Definition
A unit of pressure measured relative to a perfect vacuum, expressed in pounds of force per square inch. Because the reference point is zero pressure (a vacuum), psia readings always include the surrounding atmospheric pressure — roughly 14.7 psi at sea level on a standard day.
Plain English
A way of measuring pressure that starts counting from total emptiness. At sea level, the air around you is already pressing at about 14.7 on this scale before anything else is added.
Context Anchor
Seen in aircraft maintenance, engine, instrument, and pressure-system discussions when a pressure value must be measured from a fixed zero point.
Derivation
The 'a' stands for absolute, meaning measured from a fixed zero point — a vacuum — rather than from the surrounding air pressure. This is what separates psia from psig (gauge), which starts counting from whatever the local atmosphere happens to be.
Why Pilots Care
Mixing up psia and psig leads to readings that are off by about 14.7 psi — enough to misdiagnose a system or set a component incorrectly. Knowing which scale a manual or gauge uses keeps pressure-related troubleshooting accurate.
Analogy
Think of psia like a ruler that starts at true zero. Some pressure gauges start counting from the air pressure already around you, but psia starts from no pressure at all.
Intuition Check
Absolute does not mean perfect here. It means measured from a fixed zero-pressure reference, not from the pressure of the surrounding air.
Example Sentence 1
The engine data plate listed maximum manifold pressure in psia, so the technician added atmospheric pressure to the gauge reading before comparing values.
Example Sentence 2
Engine sensors reported manifold pressure in psia to the electronic control unit during the test run.