An open-tube manometer is a U-shaped tube with one arm open to the air and a liquid inside. The liquid levels in the two arms differ by a height h. What does that height measure?
AThe absolute pressure of the source
BThe atmospheric pressure that day
CThe density of the trapped liquid
DThe gauge pressure of the source
Answer & Solution
Correct answer: D. The gauge pressure of the source
1. With both arms open to the air, atmospheric pressure pushes down equally on each side and its effect cancels, so the levels match.
2. Connecting one arm to a source shifts the levels by whatever amount the source pressure differs from atmospheric.
3. That difference is the gauge pressure, and it equals h rho g for the liquid in the tube.
4. So an open-tube manometer reads gauge pressure, positive when the source is above atmospheric and negative when it is below.
5. Absolute pressure would need a sealed vacuum above the liquid instead of an open arm, which is what a barometer has.
_Source: OpenStax College Physics (CC BY 4.0), Ch 11 "Fluid Statics", section 11.6 Gauge Pressure, Absolute Pressure, and Pressure Measurement_
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