Pressure measurement – appropriate use of a piezometer tube A simple piezometer tube connected to a pipeline is suitable only for measuring which range/type of pressures in liquids?

Difficulty: Easy

Correct Answer: Low pressures (small positive gauge heads) only

Explanation:


Introduction / Context:
A piezometer tube is the simplest device for measuring pressure head in a liquid line by observing the height to which the fluid rises in a vertical transparent tube. Knowing its limitations helps choose the right instrument for a given application.



Given Data / Assumptions:

  • Device: open, vertical tube connected to a liquid line.
  • Fluid: essentially incompressible liquid; no entrained gas assumed.
  • Pressure indicated as static head h = p/γ, read as liquid column height.


Concept / Approach:
The tube displays positive gauge pressure as a rise above the tap point. Very high pressures would require impractically tall tubes. Gases cannot be measured accurately because their density is low and compressible. Negative gauge pressure (vacuum) cannot be read directly, since the tube would not sustain a stable column below the tap point; manometers are used instead.



Step-by-Step Reasoning:

Relate pressure to head: h = p/γ.For large p, h becomes very large → impractical tube lengths.For vacuum (p < 0 gauge), the tube would not show a downward “negative” column → alternate devices required.


Verification / Alternative check:
Common practice uses U-tube or inclined manometers for accurate small pressure differences and vacuum, and Bourdon gauges for higher pressures, confirming the piezometer’s limited role.



Why Other Options Are Wrong:

  • High, gas, vacuum, or impact pressures are beyond the practical capability of a simple piezometer.


Common Pitfalls:
Attempting to use a piezometer on gas lines or expecting it to indicate negative gauge pressure directly.



Final Answer:
Low pressures (small positive gauge heads) only

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