1. What is measurement of flow ?

Flow measurement is the process of determining the quantity of fluid (liquid or gas) passing through a cross-section per unit time. It is essential in:

• Hydraulic systems
• Power plants
• Chemical industries
• Water supply systems

2. Types of Flow Measurement

Flow can be measured in two ways:

2.1 Volumetric Flow Rate (Q)

$$Q = \frac{\text{Volume}}{\text{Time}}$$

2.2 Mass Flow Rate (ṁ)

$$\dot{m} = \rho Q$$

Where:

• $\rho$ρ = density of fluid

3. Basic Flow Equation

Flow rate through a pipe:$$Q = A \times V$$

$Q = A V$

Where:

• $A$A = cross-sectional area
• $V$V = velocity of fluid

4. Classification of Flow Measuring Instruments

• Differential pressure type
• Variable area type
• Velocity type
• Mass flow meters
• Positive displacement meters

5. Types of Flow Measuring Instruments

5.1 Venturimeter

Principle

Based on Bernoulli’s theorem.

Construction

• Converging section
• Throat
• Diverging section

Working

• Fluid velocity increases at throat
• Pressure decreases
• Pressure difference used to calculate flow

4

Advantages

• High accuracy
• Low energy loss

5.2 Orifice Meter

Principle

• Based on pressure drop across an orifice plate

4

Advantages

• Simple and inexpensive

Disadvantages

• High energy loss
• Lower accuracy than venturimeter

5.3 Rotameter (Variable Area Meter)

Principle

• Float rises in tapered tube based on flow rate

Features

• Direct reading
• Suitable for low flow rates

5.4 Pitot Tube

Principle

• Measures velocity using stagnation pressure

$$V = \sqrt{\frac{2(P_s – P)}{\rho}}$$V=ρ2(Ps​−P)​​

$V = \sqrt{\frac{2(P_s – P)}{\rho}}$V=ρ2(Ps​−P)​​

4

5.5 Turbine Flow Meter

Principle

• Flow causes turbine rotation
• Speed proportional to flow rate

4

5.6 Electromagnetic Flow Meter

Principle

Based on Faraday’s Law of Electromagnetic Induction.

Advantages

• No obstruction to flow
• High accuracy

5.7 Ultrasonic Flow Meter

Principle

• Uses sound waves to measure flow

4

Types

• Transit-time
• Doppler type

6. Comparison of Flow Meters

Instrument	Accuracy	Pressure Loss	Cost
Venturimeter	High	Low	High
Orifice Meter	Medium	High	Low
Rotameter	Medium	Low	Moderate
Electromagnetic	High	Very Low	High

7. Factors Affecting Flow Measurement

• Fluid properties (density, viscosity)
• Temperature and pressure
• Pipe diameter
• Flow conditions (laminar/turbulent)

8. Applications

• Water supply systems
• Oil and gas industries
• Chemical processing plants
• Power plants
• HVAC systems

9. Advantages of Flow Measurement

• Efficient process control
• Energy saving
• Accurate billing
• Safety monitoring

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