🔍 Key Concepts

• Difference between absolute pressure, gauge pressure, and vacuum • How pressure is measured relative to atmospheric pressure versus a perfect vacuum • Common use of pressure terms in engineering and hydraulics on ships

💭 Think About

• Ask yourself: When we say a tire is at 90 psi, is that compared to a perfect vacuum or to the surrounding atmosphere? • Which term describes pressure measured above or below atmospheric pressure? Which term describes pressure measured from zero pressure at a perfect vacuum? • If atmospheric pressure is about 14.7 psi at sea level, how would you express a pressure that is 29.4 psi above a perfect vacuum using each term?

✅ Before You Answer

• Be clear on what perfect vacuum (zero pressure) means on the pressure scale • Identify which term uses atmospheric pressure as the reference and which uses vacuum as the reference • Before choosing, match the definition in the question exactly: “difference between the pressure at a point and that of a perfect vacuum”.