Pressure measurement with a piezometer tube The pressure indicated by a simple piezometer tube connected to a pipe (open to atmosphere at the tube top) is:

Introduction:
Piezometer tubes are the most basic devices for measuring fluid pressure by observing the static head of a liquid column. This question asks which pressure reference is obtained by a plain piezometer.

Given Data / Assumptions:

• Transparent vertical tube connected to a pressurized pipe containing an incompressible liquid.
• Tube top is open to the local atmosphere.
• Capillarity and vapor pressure effects are negligible.

Concept / Approach:
The piezometer shows the height h such that p_pipe − p_atm = gamma * h. Because the tube is open to atmosphere, it inherently measures pressure relative to atmosphere—i.e., gauge pressure. Absolute pressure would require adding atmospheric pressure to this reading or using a sealed reference.

Step-by-Step Solution:
1) Read column height h above the connection centerline (with datum corrections).2) Compute p_gauge = gamma * h.3) Recognize that absolute pressure is p_abs = p_atm + p_gauge.

Verification / Alternative check:
Switching to a different local atmospheric pressure changes the absolute pressure but not the gauge reading principle, confirming the device measures relative to atmosphere.

Why Other Options Are Wrong:

• Vacuum pressure: a plain piezometer cannot read gas/vacuum directly and cannot indicate below-atmospheric pressures for gases.
• Absolute pressure: requires reference to vacuum, not atmosphere.
• Atmospheric pressure: that is the reference, not the measured value.

Common Pitfalls:
Using a piezometer for gases (impractical); forgetting elevation corrections when comparing points.

Final Answer:
gauge pressure