A simple U-tube manometer connected to a pipe in which liquid is flowing with a uniform speed will give ______.
Try it first: pick an option, then read the explanation.
Asked in SSC JE 11/10/2023 (09am-11am)
Detailed Explanation
Liquid column manometers operate by balancing fluid pressure against hydrostatic columns. The reference pressure at the open end dictates what type of pressure is measured.
1. Operating configuration of a simple U-tube manometer:
- Left limb is connected to the pressurized pipe: senses \(p_{\text{pipe}}\).
- Right limb is open to the ambient surroundings: senses \(p_{\text{atm}}\).
2. The hydrostatic column deflection \(\Delta h\) measures the difference between pipe pressure and atmospheric pressure:
\[ \Delta p = p_{\text{pipe}} - p_{\text{atm}} = p_{\text{gauge}} \]
Therefore, a simple U-tube manometer directly provides the gauge pressure.
Why “gauge pressure” Is Correct
Because one arm of the simple U-tube manometer is open to the surrounding atmosphere, the instrument naturally measures pressure with respect to local atmospheric pressure as its reference datum, which is the definition of gauge pressure.
Why the Other Options Are Wrong
- A. vacuum pressure: Vacuum pressure is merely a negative gauge pressure (when pipe pressure is below atmospheric).
- C. atmospheric pressure: Atmospheric pressure is measured by a closed-end barometer, not a pipe manometer.
- D. absolute pressure: Absolute pressure can only be obtained if the open end is evacuated to a vacuum or if p_atm is added to the gauge reading.
Key Point
Any manometer open to atmosphere reads gauge pressure: p_gauge = p_abs − p_atm.
Key Concepts for Students
- Piezometers and open U-tube manometers inherently measure gauge pressure.
- To measure absolute pressure directly, one limb must be sealed with a Torricellian vacuum above the fluid.
- Gauge pressure can be positive (above atmospheric) or negative/vacuum (below atmospheric).
Practice more: this is question 2 in Measurement of Pressure MCQs – Part 1. Try the whole set and check every answer instantly.