U–tube manometer measures :

Asked in SSC JE 2007

Detailed Explanation

U-tube manometers are constructed in several configurations: simple U-tubes open to atmosphere, differential U-tubes connecting two lines, and sealed-end U-tubes with an evacuated limb.

1. Analysis of U-tube configurations:

• Simple U-tube with sealed evacuated limb: Directly measures absolute pressure at a point (\(p_{\text{abs}} = \rho_m g h\)).
• Differential U-tube: Measures the difference in pressure between two points (Option b).
• Simple open U-tube: Measures gauge pressure at a point (\(p_{\text{gauge}} = \rho_m g h_2 - \rho_1 g h_1\)).

2. Examination Key Note:

In the official SSC JE 2007 key, option (d) ('Absolute pressure at a point') is marked. When the closed-limb configuration is used with an internal vacuum, the instrument functions as an absolute manometer measuring absolute pressure at a point.

Why “Absolute pressure at a point” Is Correct

Option (d) is designated in the official SSC JE answer key. When a U-tube has a closed evacuated arm, it measures absolute pressure at a point directly.

Why the Other Options Are Wrong

• A. Local atmospheric pressure: Local atmospheric pressure is measured with a Torricellian barometer.
• B. Difference in pressure between two points: Difference in pressure between two points is measured specifically by a differential U-tube manometer, though option (d) is the key's designated answer.
• C. Difference in total energy between two points: Total energy difference includes kinetic and potential heads, not measured by a manometer alone.

Key Concepts for Students

• An open U-tube manometer measures gauge pressure at a point.
• A differential U-tube manometer measures pressure difference between two points.
• A closed evacuated U-tube manometer measures absolute pressure at a point.

Practice more: this is question 35 in Measurement of Pressure MCQs – Part 3. Try the whole set and check every answer instantly.