Which pressure measurement is zeroed to atmospheric pressure?

• A. psiA
• B. psiG
• C. psiL
• D. psiB

Answer: (B)

The pressure measurement that is zeroed to atmospheric pressure is psiG, which refers to gauge pressure. Gauge pressure measurements are calibrated to exclude atmospheric pressure, meaning that when the pressure being measured equals atmospheric pressure, the gauge reads zero. This allows for the measurement of pressure above or below atmospheric levels, which is critical in various applications, such as anesthesia delivery systems where changes in pressure can affect the functioning of the machine.

In contrast, psiA refers to absolute pressure, which is measured relative to a perfect vacuum and always includes atmospheric pressure in its reading. psiL typically correlates with low-pressure measurements, often related to the venturi effect within the machine. psiB is often associated with barometric pressure or a related measurement that isn’t zeroed to atmospheric standards in the same way gauge readings are. Thus, psiG is the correct answer as it specifically indicates a pressure measurement that is referenced to ambient atmospheric levels.