🔍 Key Concepts

• Relationship between pressure head and vacuum (suction) head in pump systems • Common engineering units: feet of water vs inches of mercury • How positive pressure and negative (below atmospheric) pressure are usually indicated on gauges

💭 Think About

• When a suction gauge shows a pressure ABOVE atmospheric, what kind of head is that and what units are typically used in marine pump calculations? • When a suction gauge shows a VACUUM (below atmospheric), what units do gauges and textbooks usually use to express that vacuum? • Think about how compound pressure/vacuum gauges are actually marked on engine room piping—what units do you see on the pressure side and on the vacuum side?

✅ Before You Answer

• Identify which unit is more natural for water column height in hydraulic calculations • Identify which unit is most commonly used to show vacuum (below atmospheric) on gauges • Make sure your choice uses one unit for positive pressure head and the appropriate unit for suction/vacuum as normally found on marine pump gauges