Pressure is expressed as?

• A. PSIA (Pounds per Square Inch Absolute), PSIG (Pounds per Square Inch Gauge), Hg (Inches of Mercury in a vacuum)
• B. Pascal (Pa)
• C. Bar
• D. Atmosphere (atm)

Answer: (A)

Pressure is described using different scales depending on how you’re comparing it. The three common ways you’ll see in refrigeration work are absolute pressure (PSIA), gauge pressure (PSIG), and vacuum pressure measured in inches of mercury (inHg) relative to a vacuum. Absolute pressure includes the effect of atmospheric pressure, so PSIA tells you the true pressure inside a vessel. Gauge pressure, PSIG, shows how much pressure exists above the surrounding atmosphere—what you typically monitor for system operation and safety. For very low pressures, a vacuum reading is given as inches of mercury in a vacuum, indicating how close the pressure is to a perfect vacuum.

The combination of these three units—PSIA, PSIG, and inches of mercury in a vacuum—covers the main ways pressure is expressed in this field, which is why it’s the best choice. While units like Pascal, Bar, or Atmosphere are legitimate pressure units, they don’t encompass the common refrigeration-specific expressions (absolute, gauge, and vacuum) used in practice. For example, a gauge reading of 20 psig corresponds to about 34.7 psia when you add atmospheric pressure (~14.7 psi), illustrating how the different scales relate to each other.