This document discusses various techniques for measuring pressure and flow. It describes direct acting pressure transducers, manometers, elastic transducers, and types of measuring devices like LVDT, variable capacitance, and piezoelectric transducers. It also discusses high and low pressure measurement techniques, as well as devices for measuring fluid velocity like rotameters, hot wire anemometers, and laser Doppler velocimetry. Measurement ranges and operating principles are provided for different pressure and flow measurement devices.

1. Measurement of Pressure
and Flow
Presentation by Prof. S. S. Petkar
Reference: Instrumentation , Measurement & Analysis by Nakra
Choudhari

2. Direct Acting Pressure Transducers

3. Gravitational Pressure Measurement

4. Inclined Tube Manometer

5. For static calibration of pressure measuring devices, this method is used
DirectPressureMeasurement

6. Elastic Transducers
Sensing Elements

8. Types of Measuring Devices
For Dynamic Pressure Measurement
the o/p voltage can be indicated or recorded by suitable
instrument like oscilloscope or recorder

9. LVDT Type Pressure Transducer

10. LVDT Type Pressure Transducer
The device is very sensitive and is linear over wide range of motion

11. Variable Capacitance type Pressure transducer
• Here elastic diaphragm is used
• While deflection capacitance change
between electrode and elastic member
• The o/p of bridge is proportional to
pressure

12. Piezoelectric Pressure Transducer
• Piezo electric crystal is
used for dynamic pressure
• Static variable is not
measured with this type of
transducer
• Voltage is developed in
between A and B
• Crystalline material :
Quartz, barium titanite

14. High Pressure Measurement
• For measuring pressure above 1000atm.
• Electric resistance change of manganin (alloy of
Cu, Ni, Mn)or gold chrome wire with hydrostatic
pressure , due to bulk compression effect
• Coil is enclosed in bellows filled with kerosene
for proper transmission of pressure to be
measured in coil
• Change in resistance between insulators
measured by Wheatstone bridge

15. Range for various Types of pressure Measuring devices
Type of Measuring devices Pressure range
Manometer 10 to 106 Pa
Bourdon Gauge 103 to 108 Pa
Elastic Diaphragm with LVDT / Capacitance or
Resistance strain gauge transducer 100 to 108 Pa
Piezoelectric transducer 104 to 108 Pa

16. Low Pressure Measurement (Vacuum Measurement)
• Units of Vacuum measurement are Torr and Micron
• 1 Torr= 1 mm of hg
• I micron = 1/1000 torr
Devices
• Manometers and elastic gauges can be used to about 0.1 torr
• McLeod Gauge
• Pirani Gauge (Thermal Conductivity Gauge)
• Ionisation gauge
• Knudsen gauge (High Vacuum measurement)

17. McLeod gauge
• Used between 0.01 and 1000 microns.
• It is modified version of Mercury
Manometer
• It operates on principle of compressing
known volume of low pressure gas to high
pressure , and measuring the resulting
volume change.

20. Pirani Gauge (Thermal conductivity gauge)
1. it is dedicated low vacuum measurement
device
2. The resistance of the hot wire changes
with the rate of heat loss (conduction) to
the gas
3. Low pressure high temp. principle used
4. Compensating cell is used for minimizing
surrounding temp effect on filament.

22. Ionization Gauge

24. Knudsen Gauge
Knudsen gauge is a type of manometer that works with the interaction of particles with a surface.
Particles that interact with a hotter or colder surface will exert a force on that surface

25. Fluid Velocity
• Difference between stagnation pressure
and static pressure is measured.
• V= 𝟐(𝒑𝒔𝒕𝒈 − 𝒑𝒔𝒕)/𝝆
• Advantages:
• 1. Simple and low cost
• 2. does not produce pressure loss
• 3. can be easily put through small hole into
pipe or duct.
• 4. it is useful for checking mean velocities
of flow in venturi, nozzle, orifice plate

26. Rotamenter
• It is variable area flow meter device
• Float has a higher density than fluid to move up.
• The float will rise when upward forces (drag and
buoyant) is balanced by weight of float.
• The position of the float is nothing but flow rate
• the elevation of float is dependant on annular
area between it the tapered glass tube.
• It is used for metering liquid and gas
• For gases small sphere is used instead of float
• Materials:
High strength borosilicate glass
Range:- 0.1 ml/ min to more than 100 l/min
Accuracy :- +-1%
They must be mounted vertically.

27. Hot Wire Anemometry

29. Hot wire Anemometer
• It is used for measuring mean and fluctuating velocities in fluid flow
• The flow sensing element is a short length of 5micrometer in dia, platinum tungsten wire
welded between two prongs of the probe.
• The rate of cooling of wire depends upon:-
1)Shape, size & physical properties of wire
2)Difference of temperature between the heated hot wire and fluid stream
3)Properties of fluid flow
4)Velocity of the fluid.
• Constant current Method :- used @ higher frequencies & relatively smaller signals
• Constant Temperature method:- adjust current for getting initial temperature

30. Advantages
• The o/p is electrical & read out can be analog
or digital
• The time constant of the instrument is of the
order of 1/10000 to 1 /1000000 sec.
• The instrument exhibits excellent dynamic
characteristics
• Accuracy + - 0.1% for mean velocity and 2 %
for turbulent level
• Range from very low to supersonic velocity
• Does not cause pressure loss to flow as probe
is small in size
• Suitable for gases as well as liquid
Limitations
• The hot wire does not sense flow direction
• Its o/p depends on cooling of the wire in both
cases
• Accumulation of the dirt on hot wire changes
sensitivity, therefore needs frequent calibration
• The wire is very fragile.
• Clean fluid is required.
• The operating characteristics are highly non linear
• It has expensive circuitry
• Needs skilled operations

31. LASER Doppler Velocimetry

32. Materials for production of Laser Beams
• Ruby(Al oxide crystal doped with
small amount of Chromium)
• Nd Glass (Neodymium)
• CO2 Gas
• Neon Gas
• Ionised Orgon Gas
• Helium Neon gas
• Semiconductor crystal gallium
arsenide

33. Advantages
• The o/p voltage of instrument is
proportional to velocity
• No physical object is inserted in the flow
field and thus flow is undistributed by the
measurement
• Very high frequency response of order of
Megahertz is possible
• Accuracy is + - 0.2 %
• Measurements are possible in a miniature
size of volume of the order 0.2 mm side
cube
• Suitable for both gas and liquid flows
Limitations
• It involves the need of
transparent channel
• The measurement technique is
not suitable for clean flows
• The instrument is quite
expensive and requires a high
degree of experience
• Skilled operations are required
• For clean flow, the tracer
particles have to be seeded in
the flow for scattering the light.