Instrumentation in pipe flow: A simple piezometer connected to a pipe measures which quantity at the tapping point?

Introduction / Context:

Piezometers are the most basic devices used in fluid mechanics labs and field practice to sense pressure at a point in a pipeline or hydraulic system. Understanding exactly what the column height represents is essential when applying Bernoulli’s equation and constructing hydraulic grade lines (HGL).

Given Data / Assumptions:

• Piezometer is a vertical, transparent, open-ended tube connected laterally to the pipe.
• Fluid is incompressible and the tube is sufficiently large to neglect capillary effects.
• Velocity in the piezometer tube is negligible (stagnant column).

Concept / Approach:

The piezometer reading equals the static pressure head (p/γ) at the tapping elevation, measured relative to the atmosphere (gauge). It does not include velocity head (v^2/2g) because the fluid in the tube is nearly at rest. It also cannot capture negative pressures if vaporization/cavitation occurs (air may enter), limiting its range.

Step-by-Step Solution:

Verification / Alternative check:

Compare with a Pitot tube: Pitot measures total (stagnation) head, from which velocity head can be inferred. The difference between Pitot (EGL) and piezometer (HGL) confirms the specific quantity each instrument senses.

Why Other Options Are Wrong:

• Velocity head and dynamic pressure require a stagnation device (e.g., Pitot).
• Total head includes elevation and velocity components; a piezometer does not register these.
• “Negative static pressure only” is incorrect; a piezometer measures positive gauge static pressure best and has limitations under vacuum.

Common Pitfalls:

• Confusing HGL (p/γ + z) with piezometer reading (p/γ at tap elevation z).

Final Answer:

Static pressure (gauge head)