The pressure of a liquid measured with the help of a piezometer tube is :

Asked in SSC JE 2011

Detailed Explanation

A piezometer is the simplest possible device for measuring liquid pressure, consisting of a transparent vertical tube tapped into the side of a pipe or container.

1. Operating characteristics of a piezometer:

• Bottom end is connected to the point where pressure is to be measured.
• Top end is completely open to the atmosphere.
• Liquid rises in the tube until the hydrostatic weight of the liquid column balances the pressure in the pipe.

2. Mathematical relation:

\[ p_{\text{pipe}} - p_{\text{atm}} = \rho g h \;\implies\; p_{\text{gauge}} = \rho g h \]

3. Limitations of piezometers:

• Cannot measure vacuum pressure (air would be sucked into the pipe).
• Cannot measure gas pressure (gas would escape into the atmosphere).
• Cannot measure very high pressure (would require an unreasonably tall tube).

Why “Gauge pressure” Is Correct

Because the piezometer column is open to the atmosphere, the liquid column height reflects the excess pressure over local atmospheric pressure, which is precisely gauge pressure.

Why the Other Options Are Wrong

• A. Vacuum pressure: Vacuum pressure cannot be measured with a piezometer because air would enter the pipe and the liquid would drop out of sight.
• C. Absolute pressure: Absolute pressure requires adding local atmospheric pressure to the piezometer reading: p_abs = p_atm + ρgh.
• D. Atmospheric pressure: Atmospheric pressure is measured using a closed-end mercury barometer, not a piezometer.

Key Concepts for Students

• Piezometers cannot measure gas pressures because gases have no free surface.
• Piezometers cannot measure negative gauge (vacuum) pressures.
• Tube diameter must be at least 12 mm to eliminate capillary rise errors.

Practice more: this is question 30 in Measurement of Pressure MCQs – Part 2. Try the whole set and check every answer instantly.