Mouthpieces and pressure head — absolute pressure at vena contracta In external or internal mouthpieces, when the atmospheric pressure head is equivalent to 10.3 m of water, the absolute pressure head at the vena contracta can drop to zero under sufficient driving head. Is this statement correct?

Introduction / Context:
In short-tube (mouthpiece) flows, the pressure at the vena contracta can fall below atmospheric. When the vacuum equals the local atmospheric head (~10.3 m of water at sea level), the absolute pressure approaches zero, which triggers a change in flow regime (e.g., mouthpiece running full) or cavitation limits.

Given Data / Assumptions:

• Water at standard conditions; atmospheric head ≈ 10.3 m of water.
• Well-formed vena contracta occurs inside the mouthpiece.
• Neglecting dissolved gases and vapor pressure nuances for the conceptual limit.

Concept / Approach:
Absolute pressure head h_abs = h_atm − h_vac. If h_vac → h_atm (≈ 10.3 m), then h_abs → 0. At or below this limit, cavitation (or flow alteration) occurs; practical operation avoids approaching the absolute zero because vapor pressure is reached earlier (~0.2–0.3 m head for water at room temperature).

Step-by-Step Solution:

Verification / Alternative check:
Engineering texts describe that when absolute pressure at the vena contracta is reduced sufficiently, separation patterns change and full running occurs; also, vapor pressure is reached prior to zero absolute in practice.

Why Other Options Are Wrong:

• “Incorrect” conflicts with the conceptual limit statement.
• Absolute pressure cannot be negative; that option is physically impossible.
• Limiting to gas flow or sharp-edged orifices is unnecessary; the concept is general.

Common Pitfalls:
Equating the practical cavitation onset (at vapor pressure) with zero absolute pressure; zero is a theoretical limit, cavitation starts earlier.

Final Answer:
Correct