In fluid Mechanics course pressure measurement with different devices is much important. In this slides we will learn different pressure measuring devices.

1. MANOMETRY(PressureMeasurement)
Why is it important?
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2. Overview:
What is pressure in a fluid?
Types of pressure
Hydrostatic Law
Pascal’s Law
Measurement of pressure by using MANOMETERS and Mechanical Gauges
Different types of Manometers
Different types of Mechanical Gauges
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3. Pressure (P ) of a fluid can expresses as the magnitude of normal force (F in Newton) per
unit area (A in m2
) applied on a surface.
Units: Pa(= N/m2)
, psi(=lbf/in2
), bar (=105
Pa=100 kPa), etc.
Types of Pressure:
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4. Hydrostatic Law: in a static Fluid
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P.dA
(P+dP).dA
dh
Cylindrical
Element of fluid
W

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So if we use water we would require a glass tube of approximately “11 meters” which is quite
difficult, compared to a 84 cm glass tube (which we use in barometer). If we change our Gauge
liquid to mercury.

6. PASCAL’S LAW:- “The intensity of pressure at any point in a liquid at rest, is the
same in all direction.”
Consider an arbitrary fluid element of wedge shape ABC in a fluid mass at rest . The width of the element
perpendicular to the plane of paper is Unity.
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8. Different types of Pressure measuring Devices ::
Many techniques have been developed for the measurement of pressure. Instruments used to
measure pressure are called pressure gauges. Measuring of fluid pressure may be broadly
classified under following
1.Manometers
2.Mechanical Gauges
1.Manometers these are the pressure measuring devices which are based on the principle of balancing the
column of liquid(whose pressure is to be found) by the same or another column of liquid. The manometers
are classified as
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(i) Simple manometer:
 Piezometer
 U-tube manometer
 single column manometer
• Vertical single column manometer
• Inclined single column manometer
(ii) Differential manometer :
 U-tube differential manometer
 Inverted U-tube differential manometer

9. Barometer:Barometer:
An instrument for measuring local atmospheric Pressure.
A mercury barometer has a glass tube with a height of at least 84 cm, closed at one end,
with an open mercury-filled reservoir at the base. The weight of the mercury creates a
vacuum in the top of the tube. Mercury in the tube adjusts until the weight of the mercury
column balances the atmospheric force exerted on the reservoir
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10. Piezometer:Piezometer: It has a tube in which the Fluid(liquid) can rise freely, the
height of the liquid in the tube will give the value of
Pressure.
Its diameter is kept at least 12mm in order to reduce the
error.
ADVANTAGES: easy technique, accurate, economical
DISADVANTAGES: cannot measure…
1) negative pressure
2) pressure of gases
3) control of column height
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U-Tube ManometerU-Tube Manometer: To overcome limitations of piezometer

12. A U-shaped tube is half-full of liquid, one side of which is connected to the region of interest and the
other side to the reference atmospheric pressure is applied to the other.
The difference in height of the liquid represents the applied pressure.
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xx

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Single column Manometer:
(i) Vertical single column manometer

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(ii) Inclined single column manometer

15. U- Tube Differential Manometer:U- Tube Differential Manometer:
In many cases we require the difference between two pressures. For
such purposes we use a Differential manometer.
In this arrangement of manometer we connect the two ends with
different liquids whose pressure we want to measure.
It is to be noted that only those liquids should be used who are
immiscible with each other. Otherwise liquids will mix and we will not be
able to measure the difference of their heights.
Math equation:
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It is used to measure pressure difference at two points in a pipe or
between two pipes at different levels.
Case 1 - U-tube upright differential manometer connected at two points
in a pipe at same level

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Case 2 - U-tube upright differential manometer connected between two pipes at
different levels and carrying different fluids

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Inverted U-tube differential manometer
It is used for low pressure difference.

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Bourdon Tube
- The linkage is constructed so that the mechanism may be adjusted for optimum linearity
and minimum hysteresis as well as compensate for wear which may develop over a period
of time.
- An electrical-resistance strain gauge may also be installed on the bourdon-tube to sense
the elastic deformation.

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Diaphragm and Bellows Gauges
●
Represent similar types of elastic deformation
devices useful for pressure measurement applications.
●
Architecture and operation:
Diaphragm gauge:
- Consider first the flat diaphragm subjected
to the differential pressure p1-p2 as shown in
figure .
- The diaphragm will be deflected in accordance with
this pressure differential and the deflection sensed
an appropriate displacement transducer.
gauge are shown figure 4.10

26. Bellows Gauge:
- The bellows gauge is shown in figure
- A differential gauge pressure force causes displacement of the bellows, which may
be converted to an electrical signal or undergo a mechanical amplification to
permit display of the output on an indicator dial.
- Figure shows various types of bellows gauges.
●
The bellows gauge is generally unsuitable for transient measurements because of
the larger relative motion and mass involved.
●
The diaphragm gauge which may be quite stiff, involves rather small displacements
and is suit for high frequency pressure measurement.
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27. Mechanical Dead weight gauges
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28. Thank you.Thank you.
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