Contains basic pressure measurement techniques on Force Balancing Pressure Gauges and Electrical Pressure Transducers.

1. PRESSURE
MEASUREMENT
PART – III of III
ER. FARUK BIN POYEN, Asst. Professor
DEPT. OF AEIE, UIT, BU, BURDWAN, WB, INDIA
faruk.poyen@gmail.com

2. Contents:
 Force Balancing Pressure Gauge
 Dead Weight Piston Gauge
 Ring Balance Gauge
 Bell Type Pressure Gauge
 Electrical Pressure Transducers
 Strain Gauge Pressure Transducer
 Potentiometric Pressure Transducer
 Capacitive Pressure Transducer
 Reluctance Pressure Transducer
 Linear Variable Differential Transformer
 Servo Pressure Transducer
 Piezoelectric Pressure Transducer
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3. Force Balancing Pressure Gauge
 This kind of system is mostly linear.
 It is a continuous balancing system.
 They find application in calibration purposes.
 Pressure easily converted to force with introduction of surface
area.
 Dead weight piston gauge, ring balance and bell type pressure
gauge are its commonly used devices.
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4. Dead Weight Piston Gauge
 It is used in higher steady pressure measurement.
 Also find usage in calibration of bellows and diaphragms.
 The units of measurement are force and area.
 Accuracy < 0.1 %
 Range up to 300 psig.
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5. Working Principle
It consists of a very accurately machined, bored and finished piston which
is inserted into a close-fitting cylinder.
The area of cross section of both the piston and cylinder are known.
A platform is provided at the top of the piston where standard and accurate
weights are placed.
An oil reservoir with check valve is provided at the bottom.
The oil can be sucked by displacement pumps on its upward stroke.
For calibration, a known weight is first placed on the platform and fluid
pressure is applied on the other end of the piston until enough force is
developed to lift the piston weight combination and the piston floats freely
within the cylinder between limit stops.
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6. Ring Balance Gauge
 For measurement of low differential pressure.
 It consists of a hollow ring of circular section, partitioned at
the upper part and partially filled with liquid to form two
pressure chambers.
 The ring is supported at the centre of a knife edge.
 Made up of aluminium alloy or plastic moulding.
 Force operating the instrument is generated by the difference
between the pressure on two sides of partition.
 Cross section is large in case differential pressure is large.
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7. Working Principle
 The fluid under test are led into the ring through flexible connections.
 Placed such that length and movement are minimum.
 The ring balance is controlled by a weight which is at its lowest point
with same pressure on both sides.
 It is the rotation of the ring that indicates the pressure difference.
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8. Bell Type Pressure Gauge
 Range for differential pressure is between 0.06 Pa and 4 KPa.
 For static pressure, it is as high as 4 to 6 MPa.
 Force produced inside and outside the bell is balanced against a weight by compression
of the spring.
 Types: Two : - Thick Wall and Thin Wall.

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9.  Thick wall comprises a bell suspended with the open end downwards in a
sealed chamber, made of cast iron, liquid being oil or mercury.
 Open end is covered by liquid as a seal creating two chambers.
 Higher pressure acts on the inside and lower on the outside of the bell.
 The bell rises until equilibrium is maintained between upward force and
weight.
 As bell rises, a small portion of it is immersed in the sealing liquid so that the
upward thrust on it, due to buoyancy is reduced, with increase in apparent
weight.
 Since the pressure inside is greater than outside, it will cause the level of the
liquid on the outside to be greater than the level on the inside, causing the bell
to rise. The travel of the bell is proportional to the differential pressure.
 The thickness, density and area of cross section of the bell and the density of
the sealing liquid are calculated based on the range of pressure to be measured.
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10.  In Thin Wall Bell Gauge, the bell is thinner in construction
and control is achieved by means of a spring arrangement.
 Here high pressure is applied at the outside and lower
pressure is applied at the inside.
 The force difference due to pressure causes changes in the
length of the spring eventually changing the position of the
bell.
 The travel is proportional to the differential pressure.
 Range is determined here, by the modulus of elasticity of the
spring and density of the sealing liquid.
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11. Electrical Pressure Transducer
 It comprises three elements
1. Pressure sensing element viz. bellow, diaphragm, bourdon tube.
2. Primary conversion element e.g. resistance or voltage.
3. Secondary conversion element
 Mainly of the following types
1. Strain Gauge
2. Potentiometer type
3. Capacitive type
4. Reluctance type with sub classes of LVDT, Servo and Piezoelectric.
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12. Strain Gauge Pressure Transducer
 Passive resistance transducer.
 Resistance changes when compressed or stretched.
 Attached to pressure sensing device.
 There are four strain gauges connected to a bridge circuit, for two
resistance increases with increase of pressure and for the remaining two,
resistance decreases with increase of pressure.
 Under no load condition, bridge remains at balance and therefor no
current flows in the galvanometer.
 With application of pressure the strain gauges stretch or compress and
the bridge becomes unbalanced, resulting a current flow.
 The measuring the current, pressure may be calculated.
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13. 13
Strain Gauge Pressure Transducer with Bellow
Strain Gauge based Pressure Transducer on Diaphragm

14. Advantages of Strain Gauge Pressure Transducer:
 Small and easy to install
 Considerably accurate
 Offers wide range of measurement (vacuum to 20000 psig)
 Good stability
 High output signal strength
 High over range capacity
 No moving parts
 Good stability against shock and vibration
 Fast speed of response
Disadvantages of Strain Gauge Pressure Transducer:
 Cost is high
 Electrical readout is necessary
 Require constant voltage supply
 Require temperature compensation
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15. Potentiometric Pressure Transducer
 Here a potentiometer is involved.
 A movable electrical contact, called wiper, slides along the cylinder,
touching the wire at one point on each turn.
 The wiper position determines the resistance value between wiper and
wire end.
 A mechanical linkage from a bellow or a diaphragm controls the
position of the wiper.
 The wiper position determines the resistance which eventually
determines the value of pressure.
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16. Advantages of Potentiometric Pressure Transducer:
1. Resistance easily convertible to standard voltage or current
2. Simple in construction and lesser in cost
3. Easy to design
Disadvantages of Potentiometric Pressure Transducer:
1. Have finite resolution
2. Wear and tear tend to be a problem
3. Noise due to make and break of contact
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17. Capacitive Pressure Transducer
 Consists of a parallel plate capacitors coupled with a diaphragm, usually
metal and exposed to the process pressure on one side and the reference
pressure on the other side.
 Electrodes are attached to the diaphragm and are charged by a high
frequency oscillator.
 The electrodes sense any movement of the diaphragm and this changes
the capacitance.
 The change of the capacitance is detected by an attached circuit which
then outputs a voltage according to the pressure change.
 This type of sensor can be operated in the range of 2.5 Pa - 70MPa with
a sensitivity of 0.07 MPa.
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18. Working Principle
 The principle of operation of capacitive pressure transducers is based
upon the familiar capacitance equation of the parallel plate capacitor
𝐶 =
∈0∈ 𝑟 𝐴
𝑑
C = capacitance in Farad; A= area of each plate (m2); d= distance between
plates (m);
ɛ0 = 8.854 * 10-12 Farad/m2; ɛr = dielectric constant
 The capacitance of the parallel plate varies inversely with the distance
between them.
 With increase in pressure, the distance d (of the diaphragm) becomes
less and therefore C is increased and vice versa.
 Hence the bridge is unbalanced and a current flows which gives a
measure of the change of pressure.
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19. Advantages:
1. Gives out rapid response to changes in pressure
2. It can withstand a lot of vibration and shock
3. It is extremely sensitive
4. Offers a good frequency response
Disadvantages:
1. Metallic parts need insulation from each other
2. Performance affected by dirt and other contaminants
3. Temperature may lead to inaccuracy
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20. Reluctance Type Pressure Transducer
 Here the reluctance as electrical property suffers changes.
 There primarily exists three types.
1. Linear Variable Differential Transformer (LVDT)
2. Servo Pressure Transducer
3. Piezo electric Pressure Transducer
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21. Linear Variable Differential Transformer
 The device consists of a primary winding (P) and two secondary
windings named S1 and S2.
 Both of them are wound on one cylindrical former, side by side, and
they have equal number of turns.
 Their arrangement is such that they maintain symmetry with either side
of the primary winding (P).
 A movable soft iron core is placed parallel to the axis of the cylindrical
former.
 An arm is connected to the other end of the soft iron core and it moves
according to the displacement produced.
 The pressure range is 250 Pa - 70 MPa with a sensitivity of 0.35 MPa.
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22. Working Principle of LVDT
 AC voltage with a frequency of (50-400) Hz is supplied to the primary
winding. Thus, two voltages VS1 and VS2 are obtained at the two secondary
windings S1 and S2 respectively.
 The output voltage will be the difference between the two voltages (VS1-VS2)
as they are combined in series.
 Null Position – This is also called the central position as the soft iron core will
remain in the exact centre of the former. Thus the linking magnetic flux
produced in the two secondary windings will be equal. The voltage induced
because of them will also be equal. Thus the resulting voltage VS1-VS2 = 0.
 Right of Null Position – In this position, the linking flux at the winding S2 has a
value more than the linking flux at the winding S1. Thus, the resulting voltage
VS1-VS2 will be in phase with VS2.
 Left of Null Position – In this position, the linking flux at the winding S2 has a
value less than the linking flux at the winding S1. Thus, the resulting voltage
VS1-VS2 will be in phase with VS1.
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23. Working Principle Continued…
 VS1-VS2 will depend on the right or left shift of the core from the null
position.
 The resulting voltage is in phase with the primary winding voltage for
the change of the arm in one direction, and is 180 ° out of phase for the
change of the arm position in the other direction.
 The magnitude and displacement can be easily calculated or plotted by
calculating the magnitude and phase of the resulting voltage.
 The LVDT is connected to a diaphragm or bellow and with changes of
pressure, the position of the LVDT changes, producing current output
from where the pressure difference can be evaluated.
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24. 24
(a) LVDT type electrical pressure
transducer
(b) LVDT Extended to Null
Balance form of Measurement

25.  Advantages of LVDT:
1. It possesses high sensitivity
2. Very rugged in construction and therefore tolerant towards shock and
vibration
3. Stable and easy to align
4. Offers infinite resolution
5. Low hysteresis, hence repeatable
 Disadvantages of LVDT:
1. Relatively large core displacement
2. Sensitive to stray magnetic fields
3. Affected by temperature
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26. Servo Pressure Transducer
 Also called a force balance pressure transducer.
 It produces an electrical signal proportional to the pressure.
 It finds huge application in industrial differential pressure measurements.
 An increase in pressure P1 over P2 flexes the diaphragm and moves the short
end of the force beam.
 The force beam pivots and the long end moves a magnetic material in the
reluctive detector.
 There the signal is converted from A.C to D.C power and amplified via an
amplifier.
 An induction motor is activated by the amplifier which brings the force beam
back to its original position.
 The range of this type of pressure transducer is below 500 psi. It is
unresponsive to high frequency pressure oscillations.
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27. 27

28. Piezo Electric Pressure Transducer
 Piezoelectric characteristics of certain crystalline materials are used.
 Electricity is generated when pressure is applied.
 Some of these materials are Barium Titanate Sintered powder, quartz, tourmaline,
Rochelle salt.
 The main advantages of these crystals are that they have high mechanical and thermal
state capability, capability of withstanding high order of strain, low leakage, and good
frequency response.
 Each crystal has three sets of axes – Optical axes, three electrical axes OX1, OX2, and
OX3 with 120 degree with each other, and three mechanical axes OY1, OY2 and OY3
also at 120 degree with each other.
 The mechanical axes will be at right angles to the electrical axes.
 Some of the parameters that decide the nature of the crystal for the application are
1. Angle at which the wafer is cut from natural quartz crystal
2. Plate thickness
3. Dimension of the plate
4. Means of mounting
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29. Working Principle
 A force, when applied on the quartz crystal, produces electric charges on the
crystal surface.
 The charge thus produced can be called as piezoelectricity.
 Piezo electricity can be defined as the electrical polarization produced by
mechanical strain on certain class of crystals.
 The rate of charge produced will be proportional to the rate of change of force
applied as input.
 As the charge produced is very small, a charge amplifier is needed so as to
produce an output voltage big enough to be measured.
 The figure shows a conventional piezoelectric transducer with a piezoelectric
crystal inserted between a solid base and the force summing member.
 If a force is applied on the pressure port, the same force will fall on the force
summing member. Thus a potential difference will be generated on the crystal due
to its property.
 The voltage produced will be proportional to the magnitude of the applied force.
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30. 30

31.  Advantages of Piezo electric Pressure Transducer
1. Very high frequency response.
2. Self-generating, so no need of external source.
3. Simple to use as they have small dimensions and large measuring range.
4. It has a large dielectric constant. The crystal axis is selectable by orienting
the direction of orientation.
 Disadvantages of Piezo electric Pressure Transducer
1. It is not suitable for measurement in static condition.
2. Since the device operates with the small electric charge, they need high
impedance cable for electrical interface.
3. The output may vary according to the temperature variation of the crystal.
4. The relative humidity rises above 85% or falls below 35%, its output will
be affected. If so, it has to be coated with wax or polymer material.
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32. 32

33. Calibration
 It is a process of adjusting instruments’ output to match a range of
pressure with better accuracy and precision.
 It includes zero, span and linearity adjustment.
 It is thereafter tested in measuring some components whose values are
accurately known.
 Low for pressure (up to 3 inches of water gauge), micrometer or inclined
gauge is used.
 For the range up to 144 inches of water gauge, water manometer and
mercury manometers are used.
 In the range till 73 inches of mercury pressure, mercury manometer is
used.
 For ranges beyond that, dead weight piston gauge is used.
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34. Calibration of Pressure Transmitters
 Following steps are involved to calibrate pressure transmitters in the
range from 25 to 125 psig for output signal of 4 to 20 mA.
1. Transmitter allowed to reach normal temperature.
2. Dead weight piston gauge is used to supply 25 psig to instrument.
3. Zero is adjusted for an output of exactly 4 mA.
4. Next a pressure of 125 psig is supplied, and span is adjusted at 20 mA.
5. 75 psig (half scale) pressure is applied and output is checked at 12 mA.
6. These steps need iteration as zero and span may throw off against each
other.
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35. Maintenance
 Highly important to long life and good accuracy and precision of
instruments.
 Periodic maintenance is done by the following methods:
1. Visual inspection for any leak or damage in pipes and wiring.
2. Blowdown and venting in case there is clogging in the pipe or
channel.
3. Blowdown is used to remove dirt and foreign particles.
4. Venting is used to prevent gas build ups in the pipe to stop faulty
pressure reading.
5. Cleaning and lubricating keep the mechanical components smooth
and stops misalignment.
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36. Care of Instrument
 Following steps are to be taken for pressure measuring instruments.
1. Liquid filled manometers need levelling, maintaining a reasonable
constant temperature.
2. Proper allowances must be provided for pressure where liquids enter
the manometer.
3. Pressure gauges should be mounted correctly, protected from heat,
corrosion and vibration.
4. Zero setting should be periodically.
5. Recalibration should be done quarter yearly.
6. Instruments having mercury should be cleaned and refilled periodically
based on the expected accuracy.
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37. References
 Chapter 12: Pressure Measurement, “Industrial Instrumentation and Control” by S
K Singh. Tata McGraw Hill, 3rd Edition. 2009, New Delhi. ISBN-13: 978-0-07-
026222-5.
 Chapter 10: Pressure Measurement, “Instrumentation, Measurement and Analysis”.
2nd Edition, B C Nakra, K K Chaudhry, Tata McGraw-Hill, New Delhi, 2005.
ISBN: 0-07-048296-9.
 Chapter 6: Pressure Sensors, “Fundamentals of Industrial Instrumentation”, 1st
Edition, Alok Barua, Wiley India Pvt. Ltd. New Delhi, 2011. ISBN: 978-81-265-
2882-0.
 Chapter 3: Pressure Measurement, “Principles of Industrial Instrumentation”, 2nd
Edition. D. Patranabis, Tata McGaw-Hill, New Delhi, 2004. ISBN: 0-07-462334-6.
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