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PRESSURE
MEASUREMENTPART – I of IIIER. FARUK BIN POYEN
DEPT. OF AEIE, UIT, BU, BURDWAN, WB, INDIA
Contents:
1. Unit of Pressure
1. High Pressure
2. Low Pressure
2. Different Types of Pressure
1. Gauge Pressure
2. Absolute Pressure
3. Vacuum or Differential Pressure
4. Static Pressure or Velocity Pressure
3. Methods of Pressure Measurement
4. Manometers
1. U – tube Manometer
2. Well type Manometer
3. Barometer
4. Inclined Manometer
5. Micro manometer
6. Ring Balance Manometer
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Pressure: Defined as the amount of force applied to a
surface or distributed over it and is measured as force per unit
area.
Units of Pressure (P = F/A)
The basic unit of pressure in SI units is the Pascal (Pa).
It is defined as force of 1 Newton (N) per square meter (m2). That is: 1 Pa = 1 N/m2.
High pressure
1 N/m2 = 1 Pa;
1 atm = 14.696 psi = 101.325 kPa
Low pressure
1 millibar = 100 dyne/cm2 = 14.5 * 10-3 psi
1 micron = 10-6 Hg = 19.34 * 10-6 psi
1 torr = 1 mm Hg = 19.34 * 10-3 psi
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Different Types of Pressure
Absolute Pressure = Gauge Pressure + Atmospheric Pressure
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Fig 1: Different Pressure Scales
•Vacuum or Differential Pressure:
Gauges that indicate gauge pressure may be designed to indicate pressures
below zero. Such a gauge is called a “vacuum gauge”. Hauges that
indicate absolute pressure cannot indicate pressures below zero as zero is a
perfect vacuum.
• Static Pressure and velocity Pressure:
When the fluid is in equilibrium, the pressure at a particular point is
identical in all directions and independent of the orientation. This is called
static pressure.
Velocity pressure is the difference between the total pressure and static
pressure.
VELOCITY PRESSURE = TOTAL PRESSURE – STATIC PRESSURE
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TABLE I: Relationship between Units 6
Pressure units
V
T
E
Pascal Bar
Technical
atmosphere
Standard
atmosphere
Torr
Pounds per
square inch
(Pa) (bar) (at) (atm) (Torr) (psi)
1 Pa ≡ 1 N/m2 10−5 1.0197×10−5 9.8692×10−6 7.5006×10−3 1.450377×10−4
1
bar 105
≡ 100 kPa
≡ 106 dyn/cm2 1.0197 0.98692 750.06 14.50377
1 at 0.980665×105 0.980665 ≡ 1 kp/cm2 0.9678411 735.5592 14.22334
1
atm 1.01325×105 1.01325 1.0332 1 ≡ 760 14.69595
1
Torr 133.3224 1.333224×10−3 1.359551×10−3 1.315789×10−3
≡ 1/760 atm
≈ 1 mm Hg 1.933678×10−2
1 psi 6.8948×103 6.8948×10−2 7.03069×10−2 6.8046×10−2 51.71493 ≡ 1 lbF /in2
Methods of Pressure Measurement
1. Manometer Method
2. Elastic Pressure Transducer
3. Measuring Vacuum Method
4. Force Balancing Method
5. Electrical Pressure Transducer
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Manometers:Range: 0.2 MPa or 2 kg/cm2
U – tube Manometer:
Well – type Manometer:
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𝑃1 − 𝑃2 = 𝜌 − 𝜌1 ℎ1 − ℎ2 𝑔 = 𝜌 − 𝜌1 ℎ𝑔
Fig 2: U – tube Manometer
Fig 3: Well – type Mano meter
Barometer:
Inclined Manometer:
Micro manometer:
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Fig 4: Barometer
Fig 5: Inclined
Manometer
Fig 6: Micro Manometer
Ring-Balance Manometer:10
Fig 7: Ring Balance Manometer
Fig 8: manometers in different shapes
Errors in Manometer:
Effects of Temperature
Capillary Effect
Effect of Variable Meniscus
Fluids for Manometer:
Water
Red oil
Mercury
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Advantages of Manometer:
Simple and time proven
High accuracy and sensitivity
Wide range
Reasonable cost
Suitable for low pressure and low differential pressure
Disadvantages of Manometer:
Large and bulky
Levelling is required
Compatibility required between manometer fluid and measured fluid
No over – range protection
Condensation is a potential problem
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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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