To measure the static pressure in a pipe, one uses a pressure gauge connected to :
Try it first: pick an option, then read the explanation.
Correct answer: B. a piezometer tapping
Asked in SSC JE 23.03.2021 (Evening Shift)
Detailed Explanation
Static pressure represents the thermodynamic fluid pressure exerted on the pipe boundaries by the random molecular motion of fluid particles, undisturbed by the bulk flow velocity.
1. Static pressure tap requirements:
- A small hole is drilled perpendicular (normal) to the inner surface of the pipe wall.
- The edges of the tapping must be smooth, flush, and entirely free from burrs or roughness.
- Fluid streamlines must pass parallel to the opening without deflection.
2. Analysis of the options:
- Piezometer tapping: Measures pure static pressure because velocity normal to the wall is zero.
- Pitot tube: Aligns facing into the flow, measuring stagnation (total) pressure: \(p_0 = p_{\text{static}} + \frac{1}{2}\rho V^2\).
- Venturi / Orifice meter: Flow differential devices used to measure discharge, not simple static pressure.
Why “a piezometer tapping” Is Correct
A piezometer tapping drilled normal to the flow boundary measures undisturbed static pressure without introducing any dynamic impact from fluid velocity.
Why the Other Options Are Wrong
- A. a pitot tube: A Pitot tube faces upstream and measures stagnation pressure (static plus dynamic pressure).
- C. a venturi meter: A Venturi meter is a flow-rate measurement constriction consisting of a converging cone, throat, and diffuser.
- D. an orifice meter: An orifice meter is an obstruction meter used for discharge determination via pressure drop across an orifice plate.
Key Point
Static pressure is measured via a flush piezometer wall tapping normal to the fluid streamlines.
Key Concepts for Students
- Wall tap holes must be small (diameter ≤ 1/10 pipe diameter, typically 1 to 6 mm).
- Burrs or bevelled edges at the hole cause significant dynamic pressure errors (positive or negative).
- A piezometer ring (piezometer piezoring) averages readings from multiple wall taps around the circumference.
Practice more: this is question 7 in Measurement of Pressure MCQs – Part 1. Try the whole set and check every answer instantly.