
3. While Bourdon tube gauges can produce accurate results,
they cannot be used for precision applications.
4. Bourdon tubes suffer from hysteresis, a condition that
causes the tube to retract slower when moving from a high
scale reading to a low scale reading. This results in the scale
showing a slightly “higher than actual” reading.
5. Bourdon tubes are medium-sensitive, i.e., they are
usually calibrated to only work with a particular medium.
Using another medium can affect the reading owing to its
own density and weight, and thus result in an error. This
must be accounted for before changing the medium.

Any type of Bourdon tube can measure pressure proportional to the arc it subtends,
i.e., the more curved the arc, the greater is the pressure sensitivity of the device. Thus,
spiral and helical tubes are more sensitive to pressure changes than a C-type tube.
This, however, increases the complexity, and consequently, the cost of manufacturing.

• Used when Bourdon tube is inappropriate to be used
• A diaphragm is nothing more than a thin disk of material
which bows outward under the influence of a fluid pressure.
Many diaphragms are constructed from metal (Metallic
diaphragms gauge) (brass or bronze), which gives them
spring-like qualities and can either be flat or corrugated.
Some diaphragms are intentionally constructed out of
materials with little strength, such that there is negligible
spring effect. These are called slack diaphragms (Rubber) ,
and they are used in conjunction with external mechanisms
(e.g. springs) producing the necessary restraining force to
prevent damage from applied pressure.
• The Diaphragm Pressure Gauge uses the elastic deformation
of a diaphragm (i.e. membrane) instead of a liquid level to
measure the difference between an unknown pressure and a
reference pressure.
• The displacement of the diaphragm causes the pointer
deflection.

• A typical Diaphragm pressure gauge contains a capsule
divided by a diaphragm, as shown. One side of the
diaphragm is open to the external targeted pressure, PExt,
and the other side is connected to a known pressure, PRef,.
The pressure difference, PExt – PRef, mechanically deflects the
diaphragm.

• More sensitive than Bourdon and Bellows, thus used with
low pressure when high accuracy is needed (otherwise
require an impractically large and thin diaphragm).
Capsule is used in measurement of the aircraft altitude,
and for the cabin pressure.
• The sensing element of a capsule pressure gauge consists
of two corrugated diaphragms welded together at their
periphery to form a capsule. The pressure to be measured
is introduced into the capsule via an opening in the
center of the first diaphragm. The center of the second
diaphragm is connected to the transmission mechanism
so that the deflection of the measuring element can be
transmitted to the pointer.
• When the pressure rises inside the capsule, both
diaphragms will slightly deform. By making use of two
diaphragms, the total deflection of the measuring
element is twice as large.
• For the measurement of very small pressure differences,
the deflection of a single capsule may be too small.
In the pressure
gauge animation
the pressure is
going in and out
the capsule,
turning the pointer
to the right and
back to the left.

• In certain applications, especially when measuring vacuum pressure
• Bellows are also used in low-intermediate pressure measurement as
well as differential pressure measurement (dual bellows exist)
• Bellows resemble an accordion constructed from metal instead of
fabric. Increasing pressure inside a bellows unit causes it to elongate.
• Bellows are thin-walled metallic cylinders, with deep convolutions, of
which one end is sealed and the other end remains open. The closed
end can move freely while the open end is fixed. It is made of
phosphoric bronze & stainless steel. When pressure is applied to
the closed end, such as in
the animation, the bellows
will be compressed. The
closed end will move
upwards and the link, which
is the rod in between the
closed end of the bellows
and the transmission
mechanism, will go up and
rotate the pointer.

week 5-6 Pressure measurement devices lecture

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week 5-6 Pressure measurement devices lecture