Question

Which flow meter operates on Bernoulli's principle and causes a permanent pressure drop?

• A. Rotameter
• B. Venturimeter
• C. Orifice meter
• D. Orifice meter

Hint:

An orifice meter is inexpensive and simple to install but causes high permanent energy (pressure) loss due to sudden constriction and turbulence. A venturi meter is expensive but highly efficient, preserving fluid pressure.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks to identify which of the listed head-type flow meters operates based on Bernoulli's theorem and causes a high, permanent pressure loss in the piping system.

Step 2: Key Formula or Approach: Bernoulli's equation states that for steady, frictionless flow along a streamline:
\[ P_1 + \frac{1}{2}\rho v_1^2 + \rho g z_1 = P_2 + \frac{1}{2}\rho v_2^2 + \rho g z_2 \] This relationship shows that introducing a constriction inside a pipe increases fluid velocity ($v$), causing a corresponding decrease in static pressure ($P$).

Step 3: Detailed Explanation:

• Constriction Flow Meters: Both the Venturi meter and the Orifice meter restrict fluid flow to measure the resulting differential pressure ($\Delta P = P_1 - P_2$), which is proportional to the volumetric flow rate.

• Orifice Meter: Consists of a thin plate with a sharp-edged circular opening placed across the pipe. Because the constriction is sudden, flow separation occurs, creating a "vena contracta" and generating large downstream eddies. This turbulence dissipates a large portion of the fluid's kinetic energy as heat, resulting in a high permanent pressure drop (typically losing $50\% - 80\%$ of the measured differential pressure).

• Venturimeter: Designed with a gradual converging section and a streamlined diverging recovery cone. This design minimizes boundary layer separation and turbulence, allowing almost complete pressure recovery (permanent pressure drop is typically only $10\% - 15\%$).

• Rotameter: A variable-area flow meter operating under a constant pressure drop.

Step 4: Final Answer:
The orifice meter operates on Bernoulli's principle and introduces a high permanent pressure drop in the fluid stream, making option (C) the correct choice.