Measurement of pressure difference between two points is generally done by using :

Asked in SSC JE 2014, EVENING

Detailed Explanation

Measuring the pressure drop between two sections in a fluid system is fundamental for evaluating friction losses and determining flow rates across constriction meters.

1. Instrument functions:

• Venturimeter: A variable-area flow meter used to determine the rate of discharge (\(Q\)) through a closed pipe, using the pressure difference created across its throat.
• Pitot tube: An open-ended tube used to measure local flow velocity at a specific point in a stream.
• Differential manometer: A U-tube instrument whose limbs connect to two distinct points to directly measure the pressure difference (\(p_A - p_B\)).

2. Hydrostatic equation for differential U-tube:

\[ p_A - p_B = g \cdot x \cdot (\rho_m - \rho) \]

Where \(x\) is the difference in levels of the manometric fluid.

Why “Differential manometer” Is Correct

The differential manometer connects across two points in a piping system to measure the pressure difference directly from the displacement of its manometric liquid.

Why the Other Options Are Wrong

• A. Venturimeter: A Venturimeter measures discharge (flow rate), utilizing a differential manometer to read its throat differential.
• B. Pitot tube: A Pitot tube measures local fluid velocity at a point, not the pressure difference between two separate points.
• D. None of the above: Option (c) is correct; 'None of the above' is incorrect.

Key Concepts for Students

• Types include U-tube differential manometers, inverted U-tubes, and micromanometers.
• Inverted U-tubes are chosen when the pressure difference is very small and the flowing fluid is a liquid.
• Two piezometers can also measure pressure difference if the pressure is moderate and positive.

Practice more: this is question 27 in Measurement of Pressure MCQs – Part 2. Try the whole set and check every answer instantly.