In fluid mechanics instrumentation, what does a piezometer specifically measure when connected to a point in a water channel or pipe (assuming the fluid is a liquid and the pressure is not below atmospheric)?

Introduction / Context:
A piezometer is one of the simplest pressure-measuring devices in hydraulics. It is widely used in civil engineering laboratories and field work to read the pressure head at a point in a liquid system such as a pipe, channel, or tank.

Given Data / Assumptions:

• The fluid is a liquid (not a gas) and is exposed to atmospheric pressure at the piezometer tube top.
• Pressures are not below atmospheric (no cavitation or vacuum conditions).
• Steady conditions and negligible capillary effects (tube sufficiently large).

Concept / Approach:

A piezometer is simply a vertical, transparent tube tapped into the flow at the point of interest. The static liquid column rises to an elevation where the hydrostatic head balances the local gauge pressure. The piezometric reading directly gives pressure head p/(rhog) above atmospheric at that tap location.

Step-by-Step Solution:

Verification / Alternative check:

Manometers with two limbs can measure pressure differences; a single-limb piezometer reads only the local pressure head and cannot measure vacuum (sub-atmospheric) pressure correctly because the column would break.

Why Other Options Are Wrong:

(b) A piezometer does not provide differential pressure; that is the role of differential manometers. (c) Atmospheric pressure is measured by a barometer. (d) Very low (vacuum) pressure cannot be handled by a simple open piezometer. (e) Not applicable.

Common Pitfalls:

Using a piezometer for gases or for negative gauge pressures; ignoring elevation of tap point when converting head to pressure.

Final Answer:

pressure in water channels and pipes (pressure head at a point)