Pressure Measurement Part I

PRESSURE

MEASUREMENTPART – I of IIIER. FARUK BIN POYEN

DEPT. OF AEIE, UIT, BU, BURDWAN, WB, INDIA

FARUK.POYEN@GMAIL.COM

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

2

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

3

Different Types of Pressure

Absolute Pressure = Gauge Pressure + Atmospheric Pressure

4

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

5

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

7

Manometers:Range: 0.2 MPa or 2 kg/cm2

U – tube Manometer:

Well – type Manometer:

8

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:

9

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

11

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

12

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.

13