Question

Which flow-measurement device works on the principle of differential pressure created by a constriction in the pipe?

• A. Rotameter
• B. Venturi-meter
• C. Turbine flow meter
• D. Magnetic flow meter

Hint:

Obstruction flow meters like the Venturi-meter, Orifice plate, and Flow nozzle all use a constriction to create a pressure drop, which is used to calculate the flow rate.

Answer 1

The Correct Option is B

Step 1: Understanding the Problem:
The question asks to identify the flow-measurement device that operates by measuring the difference in fluid pressure caused by a physical constriction inside a pipe.

Step 2: Key Concepts and Approach:
This device relies on the Venturi effect and Bernoulli's principle of fluid dynamics.
When fluid flows through a constricted section of a pipe, its velocity increases, which causes a corresponding decrease in static pressure.
By measuring this pressure difference, the fluid's flow rate can be calculated.

Step 3: Detailed Explanation:

• Design of the Venturi-meter: A Venturi-meter consists of a converging section, a narrow throat (the constriction), and a diverging section.

• Operating Principle: As fluid enters the converging cone and passes through the throat, the cross-sectional area decreases, forcing the fluid velocity to increase.
According to Bernoulli's equation:
\[ P_1 + \frac{1}{2}\rho v_1^2 = P_2 + \frac{1}{2}\rho v_2^2 \]
The increase in velocity ($v_2 > v_1$) results in a drop in pressure ($P_2 < P_1$) at the throat.
The pressure difference ($P_1 - P_2$) is measured using a manometer or differential pressure transducer to determine the volumetric flow rate.

• Other Devices:
* A Rotameter is a variable-area flow meter.
* A Turbine flow meter uses a mechanical rotor.
* A Magnetic flow meter operates on Faraday's Law of electromagnetic induction.

Step 4: Final Answer:
Therefore, the device that works on the principle of differential pressure created by a constriction is the Venturi-meter, corresponding to Option (B).